Abundant, micrometer-scale, spherical aggregates of 2- to 5-nanometer-diameter sphalerite (ZnS) particles formed within natural biofilms dominated by relatively aerotolerant sulfate-reducing bacteria of the family Desulfobacteriaceae. The biofilm zinc concentration is about 10(6) times that of associated groundwater (0.09 to 1.1 parts per million zinc). Sphalerite also concentrates arsenic (0.01 weight %) and selenium (0.004 weight %). The almost monomineralic product results from buffering of sulfide concentrations at low values by sphalerite precipitation. These results show how microbes control metal concentrations in groundwater- and wetland-based remediation systems and suggest biological routes for formation of some low-temperature ZnS deposits.

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The Richmond Mine at Iron Mountain, Shasta County, California, USA provides an excellent opportunity to study the chemical and biological controls on acid mine drainage ͑AMD͒ generation in situ, and to identify key factors controlling solution chemistry. Here we integrate four years of field-based geochemical data with 16S rRNA gene clone libraries and rRNA probe-based studies of microbial population structure, cultivation-based metabolic experiments, arsenopyrite surface colonization experiments, and results of intermediate sulfur species kinetics experiments to describe the Richmond Mine AMD system. Extremely acidic effluent ͑pH between 0.5 and 0.9͒ resulting from oxidation of approximately 1ϫ10 5 to 2ϫ10 5 moles pyrite/day contains up to 24 g/l Fe, several g/l Zn and hundreds of mg/l Cu. Geochemical conditions change markedly over time, and are reflected in changes in microbial populations. Molecular analyses of 232 small subunit ribosomal RNA ͑16S rRNA͒ gene sequences from six sites during a sampling time when lower temperature ͑Ͻ32°C͒, higher pH ͑Ͼ0.8͒ conditions predominated show the dominance of Fe-oxidizing prokaryotes such as Ferroplasma and Leptospirillum in the primary drainage communities. Leptospirillum group III accounts for the majority of Leptospirillum sequences, which we attribute to anomalous physical and geochemical regimes at that time. A couple of sites peripheral to the main drainage, ''Red Pool'' and a pyrite ''Slump,'' were even higher in pH ͑Ͼ1͒ and the community compositions reflected this change in geochemical conditions. Several novel lineages were identified within the archaeal Thermoplasmatales order associated with the pyrite slump, and the Red Pool ͑pH 1.4͒ contained the only population of Acidithiobacillus. Relatively small populations of Sulfobacillus spp. and Acidithiobacillus caldus may metabolize elemental sulfur as an intermediate species in the oxidation of pyritic sulfide to sulfate. Experiments show that elemental sulfur which forms on pyrite surfaces is resistant to most oxidants; its solublization by unattached cells may indicate involvement of a microbially derived electron shuttle.

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Microbial metabolism is the engine that drives global biogeochemical cycles, yet many key transformations are carried out by microbial consortia over short spatiotemporal scales that elude detection by traditional analytical approaches. We investigate syntrophic sulfur cycling in the ‘pink berry’ consortia of the Sippewissett Salt Marsh through an integrative study at the microbial scale. The pink berries are macroscopic, photosynthetic microbial aggregates composed primarily of two closely associated species: sulfide-oxidizing purple sulfur bacteria (PB-PSB1) and sulfate-reducing bacteria (PB-SRB1). Using metagenomic sequencing and 34S-enriched sulfate stable isotope probing coupled with nanoSIMS, we demonstrate interspecies transfer of reduced sulfur metabolites from PB-SRB1 to PB-PSB1. The pink berries catalyse net sulfide oxidation and maintain internal sulfide concentrations of 0–500 μm. Sulfide within the berries, captured on silver wires and analysed using secondary ion mass spectrometer, increased in abundance towards the berry interior, while δ34S-sulfide decreased from 6‰ to −31‰ from the exterior to interior of the berry. These values correspond to sulfate–sulfide isotopic fractionations (15–53‰) consistent with either sulfate reduction or a mixture of reductive and oxidative metabolisms. Together this combined metagenomic and high-resolution isotopic analysis demonstrates active sulfur cycling at the microscale within well-structured macroscopic consortia consisting of sulfide-oxidizing anoxygenic phototrophs and sulfate-reducing bacteria.

