THE THERMODYNAMIC LADDER IN AQUIFER MICROBIOLOGY

The concept of a thermodynamic ladder has over the past three decades almost indelibly colored the environmental scientist’s view of the distribution of microbial activity in groundwater flows. The thermodynamic ladder reflects an energetic hierarchy of microbially mediated electron accepting processes that together control the redox state and composition of anoxic groundwater. We consider the ladder in terms of usable energy, the fraction of the available energy a functional group of microorganisms can take advantage of to drive forward its metabolism. Energy usable by a given group varies strongly with groundwater composition, especially pH and H₂ concentration, and the different groups extract it with varying efficiency. The concept of a static hierarchy of iron reducers dominating sulfate reducers, which in turn outcompete methanogens, poorly reflects the ladder of usable energy over the range of conditions commonly found in aquifers. Iron reducers, for example, have little or no advantage over sulfate reducers at neutral to alkaline pH, and may in fact be thermodynamically excluded there. Methanogens are well adapted to derive energy from oligotrophic environments and in many cases are at little or no energetic handicap, relative to sulfate reducers. From an ecologic perspective, the population dynamics of the subsurface is affected not only by competition, but mutualism among functional groups. Current concepts in aquifer microbiology call on the segmentation of aquifers into microbial zones dominated by a single group, as the result of competitive exclusion. Each point in the subsurface may in fact be characterized by the simultaneous activity of coexisting populations of a broad variety of functional groups of microorganisms.