Canadian Journal of Microbiology
Groundwater microbial diversity and antibiotic resistance linked to human population density in Yucatan Peninsula, Mexico
Journal: Canadian Journal of Microbiology
Manuscript ID cjm-2019-0173.R2
Manuscript Type: Article
Date Submitted by the 24-Sep-2019 Author:
Complete List of Authors: Moore, Anni; Morningside College, Biology and Chemistry Lenczewski, Melissa; Northern Illinois University, Geology and Environmental Geosciences Leal-Bautista,Draft Rosa Maria; Centro de Investigación Científica de Yucatán Duvall, Melvin; Northern Illinois University, Biological Sciences
Keyword: antibiotic resistance, karst, groundwater, microbial communities
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1 Groundwater microbial diversity and antibiotic resistance linked to human population
2 density in Yucatan Peninsula, Mexico
3
4
5 Anni Moore,a* Melissa Lenczewskib#, Rosa Maria Leal-Bautistac, and Melvin Duvalla,d
6
7 Department of Biological Science, Northern Illinois University, DeKalb, IL USAa
8 Department of Geology and Environmental Geosciences, Northern Illinois University,
9 DeKalb, IL, USAb
10 Water Science Unit, Centro de Investigacion Cientifica de Yucatan, Cancun, QR, Mexicoc
11 Institute for the Study of the Environment,Draft Sustainability, and Energy, Northern Illinois
12 University, DeKalb, IL USAd
13 # Corresponding Author
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14 ABSTRACT
15 Microbial community composition in selected karst groundwater sites in the Yucatan
16 Peninsula, Mexico was assessed to determine the environmental variables influencing
17 groundwater microbial diversity. The karst aquifer system is a groundwater dependent
18 ecosystem and is the world’s second largest underwater karst cave system. The area’s
19 geology allows precipitation to infiltrate into the groundwater system and prevents
20 accumulation of surface water; as such, groundwater is the only source of fresh water on the
21 peninsula. The sampling locations consisted of three karst sinkholes that extend through the
22 freshwater zone into the saline water, and an abandoned drinking water well of an ocean-side
23 resort during the dry and rainy seasons. The analysis showed that highly diverse microbial
24 communities are present in the YucatanDraft groundwater, sustained by permanently warm
25 temperatures and high nutrient input from human activity. Proximity to densely populated
26 areas, such as tourist resorts, is the most important factor influencing both diversity, presence
27 of fecal bacteria, and antibiotic resistance profile.
28
29 Key Words: microbial communities, karst, groundwater, antibiotic resistance
30
31
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32 INTRODUCTION
33
34 Yucatan Peninsula, Mexico, lacks surface water, and is completely dependent on groundwater
35 for its freshwater needs. At the same time, the Yucatan aquifer is also extremely vulnerable to
36 pollution due to the karst geology and the increased population and tourism that often lacks
37 adequate sewage treatment and disposal options. As a result, the fresh groundwater in most
38 parts of the peninsula is now unsuitable for human consumption.
39
40 The geological features that contribute to the vulnerability of the aquifer are the highly
41 permeable bedrock and inadequate soil covering. The bedrock consists of karstified soft and
42 porous marine carbonate rocks, formingDraft a highly cavernous terrain (Perry et al. 2009). Its
43 permeability is enhanced further by extensive fracturing of the terrain, allowing saltwater flow
44 into the cave system and resulting in subsurface erosion, collapse of the cave roofs, and
45 formation of an extensive network of caves and karst sinkholes (Perry et al. 2009; Gines and
46 Gines 2007). The mean depth of the soil cover of the peninsula is 7.2 cm (White and Hood
47 2004), which allows water at the surface to infiltrate quickly and directly into the aquifer
48 (Perry et al. 2003). The lack of sufficient soil cover together with the karst cannot provide
49 sufficient filtering of contaminants (chemical or biological) into the aquifer.
50
51 The aquifer of the peninsula is characterized by a thin freshwater lens overlaying the
52 seawater-derived saline layer (Lefticaru et al. 2006). This permanent stratification is sustained
53 due to the density difference between the fresh and saline waters, and a narrow mixing zone
54 (interface) exists between the two. The thickness of the freshwater layer varies throughout the
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55 peninsula from less than 10 m near the coast to over 60 m in the center of the peninsula. The
56 aquifer can easily be accessed through the karst sinkholes locally called cenotes that penetrate
57 deep into the saline layer of the aquifer.
58
59 The main threat to the aquifer comes from human activities. While the lack of large-scale
60 industrial development has kept the aquifer free from various industrial pollutants, the aquifer
61 of Yucatan Peninsula is threatened mainly by municipal human waste. This has become an
62 especially acute problem since the planned development of major tourist destinations on the
63 Caribbean coast caused the population of eastern state of Quintana Roo to explode from
64 88,150 in 1970 (Instituto Nacional de Estadística, Geografía e Informática 2001) to 1.326
65 million in 2010 (Instituto Nacional de DraftEstadística y Geografía 2013) with 17 million tourists
66 in 2017 (Instituto Nacional de Estadística y Geografía 2019). While this kind of planned
67 development has economically benefited the local population, the development of adequate
68 wastewater treatment has not kept up with the development of tourist resorts.
69
70 As a result, the freshwater aquifer, which is the only source of drinking water for the
71 peninsula, is most notably contaminated with fecal bacteria, which has great health and
72 economic impacts on the population (Pacheco et al. 2000). While the aquifer is well
73 characterized geochemically, no studies have looked at the native groundwater microbial
74 communities, their response to pollution, and their potential to deal with the high nutrient load
75 introduced into the aquifer. This information will provide both an overview of the
76 biogeochemical interactions in the aquifer and should help improve groundwater
77 management.
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78
79 This study aims to investigate the overall microbial diversity and genomic potential in the
80 groundwater, to determine which environmental variables drive the overall diversity, how
81 these factors influence the presence of fecal indicator bacteria, pathogens, cyanobacteria, and,
82 at the genomic level, antimicrobial and heavy metal resistance, and bacterial degradation of
83 aromatic hydrocarbons.