Relationships between phosphorus cycling and redox conditions in the sediments of eutrophic Missisquoi Bay, Lake Champlain were investigated over diel and seasonal timescales in three consecutive summers (2007)(2008)(2009), one of which (2007) did not experience a cyanobacteria bloom for the first time in a decade. Sediment extraction data showed that reactive phosphorus (RP) is strongly correlated to reactive iron (RFe), suggesting that the mobility of a large portion (30-40%) of the total sediment phosphorus pool is associated with nanocrystalline iron oxide minerals that may be influenced by redox conditions. RP concentrations in the top sediments increased throughout 2007 but decreased throughout 2008; calculations indicate that , 1 mm of sediment could account for the increased total dissolved phosphorus observed in overlying water. Redox conditions were measured over 24 h in situ within sediment cores and at the sediment-water interface (SWI) at different stages of each season using voltammetry. SWI redox conditions became progressively more reduced across the season and overnight and were significantly more reducing in the presence of a bloom. Soluble RP and cyanobacteria cell counts measured at five depths through the water column strongly correlated with the most reducing conditions at the SWI. Observations suggest that redox controlled nutrient flux between the sediments and the water column is variable over diel and seasonal cycles. Cyanobacteria blooms most significantly affect SWI redox conditions, suggesting that blooms may enhance RP flux from sediments, setting up a positive feedback loop that can propagate and sustain blooms in shallow freshwater systems.

Geothermal waters contain numerous potential electron donors capable of supporting chemolithotrophybased primary production. Thermodynamic predictions of energy yields for specific electron donor and acceptor pairs in such systems are available, although direct assessments of these predictions are rare. This study assessed the relative importance of dissolved H 2 and H 2 S as energy sources for the support of chemolithotrophic metabolism in an acidic geothermal spring in Yellowstone National Park. H 2 S and H 2 concentration gradients were observed in the outflow channel, and vertical H 2 S and O 2 gradients were evident within the microbial mat. H 2 S levels and microbial consumption rates were approximately three orders of magnitude greater than those of H 2 . Hydrogenobaculum-like organisms dominated the bacterial component of the microbial community, and isolates representing three distinct 16S rRNA gene phylotypes (phylotype ‫؍‬ 100% identity) were isolated and characterized. Within a phylotype, O 2 requirements varied, as did energy source utilization: some isolates could grow only with H 2 S, some only with H 2 , while others could utilize either as an energy source. These metabolic phenotypes were consistent with in situ geochemical conditions measured using aqueous chemical analysis and in-field measurements made by using gas chromatography and microelectrodes. Pure-culture experiments with an isolate that could utilize H 2 S and H 2 and that represented the dominant phylotype (70% of the PCR clones) showed that H 2 S and H 2 were used simultaneously, without evidence of induction or catabolite repression, and at relative rate differences comparable to those measured in ex situ field assays. Under in situ-relevant concentrations, growth of this isolate with H 2 S was better than that with H 2 . The major conclusions drawn from this study are that phylogeny may not necessarily be reliable for predicting physiology and that H 2 S can dominate over H 2 as an energy source in terms of availability, apparent in situ consumption rates, and growth-supporting energy.Thermophiles dominate the deepest and shortest branches of the Bacteria and Archaea domains in the tree of life, suggesting that they are likely ancestors of Earth's contemporary microbial populations (8,35). Consequently, these organisms have attracted considerable attention due to interest in the origin of enzymes and metabolic pathways that are thought to have evolved from such organisms. Chemolithotrophic metabolism is foundational to primary productivity in geothermal environments where temperatures exceed the limit of photosynthesis. The bioenergetics of such systems have been examined from the perspective of theoretical energy yield as a way of discussing the relative importance of the various electron donors and acceptors that could support primary productivity (3)(4)(5)22). Other studies have sought to link the inferred physiology of microbial populations with the predicted energy yields obtainable from the inorganic constituents present (4...