Fig. 1.

Simplified scheme of stratified microbial populations correlated to chemical zonation as result of microbial respiration (blue, aerobic respiration; green, nitrate reduction; maroon, manganese reduction; rust color, iron reduction; black-gray, sulfate reduction; light gray, methanogenesis). The dots on the right show typical downcore distributions of different microorganisms. The oxygen- and nitrate-respiring physiologies of the blue and green microbes are apparent. However, the red microbe appears to be a generalist that thrives throughout the oxic to the iron-reducing zone, whereas the sulfate reducer (in dark gray) also thrives in the metal-reducing zone. The uncultured microorganism in purple appears to be a sulfate-dependent anaerobic methane oxidizer, sandwiched between the sulfate reducers and the methanogens (light gray). Finally, the uncultured subsurface archaeon in black avoids oxygen, but is otherwise ubiquitous. These examples have actual precedents and illustrate the challenges of assigning metabolism based on geochemical stratification.