84
85
86 Draft
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87 MATERIALS AND METHODS:
88
89 Sampling sites
90
91 Three cenotes (Cenote X, U, and C) and an abandoned drinking water well were sampled in
92 the Yucatan Peninsula (Figure 1 and Table 1). Cenote X is located in a remote area,
93 surrounded by woods and agricultural fields. The freshwater layer of Cenote X is
94 characterized by eutrophic growth during the hot, rainy season, and turnover event in October
95 or November when the warm surface layer of water begins to cool, becomes denser, and
96 sinks, bringing the clear freshwater up from deeper layers. The dry season is characterized by
97 relatively clear freshwater layer. The edgeDraft of the cenote is lined by trees, allowing vegetation
98 input. Cenote U is also located in a remote area, surrounded by woods and agricultural fields,
99 and it serves as a water source for the surrounding farms. Cenote U is in a partially collapsed
100 cave, limiting the vegetation input. Cenote U is characterized by clear water throughout the
101 year. Cenote C is located at the closed territory of a limestone quarry without public access to
102 the water. The nearest resort and the coast are about 3 km distant, and the nearest town about
103 8 km east of the cenote. The groundwater flow is towards the sea, and there are no towns
104 located within the 20 km ‘upstream’ from the cenote, although there are a few single houses
105 about 10-15 km west and northwest of the site. Cenote C is partially covered with a limestone
106 ledge (uncollapsed part of the former cave roof), limiting the vegetation input. Cenote C is
107 characterized by exceptionally clear water during both dry and rainy seasons.
108
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109 The abandoned well is located on the campus of a busy tourist resort and was abandoned
110 because of saline water intrusion. The well is located about 0.5 km from the coast. It is
111 completely covered with no light or vegetation input, and no public access. The resort
112 operates its own moving bed biofilm reactor type wastewater treatment facility. This
113 technology is compact and convenient to operate, although its efficiency of nutrient removal
114 (COD, BOD, phosphate and nitrate) is half of the fixed-bed bioreactor (Choi, Lee, and Lee
115 2012). The volume of sewage handled by this specific facility and the output volume are not
116 available. The effluent of these treatment facilities (owned by the hotel group in multiple
117 tropical locations globally) has traditionally been used by that hotel chain for irrigation
118 purposes (50%) and deposited into injection wells (50%) (National Environment and Planning
119 Agency 2008). Draft
120
121 Water Sampling
122
123 Water samples were obtained from the four sites during two dry seasons (December 2008 or
124 March 2009), and one rainy season (July 2009). Cenote U was sampled only during the one
125 rainy season, once. Prior to each sampling, the geochemical profiles of the cenotes were
126 obtained using the Hydrolab Multiparameter Sonde MS5 45922 (Pedersen 2007). There is
127 very little seasonal variation in the profiles.
128
129 The water samples were obtained either by using peristaltic pump (cenotes X and U, and the
130 hotel well), or a Niskin Bottle (Cenote C). The samples were obtained from three depths
131 (freshwater, interface and saline layers) from the cenotes, and from freshwater and interface
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132 layers in the hotel well that does not have an established saline layer (Table 1; Figure 2). The
133 sediment samples were obtained from the cenotes X and C; the hotel well and Cenote U did
134 not yield any sediment. The water was filtered through 0.2 µm pore size and 47 mm diameter
135 Millipore IsoporeTM polycarbonate membrane filters. The amount of water filtered varied
136 between the samples, ranging from 0.2 to 10 liters, depending on the amount of particulate
137 matter in the water. Because of the heavy algal growth, the rainy season fresh water sample
138 from Cenote X was pre-filtered using glass fiber pre-filters (SartoriusTM). The filters were
139 placed in sterile Petri dishes and frozen until DNA extraction.
140
141 DNA was extracted from the filters usingDraft MoBIO UltraClean Soil DNA Isolation Kit 142 (currently Qiagen, Germantown, MD). A quarter of each filter was used for an initial
143 extraction; for most sites additional extractions were necessary to obtain enough DNA for
144 metagenomic analysis. DNA from the sediment samples was also extracted using the MoBIO
145 UltraClean Soil DNA Isolation Kit, using 0.5 g of the sediment.
146
147 Sequencing and data assembly
148
149 For the taxonomic classification of the communities, the 16S ribosomal rRNA V4 variable
150 region was sequenced at Argonne National Laboratories, Lemont, IL, USA in collaboration
151 with the Earth Microbiome Project (www.earthmicrobiome.org). The region was amplified
152 using 515F (GTG CCA GCM GCC GCG GTA A) and 806R (GGACTACHVGGGTWTCTAAT)
153 primers. All twenty-one samples were sequenced on the paired-end Illumina (San Diego, CA,
154 USA) MiSeq platform. The 16S rRNA V4 data were mapped to a reference sequence database
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155 using QIIME pipeline (Caporaso et al. 2010). The operational taxonomic units (OTUs) were
156 picked with a closed-reference OTU picking protocol against the Greengenes database
157 (greengenes.lbl.gov/), and the taxonomic identity was determined at 97% sequence similarity
158 level.
159
160 The metagenome sequencing was performed using the shotgun metagenomics method at
161 Argonne National laboratories on Illumina HiSeq 2000, 2x100 cycle run. The libraries of
162 eighteen samples (all sites, except Cenote U, which had low DNA yield and only one season
163 of samples) were prepared using TruSeq DNA sequence library preparation kit (Illumina, San
164 Diego, CA, USA). The metagenomic data were quality control filtered and annotated using
165 MG-RAST; the protein coding genes wereDraft annotated using the SEED Subsystems level 3
166 annotations (Meyer et al. 2008).
