Comparative ecology of H2 cycling in sedimentary and phototrophic ecosystems

The simple biochemistry of H₂ is critical to a large number of microbial processes, affecting the interaction of organisms with each other and with the environment. The sensitivity of each of these processes to H₂ can be described collectively, through the quantitative language of thermodynamics. A necessary prerequisite is to understand the factors that, in turn, control H₂ partial pressures. These factors are assessed for two distinctly different ecosystems. In anoxic sediments from Cape Lookout Bight (North Carolina, USA), H₂ partial pressures are strictly maintained at low, steady-state levels by H₂-consuming organisms, in a fashion that can be quantitatively predicted by simple thermodynamic calculations. In phototrophic microbial mats from Baja California (Mexico), H₂ partial pressures are controlled by the activity of light-sensitive H₂-producing organisms, and consequently fluctuate over orders of magnitude on a daily basis. The differences in H₂ cycling can subsequently impact any of the H₂-sensitive microbial processes in these systems. In one example, methanogenesis in Cape Lookout Bight sediments is completely suppressed through the efficient consumption of H₂ by sulfate-reducing bacteria; in contrast, elevated levels of H₂ prevail in the producer-controlled phototrophic system, and methanogenesis occurs readily in the presence of 40 mM sulfate. This revised version was published online in August 2006 with corrections to the Cover Date.