COUPLED INTERACTIONS OF A MICROBIAL COMMUNITY AND INORGANIC GEOCHEMISTRY CONTROL SULFUR AND IRON CYCLING IN AN ORGANIC-RICH SEDIMENTARY SYSTEM

Not all parts of the biosphere are equally accessible. In some cases this hinders complete understanding of the controlling processes. This paper describes coupled processes in a salt-marsh environment where, because of its accessibility and extreme youth, details of unexpected reactions have been revealed. In particular, kinetic controls can be investigated.

Siderite (iron carbonate) and iron sulfide concretions are forming now in salt-marsh sediments only 60 years old, allowing simultaneous investigation of mineralogy, aqueous and solid phase geochemistry, isotope geochemistry, microbial and molecular biology. The driving force for the chemical reactions is reduction in potential energy of the system. In this case there is a lot chemical potential energy through intimate juxtaposition of oxidizing agents and reducing organic matter, the marsh vegetation. Occurrence of reduced iron minerals and sulfides indicates that oxidized iron from the sediments and sulfate from seawater were the likely oxidants. These processes are kinetically inhibited at sedimentary temperatures but bacterial mediation allows them to proceed. Mineralogical and geochemical evidence indicates that localized, intense, microbial sulfate reduction is the focus for siderite nodule growth, despite the fact that conventional models would dictate iron reduction should occur first and siderite should not form in the presence of sulfide from sulfate reduction. Microbial bio-assay and DNA analysis revealed unexpectedly that some sulfate reducing bacteria (SRB) can undertake both sulfate and iron reduction, gaining energy from the latter process but not growing. It is possible that some SRB have developed the ability to undertake iron reduction to produce ferrous iron to react with hydrogen sulfide, which can be toxic even to SRB. This ability is an evolutionary advantage because if a small proportion of the group can switch to this alternative metabolism, the community as a whole will prosper. However, it is surprising since it would be expected that inorganic reaction of sulfide with ferric iron would be sufficiently rapid to avoid sulfide accumulation.

In this system mineral, chemical and microbial self-organization cooperatively operate to minimize the potential energy. Are the microbes agents of the chemical system or vice versa?