167
168
169 Metagenomic Data Deposition
170
171 All metagenomic data generated and used in this study are freely available on MG-RAST
172 server under project name ‘Yucatan groundwater’ as X10-mar09 (ID 4536385.3), X60-dec08
173 (ID 4536387.3), X83-dec08 (ID 4536389.3), XS-dec08 (ID 4536390.3), X10-jul09 (ID
174 4536384.3), X52-jul09 (ID 4536386.3), X70-jul09 (ID 4536388.3), C4-mar09 (ID
175 4536378.3), C15-dec08 (ID 4536374.3), C18-dec08 (ID 4536376.3), CS-dec08 (ID
176 4536379.3), C4-jul09 (ID 4536377.3), C13.5-jul09 (ID 4536373.3), C17-jul09 (ID
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177 4536375.3), H4-mar09 (ID 4536383.3), H10-dec08 (ID 4536380.3), H4-jul09 (ID
178 4536382.3), and H10.4-jul09 (ID 4536381.3).
179
180
181 Statistical analysis
182
183 Taxonomic analysis of the microbial communities was performed looking at the frequency of
184 individual groups of microbes in the groundwater, as well as alpha diversity and beta diversity
185 values. The diversity values were obtained from the QIIME analysis.
186
187 The alpha diversity was assessed usingDraft Chao1 index, phylogenetic diversity (PD) index, and
188 Shannon entropy. Chao1 index is used as a non-parametric estimator for microbial species
189 richness. This metric estimates the true number of species from the observed number of
190 species based on the probability of the particular species being observed repeatedly upon
2 푛1 191 resampling. Chao1 index is calculated as SChao = Sobs+ , where n1 is the number of species 2푛2
192 observed once, and n2 is the number of species observed during both sampling events (Hughes
193 et al. 2001).
194
195 Phylogenetic diversity (PD) considers not just the number of different species observed, but
196 also how phylogenetically diverse the community is. This measure is defined here as the
197 minimum total of branch lengths connecting all the organisms of the community in the
198 phylogenetic tree (Faith and Baker 2006).
199
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200 Shannon entropy is a diversity measure that considers both the species richness as well as
201 their abundance. This assures that neither abundant nor rare species are favored in
202 calculations, making it a fair measure (Tuomisto 2010). Shannon entropy is calculated as H =
203 - ∑pi log pi, where pi is the observed relative abundance of fragment i, which is calculated as
204 Ni /N (Yang et al. 2004).
205
206 The statistical analysis of the results was performed using multivariate regression method on
207 SAS® Enterprise Guide® ver 4.3. The level of significance α was set to 0.05, the probability
208 value calculated, and, compared to two-tailed probability (α/2). Eight variables were
209 evaluated to see what influenced the alpha diversity measures and the microbial composition
210 of the communities (phylum, genus, orDraft specific group level) (Table 2).
211
212
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213 RESULTS
214
215 Overall, the metagenomic sequencing of 18 samples produced approximately 286,728,222
216 raw sequence reads with total of 33,591,513,568 bp. After quality control and removal of
217 artificial duplicate reads, 239,157,337 sequences (27,743,268,124 bp) with an average length
218 of 130 bp were used for analysis. From the amplicon sequencing, the 21 samples yielded
219 1,729,428 sequences and 86,076 OTUs with 97% sequence identity. The number of sequences
220 per sample can be seen in Supplemental Table S1.
221
222
223 DiversityDraft at phylum level
224
225 Overall, 79 different phyla were present in the samples. In addition, three unclassified
226 categories were recognized: the unclassified bacteria, unclassified archaea, and the sequences
227 that could not be classified as either bacteria or archaea. Thirteen of the identified phyla were
228 present in the samples over (or near) 1% and were used for distribution analysis (Figure 3).
229 The summary of the significant trends as response to environmental variables on the phylum
230 level is shown in Table 3.
231
232 Genus level abundance
233
234 Overall, 1502 different OTUs were recovered from the samples, and 525 of these (35%) were
235 identified at the genus level according to the Greengenes ribosomal database (Desantis et al.
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236 2006). Eight genera each had the relative abundance over 1%: Flavobacterium (relative
237 abundance 1.46%), Prochlorococcus (1.58%), Brevundimonas (1.46%), Rhodobacter
238 (1.73%), Novosphingobium (1.81%), Desulfobacterium (1.42%), Acinetobacter (17.10%), and
239 Pseudomonas (5.45%). Of these, Acinetobacter was significantly more prevalent in Cenote X
240 and the inland sites. Pseudomonas was significantly more prevalent in Cenote U (Table 4).
241
242
243 Fecal bacteria
244
245 Five groups of fecal indicator organisms – enterobacteria (total coliforms), Escherichia coli
246 (fecal coliform), enterococci, BacteroidesDraft, and Bifidobacterium – were detected in the cenotes
247 and the hotel well. Fecal indicator bacteria were significantly more prevalent during the rainy
248 season (p = 0.002); the prevalence of individual indicator organisms varied (Figure 4 and
249 Figure 5). Enterobacteriaceae are present at each site in both seasons, although they are
250 almost absent from the Cenote X sediment. Enterobacteriaceae are also the most prevalent
251 indicator organisms for all the sites except the sediment of Cenote X, although they are
252 present in significantly higher numbers during the rainy season (p = 0.0009).
253
254 The exclusively fecal coliform bacteria (represented here by genus Escherichia) were absent
255 during both seasons from the freshwater and interface layers at Cenote X and the saline layers
256 of Cenote C and Cenote U. The hotel well site had significantly more E. coli than Cenote X (p
257 = 0.047). Looking at the human impact, the tourist impact site (hotel well) had marginally
258 more E. coli than the minimal impact site (Cenote C; p = 0.058). Enterococci were absent
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259 from many samples; the highest amount of enterococci were found in the interface sample of
260 the hotel well in rainy season. Both Cenote X and C had marginally less (p = 0.053 and p =
261 0.054, respectively) enterococci than the hotel well, and the tourist-impacted site (hotel well)
262 had significantly more (p = 0.047) than minimal impact sites.
263
264 The genus Bacteroides was similarly sparsely distributed among the sites, and absent from
265 Cenote X except for traces in the dry season fresh water sample. However, it made up
266 magnitudes higher fractions in the rainy season samples of the hotel well, where this tourist-
267 impacted site had significantly higher amounts (p = 0.034) than other sites.
