Microbial Physiology and Biosignature Production: Mineralogical, Morphological and Geochemical Examples

Microbial processes have driven biogeochemical cycles and modified the Earth?s surface throughout geologic time. Some microbial physiologies created unique chemo-physical conditions to influence mineral precipitation and trace element cycling. Such physiology-specific products may be preserved as long-lived biosignatures in the geologic record and could be used as a new evidence to detect ancient life.

Microbial iron reduction is one of the most important geobiochemical process in nature and can promote carbonate precipitation. However, little is known about how iron reducing bacteria induce carbonate formation, and control carbonate composition and morphology. Direct observation and geochemical modeling indicated actively metabolizing cells locally raised pH and provided essential nucleation sites for carbonate precipitation. By altering metal ion concentrations around cell surfaces, and inducing metastable carbonate, iron reducing microorganisms could produce a wide range of carbonate cements in natural sediments.

In addition, iron reducing bacteria can use diverse physiologies (membrane bound enzymes, soluble electron shuttles, and nanowires) to transfer electrons to insoluble iron oxide minerals. This physiologies may create a variety of physicochemical microenvironments in which mineral dissolution and precipitation can occur. The incubation of S. oneidensis MR-1, G. fermentans, and G. metallireducens GS-15, representing three different physiologies, showed that some special carbonate shapes and micropores are exclusive to a certain physiology. This suggests the information of iron reducing physiology can be recorded in carbonate minerals, which may be used as a biomarker to indicate the presence of life on the early Earth.

Cerium is an important paleoredox probe, since it is the only rare earth element with an active redox chemistry at Earth surface temperatures. However, lack of details about mechanism of Ce redox and its anomalous behavior obstructs the use of Ce as a redox tracer to understand past sedimentary environments. The experimental results indicated Roseobacter sp. AzwK-3b, a superoxide producer, is able to oxidize Ce(III) through the superoxide pathway. On the other hand, the iron reducer S. oneidensis MR-1 can reduce cerium(IV) directly, and uses cerium as the sole electron acceptor for anaerobic respiration. This study indicates a new biological mechanism of Ce redox.

Volcanic ash contains 1-10% FeO by weight and could be a significant contributor of Fe to fertilize oceans in the past. A 49 meter Eagle Ford core (late Cretaceous), containing 51 visualized ash beds with varying thickness, was scanned with X-ray fluorescence microscopy to determine burial of Fe, trace elements delivered to the sediment by sinking organic matter (Ni), and paleoredox proxies (Mo and Cr) below, in, and above ash beds. Ash beds contain much more Fe, Mo and Ni, but less Cr than interbedded black shales. This suggests that input of ash increased marine primary productivity (Ni), which in turn enhanced oxygen demand and promoted euxinia (Mo and Cr). We conclude that Fe-bearing volcanic ash fertilized the Late Cretaceous Western Interior Seaway.