Microbial Sulfur Transformations in Sediments from Subglacial Lake Whillans
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ORIGINAL RESEARCH ARTICLE published: 19 November 2014 doi: 10.3389/fmicb.2014.00594 Microbial sulfur transformations in sediments from Subglacial Lake Whillans Alicia M. Purcell 1, Jill A. Mikucki 1*, Amanda M. Achberger 2 , Irina A. Alekhina 3 , Carlo Barbante 4 , Brent C. Christner 2 , Dhritiman Ghosh1, Alexander B. Michaud 5 , Andrew C. Mitchell 6 , John C. Priscu 5 , Reed Scherer 7 , Mark L. Skidmore8 , Trista J. Vick-Majors 5 and the WISSARD Science Team 1 Department of Microbiology, University of Tennessee, Knoxville, TN, USA 2 Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, USA 3 Climate and Environmental Research Laboratory, Arctic and Antarctic Research Institute, St. Petersburg, Russia 4 Institute for the Dynamics of Environmental Processes – Consiglio Nazionale delle Ricerche and Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, Venice, Italy 5 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, USA 6 Geography and Earth Sciences, Aberystwyth University, Ceredigion, UK 7 Department of Geological and Environmental Sciences, Northern Illinois University, DeKalb, IL, USA 8 Department of Earth Sciences, Montana State University, Bozeman, MT, USA Edited by: Diverse microbial assemblages inhabit subglacial aquatic environments.While few of these Andreas Teske, University of North environments have been sampled, data reveal that subglacial organisms gain energy for Carolina at Chapel Hill, USA growth from reduced minerals containing nitrogen, iron, and sulfur. Here we investigate Reviewed by: the role of microbially mediated sulfur transformations in sediments from Subglacial Aharon Oren, The Hebrew University of Jerusalem, Israel Lake Whillans (SLW), Antarctica, by examining key genes involved in dissimilatory sulfur John B. Kirkpatrick, University of oxidation and reduction. The presence of sulfur transformation genes throughout the top Rhode Island, USA 34 cm of SLW sediments changes with depth. SLW surficial sediments were dominated *Correspondence: by genes related to known sulfur-oxidizing chemoautotrophs. Sequences encoding the Jill A. Mikucki, Department of adenosine-5-phosphosulfate (APS) reductase gene, involved in both dissimilatory sulfate Microbiology, University of Tennessee, M409 Walters Life reduction and sulfur oxidation, were present in all samples and clustered into 16 distinct Sciences, 1414 West Cumberland operational taxonomic units.The majority of APS reductase sequences (74%)clustered with Avenue, Knoxville, TN 37996, USA known sulfur oxidizers including those within the “Sideroxydans” and Thiobacillus genera. e-mail: [email protected] Reverse-acting dissimilatory sulfite reductase (rDSR) and 16S rRNA gene sequences further support dominance of “Sideroxydans” and Thiobacillus phylotypes in the top 2 cm of SLW sediments. The SLW microbial community has the genetic potential for sulfate reduction which is supported by experimentally measured low rates (1.4 pmol cm−3d−1) of biologically mediated sulfate reduction and the presence of APS reductase and DSR gene sequences related to Desulfobacteraceae and Desulfotomaculum. Our results also infer the presence of sulfur oxidation, which can be a significant energetic pathway for chemosynthetic biosynthesis in SLW sediments. The water in SLW ultimately flows into the Ross Sea where intermediates from subglacial sulfur transformations can influence the flux of solutes to the Southern Ocean. Keywords: Antarctic subglacial aquatic environments, geomicrobiology, chemosynthesis, sulfur oxidation, sulfate reduction INTRODUCTION Initial analyses of samples collected from SLW show the pres- Subglacial aquatic environments exist beneath the Antarctic Ice ence of an active community of diverse heterotrophic and Sheet as lakes, streams, marine brines, and water-saturated autotrophic microorganisms in the water column and surfi- sediments (Priscu et al., 2008; Fricker and Scambos, 2009; cial sediments (Christner et al., 2014). Substrates for subglacial Mikucki et al., 2009; Skidmore, 2011; Wright and Siegert, 2012). growth are primarily derived from minerals and organic mat- Recently, the Whillans Ice Stream Subglacial Access Research ter in the underlying sediments (Tranter et al., 2005). Recent Drilling (WISSARD) project directly sampled water and sed- evidence for the presence of sulfur cycling microorganisms in iment from Subglacial Lake Whillans (SLW), one of the 379 Antarctic subglacial environments (Christner et al., 2006, 2014; identified Antarctic subglacial lakes (Wright and Siegert, 2012). Lanoil et al., 2009; Mikucki et al., 2009), implies that sulfur trans- formations may provide chemical energy for growth in these