268
269 Bifidobacterium was absent from half Draftof the sites, but like E. coli, it was more evenly
270 distributed. Bifidobacterium was completely absent from Cenote U and the dry season water
271 samples at Cenote X. Bifidobacterium followed the same trends as other indicator organisms
272 – the tourist-impacted hotel well had significantly higher amounts (p = 0.025) than the other
273 sites.
274
275 Prevalence of antibiotic and heavy metal resistance genes
276
277 The metal resistance pathways observed in the cenotes included resistance to arsenic, copper,
278 chromium, mercury and cobalt, zinc, and cadmium. Overall, Cenote C had significantly fewer
279 resistance genes associated with copper (p = 0.008), cobalt, zinc and cadmium (p = 0.013),
280 chromium (p = 0.0037) and mercury (p = 0.018) than the other sites.
281
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282 A similar trend was observed in the case of antibiotic resistance, although here the trend was
283 not as clear-cut. The antibiotic resistance pathways detected included many multidrug
284 resistance operons and associated structures, such as the mtdABCD multidrug resistance
285 cluster, multiple antibiotic resistance mar-locus, and various other multidrug resistance efflux
286 pumps and operons. Of the individual antibiotic resistance pathways, fluoroquinolone, beta-
287 lactamase, erythromycin, fosfomycin, streptothricin, vancomycin, methicillin and
288 aminoglycoside resistance genes were detected. Cenote C had significantly lower amount of
289 fluoroquinolone resistance (p = 0.044) together with fewer sequences associated with
290 multidrug resistance pathways, while the hotel well had higher number of sequences
291 associated with vancomycin (p = 0.0051) and fluoroquinolone (p = 0.056) resistance.
292 Draft
293 Aromatic hydrocarbon degrading bacteria
294
295 While the ability to degrade various hydrocarbons and aromatic compounds can be assessed
296 better by looking at the genomic information of the communities, 25 genera that exclusively
297 contain the known hydrocarbon or aromatic hydrocarbon degrading bacteria were identified
298 and analyzed for their abundance across the sites. These genera include Rhodococcus
299 (Actinomycetales), Sporotomaculum and Syntrophomonas (Clostridiales), Asticcaulis,
300 Brevundimonas, Caulobacter, Mycoplana, and Phenylobacterium (Caulobacterales),
301 Thalassospira (Kiloniellales), Aminobacter and Xanthobacter (Rhizobiales), Oleomonas
302 (Rhodospirilliales), Novosphingobium and Lutibacterium (Sphingomonadales), Methylibium,
303 Variovorax, and Xenophilus (Burkholderiales), Dechloromonas and Propionivibrio
304 (Rhodocyclales), Marinobacter (Alteromonadales), Alcanivorax and Oleibacter
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305 (Oceanospirillales), Acinetobacter and Alkalindiges (Pseudomonadales), and
306 Hydrocarboniphaga (Xanthomonadales). These bacteria make up a surprisingly large fraction
307 (20%) of the total bacteria due to the prevalence of Acinetobacter, Novosphingobium and
308 Brevundimonas.
309
310 Acinetobacter dominates all the water samples from Cenote X (except the dry season
311 freshwater sample), all the samples from Cenote U, both the interface samples from Cenote C,
312 and except for the dry season interface sample from the hotel well. Novosphingobium is
313 prevalent only in the rainy season fresh water sample from Cenote X, and Brevundimonas
314 composes a large fraction only in the rainy season interface sample from Cenote C. Overall,
315 both the inland cenotes (X and U) hadDraft more hydrocarbon and aromatic compound degraders
316 than the coastal sites (p = 0.0084).
317
318 The focus of functional genomic comparison was on the degradation pathways that deal with
319 aromatic compounds that are exclusively derived from coal tar or petroleum. The hotel well
320 had more sequences associated with anaerobic toluene and ethylbenzene degradation (p =
321 0.034) and toluene-4-monooxygenase (p = 0.0004). Cenote X had more sequences associated
322 with biphenyl (p = 0.043) and chlorobenzoate (p = 0.044) degradation. Cenote C had
323 significantly fewer sequences associated with naphthalene (p = 0.017), and chlorobenzoate (p
324 = 0.026) degradation. Additionally, freshwater layer had more cresol degradation (p = 0.043).
325
326
327
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328 Special groups of interest: Cyanobacteria
329
330 Cyanobacteria constituted a total of 1.76% of the organisms present with Prochlorococcus
331 (Synechococcales) being the most abundant cyanobacterium. While cyanobacteria were
332 present in every sample, they were more prevalent in the freshwater sites with sunlight
333 exposure, although the statistical significance was only marginal (p = 0.06). The relative
334 abundance of cyanobacteria was highest at Cenote C (Figure 6). The fresh and saline layers in
335 Cenote U were the only sites showing the presence of Microcoleus (Chroococcales), which is
336 one of the cyanobacteria that can switch to sulfur reduction in dark conditions (Stal and
337 Moezelaar 1997). The overall presence of the order Chroococcales (that include other
338 common genera, such as ChroococcusDraft and Microcystis) was significantly higher in the inland
339 cenotes (X and U, p = 0.045) and in the sediment samples (p = 0.0029). Pseudoanabaenales
340 had significantly higher presence in the sediment than in the water column (p = 0.0035).
341
342 Other relatively abundant cyanobacteria include the uncultured early diverging, possibly non-
343 photosynthetic lineages belonging to the 4c0d-2 clade, notably the MLE1-12, that has been
344 observed in drinking water (Poitelon et al. 2009), and YS2 that has been detected in the
345 human and animal gut (Lin et al. 2013; Chen et al. 2011). These uncultured lineages were
346 significantly more prevalent (p<0.001) in the hotel well in both depths and seasons.
