![]() In the Cu-Zn Kidd Creek Mine, the presence of SRP corresponded to the enrichment of pyrite in the sediments ( Fortin and Beveridge, 1997), while in the Zn-Pb Piquette mine, only sphalerite (ZnS) precipitated in a SRP-rich biofilm to the exclusion of other metal sulfides ( Labrenz et al., 2000 Druschel et al., 2002 Labrenz and Banfield, 2004). There are few environmental studies closely linking microbial sulfate-reducing activity and specific sulfide minerals, even though it has been 100 years since the first suggestion that SRP might be responsible for metal sulfide ore formation ( Siebenthal, 1915). Although black sulfide precipitates in anoxic sediments are used as an indicator for the presence of SRP, their abundance isn't necessarily linked to large metal sulfide deposits. SRP are present in an enormous diversity of environments including freshwater ( Ramamoorthy et al., 2009 Sass et al., 2009), hypersaline ( Foti et al., 2007), hydrothermal sediment ( Jørgensen et al., 1992), polar ( Ravenschlag et al., 2000 Karr et al., 2006), and oceanic crust ( Robador et al., 2015) habitats. Ninety-seven percent of the sulfide produced on Earth is attributable to the activity of sulfate-reducing prokaryotes (SRP) in low-temperature environments ( Trudinger et al., 1985 Rickard, 2012b), while the remaining three percent are produced at volcanoes and deep-sea hydrothermal vents ( Elderfield and Schultz, 1996 Andres and Kasgnoc, 1998). In these low-oxygen environments, sulfur and iron can be immobilized in the form of iron sulfide minerals, primarily the iron(II) monosulfide mackinawite (tetragonal FeS), the iron(II,III) sulfide greigite (Fe 3S 4), and the iron (II) disulfide pyrite (FeS 2) ( Schoonen, 2004 Rickard, 2012a, c). Sulfate is the dominant sulfur species at 28 mM in the modern oxic oceans, while reduced sulfur species, including hydrogen sulfide and organosulfur compounds, are often abundant where oxygen is low or absent. Microorganisms can take advantage of this diversity of oxidation states for energy conservation, which can be achieved by: (1) coupling the oxidation of organic compounds or dihydrogen to the reduction of oxidized organic and inorganic sulfur compounds (e.g., dimethyl sulfoxide, sulfate, elemental sulfur, and thiosulfate) ( Widdel and Bak, 1992 Rabus et al., 2013) (2) disproportionating elemental sulfur, thiosulfate and sulfite ( Bak and Pfennig, 1987 Thamdrup et al., 1993) and (3) oxidizing organosulfur compounds, hydrogen sulfide, sulfur, sulfite, and thiosulfate chemosynthetically with oxygen and nitrate during respiration, or by anoxygenic photosynthesis ( Jørgensen and Nelson, 2004). Much of the interest in sulfur is due to its redox versatility-from sulfide (−2) to sulfate (+6), with numerous redox transformations possible in between ( Zopfi et al., 2004). In Earth's biosphere, sulfur may be gaseous (e.g., sulfur dioxide), dissolved (e.g., sulfide, polysulfides, thiosulfate, sulfite, or sulfate) or solid (e.g., metal sulfides, elemental sulfur). Though the sulfur cycle was the first elemental cycle to be studied ( Beijerinck, 1895), research on sulfur biogeochemistry is far from complete, and novel aspects of sulfur's transformations on Earth are still being discovered ( Canfield et al., 2010). While iron is one of the most abundant elements on Earth, sulfur represents < 1% of the Earth by mass ( Allègre et al., 1995), although its importance to life and earth systems is greater than its abundance would suggest. Throughout Earth's history the burial of solid phases of Fe and S has controlled the redox state of Earth's surface environments ( Berner, 1984). These inquiries have revealed the need for additional thorough, mechanistic and high-resolution studies to understand microbially mediated formation of a variety of sulfide minerals across a range of natural environments.Ĭo-Occurence of Microorganisms and Sulfide Minerals in Nature We discuss whether biologically derived minerals are distinguishable from abiotic minerals, possessing attributes that are uniquely diagnostic of biomineralization. We summarize the evidence that links sulfur-metabolizing microorganisms and sulfide minerals in nature and we present a critical overview of laboratory-based studies of the nucleation and growth of iron sulfide minerals in microbial cultures. Here we review the role of microorganisms in the precipitation of extracellular iron sulfide minerals. Iron sulfide mineralization in low-temperature systems is a result of biotic and abiotic processes, though the delineation between these two modes of formation is not always straightforward. 1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA. ![]()
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