Trace Metal Chemistryand Silicification of Micro
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TRACE METAL CHEMISTRY AND SILICIFICATION OF MICRO-ORGANISMS GEOTHERMAL SINTER, TAUPO VOLCANIC ZONE, NEW ZEALAND K. L. BROWN’, E. J. S. L. K. A. CAMPBELL’ ‘Geology University of Auckland of Geology, Portland State University, Portland, Oregon, USA SUMMARY part of a pilot study investigating the role of microorganisms in the of As, Sb, and Hg, the inorganic geochemistry of seven different active sinter deposits and their contact fluids were characterised. The extractions showed whether metals were loosely exchangeable or bound to carbonate, oxide, organic or crystalline fractions. Hyperthermophilic microbial communities associated with sinters deposited from high temperature (92-94°C) fluids at a variety of geothermal sources were investigated using SEM. The rapidity and style of silicificationof the can be correlated with the dissolved silica content of the fluid. There was little evidence to suggest that any of the heavy metals were associated preferentially with the hyperthermophiles at the high temperature ends of the terrestrial thermal spring ecosystems studied. 1.0 INTRODUCTION rates, pre-sterilised glass microscope slides were deployed as artificial substrates at some sites. Arsenic, antimony, boron, thallium and mercury Bulk concentrations of trace elements were (As, Sb, B, Hg) are commonly found analysed semi-quantitatively using X-ray concentrated in silica sinter deposited around fluorescence spectrometry. Bulk samples natural New Zealand hot springs (Weissberg, also were analysed to determine whether the trace and in the sulphide scales of geothermal metals in the sinters were exchangeable, or bound pipelines of some power stations of the Taupo to oxides, carbonates, organics, or incorporated Volcanic Zone (Brown and McDowell, 1983). into the crystalline fractions, by using a series of Release of these elements from geothermal waters, sequentialextractionsoriginally developed for soil steam condensate and leachate into waterways can analysis by Tessier et al. with pose an environmental risk. Investigation of the modifications by Kim and Fergusson (1991). manner in which these elements are Biological samples for SEM were preserved in in silica sinter may reveal an avenue of gluteraldehyde, then rinsed twice in distilled remediation where elements such water, subjected to an ethanol dehydration series, I as these cause contamination. The surfaces and and critical point dried to prevent volume loss extracellular components of bacteria are highly (Ruffolo, 1974). reactive and they could provide ideal sorption surfaces for the nucleation of inorganic phases, 2.2 DNA preparation however, so far, the interactions that might occur between metals and microbiota surfaces have not To investigate whether DNA could be extracted been extensivelystudied. fiom vitreous sinter, geyserite fkom Tokaanu was subjected to molecular analysis. The presence of 2.0 SAMPLE SITES METHODS bacterial DNA in the sample was investigated by i polymerase chain reaction (PCR) amplification of The sampling sites comprised the natural hot a 200 base pair of the small springs at Waikite (WKT), man-made ribosomal RNA gene, using Bacterial-specific environments such as primers. Samples with Bacterial primer were and weirboxes at the placed in wells in Fig. 2F. included Wairakei Geothermal Power Station and the a positive control to ensure that the amplification Ohaaki Pool (OHK), and an abandoned was properly (wells 5 and a blank of geothermal bore at Tokaanu (TK). distilled water as a contamination check (well 7), and a ‘ladder’ to ensure complete sample 2.1 Analyses migration (well 8). PCR products were separated by gel electrophoresis and identified under W The contact fluids were analysed for As, Sb, B, illumination. and Hg by ICP-MS. In order to qualitatively assess sinter and microbial biofilm accumulation 39 Table 1 Chemistry ofgeothermalwaters (allvalues in below detection limits) 3.0 RESULTS low Fe concentrations had low As concentrations. It was also noted that when the arsenic and iron 3.1 Fluids concentrations antimony concentrations increased. Results for trace metal concentrations in geothermal fluids analysed by ICP-MS are shown 3.2 Fluid-sinter ratios in Table 1. The silica concentrations in the fluid collected the geothermal power To determine whether As, Sb, B, Tl and Hg were developments were considerably greater than selectively concentrated in sinter, the trace metal those of the natural features. Except for the concentrations in the fluid and sinter phases were Waikite site, the water samples display reasonably normalised to the silica concentration of each Uniform concentrationsof B, As,Hg, Sb, and Tl. sample, in the Waikite and Hg in the Tokaanu geyserite and Flash