347
348
349
350
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351 Diversity
352
353 Alpha diversity was assessed using three indices: Shannon entropy, Chao1 index, and
354 phylogenetic diversity (PD). Overall, the human-impacted hotel well had significantly higher
355 diversity values for all three indices than the sites with limited or no human impact. The
356 diversity was also significantly higher during the dry season (October-April). Sediment
357 samples (which yielded overall greater amounts of DNA) also had greater diversity values
358 than water samples for all three indices used. Higher sunlight exposure resulted in reduced
359 phylogenetic diversity, but not overall species richness. All the diversity metrics can be found
360 in supplemental data. The summary of the significant (and marginal) results is displayed in
361 Table 5. Draft
362
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363 DISCUSSION
364
365 Overall, the results indicate that the samples from human-impacted sites (hotel well) are
366 significantly different from samples from sites where the human impact is lesser or virtually
367 absent. The DNA evidence indicates that a rich community of microbes is present in the
368 cenotes. Overall, 1502 OTUs were detected in Yucatan groundwater, making the diversity
369 similar to the higher estimates from other karst aquifers (Gray and Engel 2013). It should be
370 noted that although the overall diversity is on the high end for this type of an environment, the
371 true diversity (including novel diversity) is likely even higher. Since the closed OTU picking
372 only maps the sequences to the reference sequence collection (in our case Greengenes),
373 sequences with no reference match areDraft discarded, resulting in underestimation of the true
374 microbial diversity in the environment.
375
376
377 Taxonomic composition and functional diversity
378
379 When looking at the composition of the microbial communities, our results indicate that the
380 cenotes differ from most groundwater and surface freshwater sites, which are mainly
381 dominated by Betaproteobacteria (Garrido et al. 2014; Röske et al. 2012). In contrast, marine
382 environments are dominated by Alphaproteobacteria (Gilbert et al. 2012), and where
383 Betaproteobacteria make up only a minute fraction of the total bacteria (Glöckner, Fuchs, and
384 Amann 1999). Additionally, except for the families Enterobacteraceae and Xantomonadaceae,
385 and the orders Pseudomonadales and Legionellales, Gammaproteobacteria are also more
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386 dominant in the saline environments than freshwater (Newton et al. 2011). Considering the
387 constant marine water intrusion, it is likely that the cenotes and the whole groundwater
388 microbial communities may resemble the ocean microbiome, especially considering the lack
389 of surface water influence.
390
391 Dominance of Acinetobacter has been documented mainly in sewage and wastewater
392 treatment influent (Shanks et al. 2013; Karadaget al. 2013), where the phosphate
393 accumulating Acinetobacter is expected to reduce the phosphate load in the water as well as
394 degrade various hydrocarbons. However, these samples are usually characterized by the co-
395 dominance of other groups, such as Lachnospiraceae (Shanks et al. 2013), or Trichococcus
396 and Aeromonas (VandeWalle et al. 2012),Draft none of which are seen in the cenotes.
397
398 The sediment communities of both Cenote X and C are dominated by Deltaproteobacteria.
399 Sediment domination by Gamma- and Deltaproteobacteria has been shown in the anoxic
400 sediments with high rates of sulfate reduction (Wang et al. 2012; Zinger et al. 2011). Because
401 this phylum includes majority of the sulfate reducing microbes, the prevalence of
402 Deltaproteobacteria is expected, although considering the highly sulfidic water of the cenotes,
403 it is surprising that Deltaproteobacteria are not more prevalent in the interface and saline
404 layers of other sites.
405
406 Euryarcheota were also most prevalent in the sediment. This phylum was dominated by the
407 WCHD3-30 cluster of archaea, an uncultured group that has been found in methanogenic
408 zones of sediments, and are related to type II methanogens, such as Methanosaeta (Dojka et
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409 al. 1998). Although the number of known methanogens in the cenotes is very low, it is likely
410 that methanogenesis is more prevalent in the sediment considering the uncultured and
411 uncharacterized methanogenic archaea.
412
413 Actinobacteria, which have been shown to be abundant in freshwater (Holmfeldt et al. 2009;
414 Newton et al. 2007), are expectedly prevalent in the freshwater layers of Cenotes X, C, and U,
415 but not in the hotel well. The abundance of Actinobacteria is mainly due to two families, the
416 uncultured C111 and ACK-M1, both of which are commonly found in (and often dominating)
417 the freshwater habitats around the world (Urbachet al. 2001; Gucht et al. 2005).
418
419 Phylum Bacteroidetes is present in all Draftsites, although it is more numerous in the coastal sites
420 than in the inland cenotes. This phylum is often prevalent in freshwater lakes (Newton et al.
421 2011). Because many members of Bacteroidetes have been associated with degradation of
422 complex biopolymers, many studies have noted an increased level of Bacteroidetes in the
423 water during cyanobacterial blooms and in the presence of high amounts of dissolved organic
424 carbon (Eiler and Bertilsson 2007). One Bacteroidetes lineage often found elevated during
425 cyanobacterial blooms is Flavobacterium (Newton et al. 2011). While most of the
426 Bacteroidetes in the cenotes are present in low amounts, there was a significant
427 Flavobacterium presence in Cenote C, which also has a significant fraction of cyanobacteria.
428
429 The only phylum with significantly higher presence in the saline layer was Planctomycetes.
430 While Planctomycetes are found in freshwater habitats and even soil, abundance and high
431 diversity of these bacteria is characteristic of anoxic marine water and sediment (Kirkpatrick
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432 et al. 2006), and therefore it is not surprising to find it in high abundance in saline
433 groundwater with constant marine water intrusion.
434
435 Fecal bacteria
436 The presence of fecal bacteria in the Yucatan aquifer is an established problem, owing to the
437 geological setting and the lack of water treatment infrastructure, adequate sewage treatment
438 and disposal options in the rural towns and villages, and the questionable wastewater effluent
439 disposal practices in the bigger towns and tourist resorts. While the previous studies have
440 evaluated the presence of fecal bacteria by traditional culture-based and biochemical methods,
441 this study assessed fecal indicator bacteria in the context of the whole community microbial
442 composition. Draft
443
444 Overall, the amount of fecal indicator bacteria in the cenotes makes up less than 1% of the
445 total microbes. Only fecal coliforms (Escherichia), fecal streptococci, Bacteroides, and
446 Bifidobacterium were used for the trend analysis. While total coliforms (Enterobacteriaceae)
447 are most prevalent of all the indicator bacteria in all the samples, they do not follow the same
448 trends as the other fecal indicators, confirming once again that total coliforms are not the best
449 indicators of fecal contamination, since some coliforms are not of fecal origin, and because of
450 their ability to persist in the environment (Carrero-Colon et al. 2011).
