Microbial nitrate-dependent, Fe(II) oxidation (NDFO) is a ubiquitous biogeochemical process in anoxic sediments. Since most microorganisms that can oxidize Fe(II) with nitrate require an additional organic substrate for growth or sustained Fe(II) oxidation, the energetic benefits of NDFO are unclear. The process may also be self-limiting in batch cultures due to formation of Fe-oxide cell encrustations. We hypothesized that NDFO provides energetic benefits via a mixotrophic physiology in environments where cells encounter very low substrate concentrations, thereby minimizing cell encrustations. Acidovorax sp. strain 2AN was incubated in anoxic batch reactors in a defined medium containing 5 to 6 mM NO 3 ؊ , 8 to 9 mM Fe 2؉ , and 1.5 mM acetate. Almost Microbially driven, Fe(III)/Fe(II) redox transitions in the environment have a dramatic impact on iron's solubility, mineralogy, sorption characteristics, and overall geochemical properties (21,29,45). Microbially mediated redox transformations of Fe can also affect the biogeochemical cycling of other key nutrients (e.g., C, S, P, and N) (15, 29), trace metals (9, 49), metalloids (12), and the fate of organic pollutants (25) and contaminant metals (8), including those released from industrial and mining areas (30). Anaerobic, microbial Fe(III) reduction has been studied extensively over the last 30 years (31, 45), and much is known about the microbiology and geochemistry of that process.Chemotrophic, anoxic Fe(II) oxidation under circumneutral conditions, on the other hand, was first described only 15 years ago (42), and little is known about the microbiology of the process or its actual significance in the environment. Microbial nitrate-dependent, Fe(II) oxidation (NDFO) has been demonstrated in a variety of sediments, microbial consortia, and pure cultures, including both autotrophic and heterotrophic cultures. With respect to autotrophic growth via NDFO, for example, the mesophilic enrichment culture originally described by Straub et al. (42) has been maintained for over a decade under autotrophic conditions and is capable of near complete oxidation of Fe(II) and reduction of NO 3
Mudstone--the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles--contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated--rather than detrital--quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.
Microbes are ubiquitous in modern sediments, and must have been a similarly common constituent in the past. After death, however, they degrade readily and usually do not become part of the rock record. Especially for our understanding of early earth history and the evolution of life, however, finding preserved cellular remains has been critical. Verified microbial remains from early earth history (Awramik et al., 1983) have all been reported from sediments that experienced very early cementation by fine crystalline quartz (Schopf and Walter, 1983). The quartz cement renders these rocks (cherts) transparent and allows examination of microbial fossils in the context of the rock matrix (3). Because such cherts are rare, the microbial record of early as well as later earth history is still poorly known. Pyrite, another early diagenetic mineral that forms as a result of microbial processes in sediments, has so far received little attention as a source of microbial fossils. Pyrite grains ranging in age from Archean to Jurassic were examined by scanning and transmission electron microscope, and most of them show coccoid, rod-shaped, and even filamentous features that are interpreted as microbial. Although pyrite represents a much more common sedimentary mineralization than chert, its opaque nature has in the past rendered the search for contained microbial remains very difficult. The identification of microbial remains in sedimentary pyrite opens up the prospect to greatly expand our knowledge of microbial life in very old sediments, as well as allowing us much more detailed analysis of microbial life throughout Earth history. Sedimentary pyrite grains may also represent a good prospect to find traces of past life on Mars.
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