Abbreviations: SLW, Subglacial Lake Whillans; WIS, Whillans Ice Stream; SOP, cold, dark ecosystems. Measurements of metabolic substrate sulfur-oxidizing prokaryotes; SRP, sulfate-reducing prokaryotes; APS, adenosine- concentrations, enrichment cultures, molecular surveys, sulfur 5-phosphosulfate reductase; DSR, dissimilatory sulfite reductase; rDSR, reverse dissimilatory sulfite reductase. and oxygen isotopic composition, and microcosm experiments www.frontiersin.org November 2014 | Volume 5 | Article 594 | 1 Purcell et al. Subglacial Lake Whillans sulfur cycling have been used to infer the presence of sulfate reduction and release of metabolic byproducts into the Ross Sea and possibly the sulfide oxidation beneath Arctic and Alpine glaciers (Skidmore Southern Ocean. et al., 2000, 2005; Bottrell and Tranter, 2002; Wadham et al., 2004; Montross et al., 2013). Similarly in Antarctic subglacial systems, MATERIALS AND METHODS sulfide oxidation has been inferred as an important microbial pro- SITE DESCRIPTION AND SAMPLE COLLECTION cess from 16S rRNA gene sequence data (Mikucki and Priscu, Subglacial Lake Whillans is located beneath the downstream por- 2007; Lanoil et al., 2009) and geochemical measurements (Skid- tion of the WIS (S 84.237◦, W 153.614◦; Christianson et al., 2012), more et al., 2010; Wadham et al., 2010). However, there have been ca. 100 km from the grounding zone, where the ice sheet transi- few studies on functional gene diversity in Antarctic subglacial tions into the Ross Ice Shelf (Figure 1). SLW is a shallow (∼2.2 m systems. deep; Tulaczyk et al., 2014) lake located in what appears to be Sulfur is required for cellular components, such as the amino a large wetland along the Siple Coast of West Antarctica (Priscu acids cysteine and methionine; however, some microorgan- et al., 2010; Fricker et al., 2011). SLW is considered an ‘active’ lake isms utilize sulfur compounds in dissimilatory, energy-yielding as it drains and refills on a sub-decadal time scale discharging water metabolic processes. The sulfur-oxidizing prokaryotes (SOP) are towards the Ross Sea (Fricker et al., 2007; Carter and Fricker, 2012; metabolically and phylogenetically diverse (Friedrich et al., 2001, Siegfried et al., 2014). In January 2013, the WISSARD Project1 2005), and can fix CO2 utilizing a variety of electron acceptors − +/ + + used hot water drilling to penetrate 801 ± 1 m of glacial ice to including O ,NO ,Mn3 4 , and Fe3 (Mattes et al., 2013). 2 3 access SLW. Details of drilling operations are described elsewhere Sulfate-reducing prokaryotes (SRP) respire organic material for (Tulaczyk et al., 2014). A clean access protocol (Priscu et al., 2013) energy and use sulfate as an electron acceptor when oxygen is was followed to maintain both sample integrity and environmen- absent (Jørgensen, 1982; Jørgensen and Postgate, 1982). Reduced tal stewardship. Briefly, drilling water was passed through two sulfur compounds generated by sulfate reduction can provide filtration units (2.0 and 0.2 μm) to remove large particulates and energy for the SOP component of the community, although a microbial cells. Water was then subjected to ultraviolet irradiation larger fraction of reduced sulfur for microbial oxidation may come of 185 nm for organic matter destruction and germicidal 254 nm. from mineral sources. This may be important subglacially, where Finally, drilling water was pressurized and heated to 90◦C and used the grinding of glacial ice over bedrock would expose reactive to melt an access borehole. Instruments were cleaned with 3% mineral surfaces (Anderson, 2007). Sulfate reduction is widely hydrogen peroxide and cables and hoses were deployed through recognized as an important process in other dark and cold environ- a UV collar during deployment down the borehole (Priscu et al., ments including anaerobic marine sediments, where it contributes 2013; Christner et al., 2014). to greater than 50% of total organic carbon oxidation globally Sediments from SLW were collected using a gravity driven (Canfield et al., 1993; Thullner et al., 2009; Bowles et al., 2014). multi-corer (Uwitec) built for recovery of sediment cores and The linkage of the sulfur and carbon cycles in Antarctic subglacial water from the sediment-water interface. The multi-corer suc- environments has not been well characterized. Here we aimed to cessfully recovered ∼40 cm of sediment and 20 cm of basal water survey the potential role of sulfur transforming communities in in most deployments (Tulaczyk et al., 2014). Sediment cores ana- SLW sediments. lyzed in this study were collected from the second (identified We measured rates of sulfate reduction and analyzed the pres- as core ‘MC-2B’) and third multicore deployment (core ‘MC- ence and diversity of three dissimilatory sulfur cycling