451
452 The removal of total coliforms from the analysis shows that most fecal bacteria are present in
453 the hotel well and in the freshwater zone of Cenote U. The increased amounts of fecal bacteria
454 in the hotel well is not surprising, considering the population density in the area, the number
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455 of tourists, the shallowness of the well, and the sewage effluent disposal practices. In Cenote
456 U, which is a popular swimming place, the fecal bacteria are significantly more prevalent in
457 the freshwater sample.
458
459 In addition to human-impacted sites, the fecal bacteria are also more abundant during the
460 rainy season, when there is an increased surface runoff, accelerating the transport of fecal
461 contaminants into the groundwater. It may also be that increased recreational use of the
462 cenotes during the hot summer months contributes to this problem. Even in the hotel well, the
463 rainy season has considerably more fecal bacteria than the dry season, even though the
464 occupancy rates of the coastal resorts are actually lower during the summer months.
465 Draft
466
467 Microbial defense mechanisms – antibiotic and heavy metal resistance
468
469 Antibiotic resistance is a serious problem worldwide, and the sub-therapeutic concentrations
470 of various antibiotics in the environment contribute to this issue by putting a selective
471 pressure on the microbes in natural environments to acquire antibiotic resistance genes
472 (Andersson and Hughes 2014). Antibiotics and antibiotic resistance genes end up in the
473 groundwater through improper disposal of pharmaceuticals, through human waste from the
474 antibiotic-consuming population, agricultural runoffs, and wastewater spills (Sui et al. 2015).
475 The latter is especially worrisome in freshwater-limited areas, such as Yucatan Peninsula,
476 where the fresh groundwater serves both as recreational and drinking water reservoirs, and
477 where the fast groundwater flow can distribute these resistance genes over long distances.
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478
479 When looking at the distribution and prevalence of antibiotic resistance genes and pathways,
480 the most common features are various multidrug resistance efflux pumps (and similar
481 operons), followed by fluoroquinolone and methicillin resistance, and beta lactamases, all of
482 which are relatively uniformly distributed throughout the sites. The hotel well that has the
483 highest human impact also has significantly higher levels of vancomycin and fluoroquinolone
484 resistance. This prevalence of fluoroquinolone resistance in Yucatan groundwater is
485 worrisome, although not uncommon, since the rapid development of fluoroquinolone
486 resistance is a global trend (Dalhoff 2012).
487
488 In addition to antibiotic resistance, thereDraft is also relatively high level of metal resistance genes
489 present in the cenotes, which is somewhat surprising considering the virtual absence of heavy
490 and transitional metals in these environments. Genes and pathways encoding for the resistance
491 to arsenic, cobalt, zinc copper and cadmium are prevalent in all the sites, while the resistance
492 pathways to mercury and chromium are present in some sites. It is possible that an
493 unknown/undetected factor is responsible for the selective pressure keeping these genes in the
494 microbial community, or that the prevalence of these genes in the virtually metal free cenotes
495 is an additional evidence of how the absence of environmental selection does not mean the
496 genes will be lost.
497
498
499
500
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501 Degradation of aromatic compounds
502
503 While the bacteria known to degrade aromatic hydrocarbons were more prevalent in Cenote
504 X, the genomic data showed significantly increased amounts of degradation pathways both in
505 the Cenote X and the hotel well. The overall most prevalent pathways were the
506 chloroaromatic degradation and biphenyl degradation. However, these pathways are present
507 naturally in various bacteria in the environment (Bugg et al. 2011), and do not necessarily
508 indicate the presence of the aromatic hydrocarbons.
509
510 The degradation pathway is the same for naphthalene and anthracene and encoded on the
511 NAH plasmid that is relatively commonDraft in various Pseudomonas strains as well as in
512 Aeromonas, Flavobacterium, Bacillus and others (Sanservino et al. 1993; Cerniglia and
513 Heitkamp 1989). Because naphthalene and anthracene are relatively soluble in water, the
514 degradation of these compounds is fast, and the ability to degrade them is widespread in both
515 pristine and contaminated environments (Cerniglia and Heitkamp 1989). Since Yucatan
516 Peninsula lacks industries where naphthalene or anthracene are used or produced, the
517 fluctuation of naphthalene degrading pathways in the water is not tied to industrial
518 contamination.
519
520 The pathways for toluene and ethylbenzene degradation are encoded on the TOL-plasmid that
521 is relatively common in Pseudomonas and various other organisms. The prevalence of both
522 toluene monooxygenase and anaerobic toluene and ethylbenzene degradation pathways
523 identified in this study only in the hotel well microbial community may suggest the presence
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524 of these compounds in the water. As both of these chemicals are found in gasoline, and as the
525 resort has more motorized traffic than either Cenote C or X, there are various sources that can
526 leak low levels of toluene and ethylbenzene to the ground.
527
528 Two trends were specific to the redox conditions: cresol degradation was more prevalent in
529 the aerobic freshwater zone and the anaerobic degradation of toluene and ethylbenzene was
530 more prevalent at the interface. Anaerobic toluene and ethylbenzene degradation are common
531 to some sulfate reducing bacteria, such as the Desulfobacteraceae family (Koizumi et al.
532 2002) as well as various denitrifying and iron reducing bacteria (Heider et al. 1998).
533 Considering the prevalence of Desulfobacteraceae throughout the sampling sites, the
534 prevalence of these pathways in the reducingDraft environments is not surprising, regardless of the
535 presence or absence of the chemicals.
