Microbial Geochemistry: The influence of microbes on geochemistry; the influence of geology on microbial ecology

Category Geochemistry
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Bennett, Philip; Omelon, Christopher
Holding Date 11 October 2008

Microbial geochemistry studies the interactions between microbes, metals and minerals. It includes understanding how microbes influence geological processes, as well as how mineralogy and geochemistry affect microbial populations. The abundance and metabolic diversity of microbes has resulted in their being found in almost every recognized habitat on Earth; their chemical reactivity greatly impacts local surroundings through direct and indirect energy and chemical exchanges, acceleration of slow redox reactions, diagenesis of rocks, soils and sediments, and dissolution and precipitation of minerals. Microbial geochemistry links biology to geochemistry, hydrology and geology and identifies relationships in this complex system responsible for the evolution of water and rocks at the Earth’s surface. The distinction between focused disciplines such as aqueous geochemistry and the more interdisciplinary field of microbial geochemistry is the recognition that microbes are more than simply contributors to a set of chemical reactions. The microbial side of geochemistry is actually a dynamic community of microbes that compete for common nutrients or habitat; the net result is a constantly changing perturbation of the geochemical and geological surroundings.
Chemical disequilibria driven by microbial activity can be either direct or indirect: direct examples include cell-metal adsorption as well as proton exchange, which influence chemical interactions with dissolved ions and suspended solids. These are in contrast to indirect processes related to bacterial metabolism that cause changes in pH, redox state or concentration of aqueous ions.
One important scientific advance is the application of molecular techniques that focus on specific gene sequences to identify members within a complex microbial community. These and related techniques such as FISH allow us to build a picture of community structure and of the flow of energy, carbon, and other nutrients from primary producers to consumers. One critical development in this area is the ability to link phylogeny to function in environmental samples, i.e. the ability to attribute a specific geochemical function to a specific population without relying on laboratory cultures or experiments.
A key realization is that microbes competing for scarce resources do little for fun or by accident; biogeochemical reactions occur because it offers an advantage, and that these reactions take advantage of the chemical composition of the surrounding environment. And the future of this field will depend on our ability to understand not just which reactions occur at the interface between a microbe and a mineral, but why. Why does a microbe precipitate a mineral on its cell wall? Or attach to and transform one mineral and not another? Such questions reveal the fundamental link between aqueous geochemistry and microbial ecology, and will ultimately lead to functional models of geochemical processes at the Earth’s surface.