To identify drivers of microbial community structure over the continuum of environmental conditions my research focuses on two complementary perspectives: the ability of communities to withstand stressors, and their ability to remediate environmental stressors.  As an example for adaptation to stress a subsurface salinity gradient spanning 8 to 3490 mM Cl^- was investigated for Archaeal diversity. We investigated an organic matter-rich sedimentary rock that produces economically significant quantities of methane of biological origin from depths several hundred meters below the earth's surface. Detailed community analyses indicated links between Archaeal communities and pore water salinity. Differences in methanogen substrate utilization were associated with porewater salinity.  Our findings indicate that several distinct communities of methanogens persist, each adapted to particular conditions of pore water salinity and preferential substrate use.  As an example of adaptation that overcomes an environmental stressor a chronosequence of recently deposited lava was investigated for primary and secondary succession of microbial communities.  Primary succession was investigated by the analysis of eleven sites representing the succession of 300 yr of life history on lava deposits. Microbial colonization of and succession on volcanic deposits depended directly on the intrinsic quality of the site, such as substrate availability. The number of species shared along soil biogeochemical parameters indicated the survival of several phylotypes from one successional stage to the next. A parallel investigation of the extent of secondary succession was made on a nearby geothermal gradient established after a drastic increase in soil temperature within the young rainforest created a new habitat and provided an opportunity for thermophiles and thermotolerants to establish communities. In all gradients changes in the community structure reflected the response of populations to shifting environmental conditions. Our future challenge is to detail microbial species function in an ecosystem, especially the extent of functional redundancy in ecosystem processes.