536
537 Cyanobacteria
538
539 Although cyanobacteria, which are present in most aquatic environments, are not generally
540 used as primary water quality indicators, the prevalence of cyanobacteria can indicate specific
541 pollutant problems, such as high levels of phosphate (Conley et al. 2009). Based on the 16S
542 analysis, cyanobacterial taxa are not prevalent in the cenotes, with the exception of Cenote C
543 that receives the most sunlight. The cyanobacteria are not numerous in the freshwater zones of
544 Cenote X and Cenote U where the light conditions are favorable. Since iron is one of the
545 limiting nutrients for cyanobacterial growth (Mills et al. 2004), the lack of metals is likely one
546 of the major contributing factors to the lack of these organisms.
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547
548 Diversity within the communities
549
550 Besides looking at the taxonomic composition of the microbial communities, an equally
551 important indicator in ecosystem health is the overall diversity. Most of the factors
552 influencing microbial diversity are predictable. The biggest impact to all the three diversity
553 metrics came from the substrate where the microbes were growing (sediment had more than
554 water), and from the tourist impacted site (hotel well had more than other sites).
555
556 The increased diversity in the sediment is not surprising, because the sediment harbors higher
557 number of microbes than water. The impactDraft that a busy resort with a wastewater injection
558 well and effluent usage for irrigation purposes have on the groundwater and its microbial
559 content and diversity is also predictable. Location of the well less than half a kilometer from
560 the coast can also enhance the diversity, as the tides can push marine water into the well
561 (hence the saline water intrusion). The impact of marine water intrusion can also be a factor in
562 the composition of the microbial communities – the hotel well is the only site with the
563 presence of exclusively marine microbes, such as Mariprofundus (Zetaproteobacteria).
564
565 However, in this study only the phylogenetic diversity was affected by the sunlight, while
566 Shannon entropy and the Chao1 index were not. This indicates that while Cenote C (the only
567 site that received sunlight throughout the water column) may not necessarily have an overall
568 lower number of microbes present, but rather, as the data indicated, these microbial
569 communities consist of closely related photosynthetic organisms. Cenote C had the highest
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570 number of photosynthetic bacteria (both cyanobacteria, and purple and green sulfur bacteria)
571 of all the sampled sites.
572
573 Overall, the microbial diversity in Yucatan groundwater is high similar to other tropical karst
574 environments that have some human impact. The biggest factors that influence the microbial
575 community composition and the diversity in Yucatan groundwater are anthropogenic
576 influence and sunlight. While there are also minor seasonal differences between the dry and
577 rainy season communities that are likely influenced by increased runoff during rainy season,
578 and the coastal sites are influenced by the seawater intrusion, these effects are minimal.
579 Additionally, the sediment community is more diverse than the planktonic community, with
580 the sediment community having a significantlyDraft higher proportion of sequences from unknown
581 organisms. While the taxonomic difference was mainly influenced by human impact, the
582 metagenomic difference had much clearer differences between the sites, light input and
583 salinity/redox potential.
584
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585 ACKNOWLEDGEMENT
586
587 The authors would like to thank the Northern Illinois University Center for Latino and Latin
588 American Studies for the financial support. We would like to thank the Earth Microbiome
589 Project, especially Jack Gilbert and Sarah Owens, for allowing us to participate and for their
590 great insights. We would like to thank the following people who have contributed to this
591 research: Andrew Thompson, Dave Breed, Cheyenne Johnson, Rich Becker, and Bianca
592 Pedersen. We also like to thank the two anonymous reviewers for their thoughtful comments.
593
594
595 Draft
596
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798 799 800 801 802 803 Table 1: Comparison of the four sampling sites. 804 Cenote X Cenote U Cenote C Hotel Geographical coordinates 20.909567 20.989455 20.585669 20.370214 (Latitude/Longitude) -88.866947 -88.601856 -87.174442 -87.333573
Location Inland Inland Coast Coast
Type of cenote Pit cenote Pit cenote Cavernous Water well cenote Depth 120 m 118 m 20 m 20 m
Interface 50-60 m 68-75 m 13-15 m 9 m and below
Light input Low Low High None
Vegetation carbon input High DraftLow Low None
Human access to water Recreational Recreational None None 805 806
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807 Table 2: Environmental variables evaluated in the study. 808
Site Cenote X Cenote C Cenote U Hotel Well
Season dry vs. rainy dry vs. rainy rainy only dry vs. rainy
Substrate water vs. sediment water vs. sediment water only water only
Location inland coastal inland coastal
Salinity freshwater freshwater freshwater freshwater interface interface interface interface saline saline saline
Light high light (fresh water) high light low light no light low light (interface and saline)
Type of human agricultural impact Draftminimal impact agricultural impact tourist impact impact 809 810
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811 Table 3: Summary of significant trends observed on the phylum level. 812 Phylum Fraction of Significant trend total microbes Actinobacteria 4.13 Freshwater has more than interface (p = 0.028) and saline layer (p = 0.015)
Bacteroides 7.66 Coast has more than inland (p = 0.009)
Chlorobi 1.3 Coast has marginally more than inland (p = 0.0528)
Chloroflexi 5.7 None
Cyanobacteria 2.13 None
Firmicutes 2.77 Coast has more than inland (p = 0.0228)
OP3 1.07 None