plant 11 sinter were 3.2 Sinter the only trace metals that were selectively concentrated in the sinter. Results for trace metal concentrations in the bulk sinter, as analysed by show that except for 3.3 Selectiveleaching the and the Tokaanu vent geyserite samples, the silica concentrations are all greater The sequential extraction analyses showed that As than 80%. The Waikite sample is a mixture of does not exhibit preferential association with any silica and calcite. Among the trace metals, only of the extractable Sb is bound As and Sb were found to be above the detection preferentially with the crystalline of each limits for However, total concentrations of of the sinters. Although B is predominantly all of the trace metals were calculated during the associated with the crystalline there are sequential extraction analysis (Table 2). High significant quantities associated with some of the iron concentrations were found in the Tokaanu other extracted especially at Ohaaki vent geyserite, which also contained the highest where it is found in anomalous concentrations in arsenic concentration. The antimony all of the relative to the other sinter concentrations are reasonably Uniform for samples. Tl occurs variably in all except samples. the exchangeable The highest concentrations of Tl are associated with the After this study began, water draining from an organic fraction in the high temperature acid iron-rich natural geothermal was geyserite Tokaanu. The only significant diverted away the Wairakei main borefield concentrations of Hg occur primarily with the drain consequently, two sinter organic fraction from the moderate temperature samples from this site were one taken silica oncoids at from the drain before, and one taken after the diversion. The showed that the sample with high Fe concentrations also had elevated As concentrations, while the sample with 40 Table 2 Trace element total concentrations in the sinters calculatedfrom the sum of the sequential extractions. A single total (nonsequential) digest of TKI is also shown (ppm) = below detection limits) Element Sb As B TI TKI 269 1210 314 0.47 6.6 TKI single 504 2360 458 0.3 9.2 digest 150 39 346 0.01 1.3 TK3 198 214 622 3.88 OHK 138 26 1703 2.1 WKFPII 116 IO 91 0.07 3.0 16 93 7 73 WKMD 68 1237 258 0.05 1.5 WKEB 105 24 195 0.01 1.6 WKT 35 85 479 0.02 0.5 3.4 Microbiota subaqueous geyserite, the hyperthermophiles that colonise Flash Plant 11 subaqueous sinter were In this study we have grouped the microbial often extensively and display a range of communities investigated according to their apparent diameters and lengths as shown in Fig. environmental niche growthhabitats described by Golubic et al. (1981) as epilithic (on upper surface), hypolithic (underneath), and endolithic Figs 2A-2D show an example of a hypolithic We therefore identified the microbial community that colonised the underside of a communities associated with the outennost columnar geyserite knob Tokaanu. Prior to accretionary surfaces as epilithic, and the collection, the knob was communities associated with the undersides of directly to periodic splashes of hydrothermal fluid sinter as hypolithic. ejected the vent SEM investigation revealed that the hypolithic thatgrow attached to the community consists primarily of organisms upper- or outermost of sinters may be by two different morphotypes. One located either in subaerial, subaqueous, or mixed morphotype is comprised of long rods in regimes. length and in width. The other Figs. and show examples of the epilithic morphotype is by long filaments that communities that colonised the reach up to or more in length and subaqueous geyserite Tokaanu. Prior to in width. The of a thin, at least collection, the Tokaanu geyserite sample was silicified microbial biofilm matrix is shown in continuallysubmerged in water with a of Fig. 2E. Molecular analysis of taken 7.7. Hyperthermophilic microorganisms that the surface on the underside of the Tokaanu adhere to the geyserite surface, or to each other, columnar geyserite shows the presence of display a similar morphotype that consists of long bacterial DNA (Fig. 2E). of the thin filaments. Shown in gene a 200 base pairs long, Fig. is an example of the filamentous typical of bacterial DNA. hyperthermophiles diameter) that attached to a glass slide deployed at the Endolithic phototrophic microorganismswere also effluent of Flash Plant 11 located at the observed to occur within the outermost few Geothermal Power Station. The glass slide was millimetres of columnar geyserite at Tokaanu. continuously submerged in water = 8.1) Green and orange pigments of the phototrophic for two weeks. The epilithic community that endoliths visible in views of the subaqueous sinter Surfaces at Flash the sinter. Within a few millimetres of the Plant 11, and the effluent at the