Planctomycetes 2.54 Saline layer has more than freshwater and interface (p = 0.034), and sediment has more than water (p = 0022) Proteobacteria * 55.7 DraftTotal Proteobacteria Alpha 11.49 Rainy season has more than dry season (p = 0.0085) Proteobacteria Freshwater has more than interface (p = 0.005) and saline 5.28 layer (p = 0.0034); Cenote X has less than hotel well (p = Beta 0.0311) Proteobacteria
Gamma 31.2 Coast has less than inland (p = 0.0045) Proteobacteria
Delta Rainy season has less than dry season (p = 0.0116); Proteobacteria 5.47 sediment has more than water (p = 0.0083)
Epsilon 1.3 None Proteobacteria
SAR406 1.13 None
Verrumicrobia 1.5 None
Bacteria, Phylum 6.7 Cenote U has less than hotel well (p = 0.0164); dry season unidentified has more than rainy season (p = 0.0247); and sediment has more than water (p = 0.0026)
Euryarcheota 2.37 Sediment has more than water (p < 0.0001)
Unclassified 0.64 Sediment has more than water (p = 0.0003)
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813 * The phylum Proteobacteria was broken down to classes Alpha, Beta, Gamma, Delta and Epsilon 814 Proteobacteria. 815 816 Table 4: Relative abundance of the dominant genera in specific samples. 817 Genus Relative Site Season Salinity abundance Novosphingobium 29.8% Cenote X Rainy Fresh
Prochlorococcus 22% Cenote C Rainy Fresh
Brevundimonas 21.7% Cenote C Rainy Interface
Rhodobacter 11.8% Cenote C Dry Saline 16.4% Hotel well Rainy Fresh
Flavobacterium 12.8% Cenote C Dry Saline
Desulfobacterium 9.8% Cenote C Rainy Interface 818 Draft
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820 Table 5: The change in alpha diversity as a response to environmental variables. 821 Chao1 PD Shannon entropy Site (compared to hotel well): Cenote X less (p = 0.045) None Cenote C less (p = 0.013) Cenote U less (p = 0.02) less (p = 0.052)
Season Dry (compared to rainy season): more (p = 0.053) more (p = None 0.056) Substrate Water (compared to sediment): less (p = 0.0024) less (p = less (p= 0.01) 0.0055) Location Coast (compared to inland): None None more (p = 0.058) Salinity (compared to saline layer): Fresh None None None Interface Draft Light (compared to no light): High light Low Light None Less (p = 0.057) None
Human impact Tourist (compared to minimal impact): More (p = 0.048) More (p= More (p= 0.007) 0.042) 822 823 824
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825
826 Figure Captions
827
828
829 Figure 1: Sampling sites in Yucatan Peninsula, Mexico.
830 Figure 2: Sampling plan with details of location, time and depth.
831 Figure 3: Distribution of the thirteen most prevalent phyla together with unidentified
832 organisms and unclassified bacteria.
833 Figure 4: Relative abundance of the fecal indicator bacteria in the cenotes.
834 Figure 5: Relative abundance of fecal indicator bacteria other than coliforms.
835 Figure 6: Relative abundance of prevalentDraft cyanobacteria
836
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837
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839 Figure 1: Sampling sites in Yucatan Peninsula, Mexico.
840 Draft
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841
842
843 844 Figure 2: Sampling plan with details ofDraft location, time and depth. 845
846
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1.00E+00
9.00E-01
8.00E-01
7.00E-01
6.00E-01
5.00E-01
4.00E-01
3.00E-01 Relative frequency (%) 2.00E-01
1.00E-01
0.00E+00
C4jul09 H4jul09 X10jul09X52jul09X70jul09 XSdec08CSdec08 X60dec09X83dec08 C4mar09C15dec08C17dec08 C18jul09U10jul09U70jul09U80jul09H4mar09 X10mar09 DraftC13.5jul09 H10mar09 H10.4jul09 Unclassified Euryarchaeota Bacteria;Other Actinobacteria Bacteroidetes Chlorobi Chloroflexi Cyanobacteria Firmicutes OP3 Planctomycetes Proteobacteria SAR406 Spirochaetes Verrucomicrobia
X – Cenote X; C – Cenote C; U – Cenote U, H – hotel well. The number indicates sampling depth (m), S: 847 sediment. December-March: dry season, July: rainy season
848
849 Figure 3: Distribution of the thirteen most prevalent phyla together with unidentified
850 organisms and unclassified bacteria.
851
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852
100%
90%
80%
70%
60%
50%
40%
30% Relative frequency (%) 20%
10%
0% Draft C4jul09 H4jul09 X10jul09X52jul09X70jul09C4mar09 C18jul09U10jul09U70jul09U80jul09H4mar09 XSdec08CSdec08 X10mar09X60dec09X83dec08 C15dec08C17dec08 C13.5jul09 H10mar09 H10.4jul09 Enterobacteraceae (Coliforms) Escherichia (Fecal coliform) Enterococcus Bacteroides X – Cenote X; C – Cenote C; U – Cenote U, H – hotel well. The number indicates sampling depth (m), S: 853 sediment. December-March: dry season, July: rainy season
854 Figure 4: Relative abundance of the fecal indicator bacteria in the cenotes.
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855
100% 90% 80% 70% 60% 50% 40%
Relative frequency (%) 30% 20% 10% 0%
C4jul09 H4jul09 X10jul09X52jul09X70jul09C4mar09 C18jul09U10jul09U70jul09U80jul09H4mar09 XSdec08CSdec08 X10mar09X60dec09X83dec08 C15dec08DraftC17dec08 C13.5jul09 H10mar09 H10.4jul09 Escherichia (Fecal coliform) Enterococcus Bacteroides Bifidobacterium X – Cenote X; C – Cenote C; U – Cenote U, H – hotel well. The number indicates sampling depth (m), S: 856 sediment. December-March: dry season, July: rainy season
857 Figure 5: Relative abundance of fecal indicator bacteria other than coliforms.
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858
859
100%
90%
80%
70%
60%
50%
40%
Relative frequency (%) 30%
20% 10% Draft 0%
C4jul09 H4jul09 X10jul09X52jul09X70jul09C4mar09 C18jul09U10jul09U70jul09U80jul09H4mar09 XSdec08CSdec08 X10mar09X60dec09X83dec08 C15dec08C17dec08 C13.5jul09 H10mar09 H10.4jul09 Uncultured 4C0d-2 Uncultured S15B-MN24 Nostocales and Stigonematales Croococcales X – Cenote X; C – Cenote C; U – Cenote U, H – hotel well. The number indicates sampling depth (m), S: 860 sediment. December-March: dry season, July: rainy season
861 Figure 6: Relative abundance of prevalent cyanobacteria.
862
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864
865
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868 869 870
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