Shallow subsurface temperatures can reach extreme levels in just 40 cm depth in Guaymas Basin sediments, limiting microbial colonization to thermally tolerable surface sediments. At temperatures beyond approximately 80°C and 100°C, respectively, the 13C-isotopic signatures of microbial anaerobic oxidation of methane and organic matter remineralization appear to be thermally restricted. Putative methane consuming archaea dominate the archaeal clone library while sulfur cycling bacteria and Chloroflexi-related sequences dominate the bacterial clone library. Archaeal clone library data suggest that the ANME-1 Guaymas archaea tolerate high in situ temperatures up to approximately 80°C, thereby gaining an advantage in access to the geothermal methane pool in hot Guaymas Basin sediments. Lastly, the results indicate that in situ thermal and/or geochemical gradients structure archaeal community composition and biogeography more than bacterial biogeography. While the average upper thermal temperature for detectable microbial life by RNA recovery in Guaymas Basin sediments appears to be around 80°C, temperatures may fluctuate by 25°C in as little as a day. Isotopic evidence for microbially mediated methane oxidation is only slight, yet putative methanotrophic archaea are commonly recovered in nearly all samples suggesting they may perform other physiological modes or isotopic signatures are not detectable because of high methane concentrations. High temperature associated archaea appear to be represented by OTUs related to uncultured MCG and ANME-1 Guaymas groups. For bacteria the dominant high temperature associated OTU was phylogenetically associated with the Thermodesulfobacteriaceae. Two of the four main themes of C-DEBI research are “Extent of Life” and “Limits of Life”. Using sediment samples acquired from Guaymas Basin, my C-DEBI research links these two themes by examining how the biogeographical distribution of sedimentary microorganisms is shaped by severe, life-limiting conditions. Although these samples are not from deep sediments, they exemplify deep biogeochemical processes that have been compressed to shallower depths by elevated hydrothermal activity. My research demonstrates how thermal and geochemical regimes interact to control the spatial extent of life by focusing on microbial zonation in an energetically diverse hydrothermal environment. My intention with this research was to accurately describe microbial biogeography and the physicochemical factors controlling it in these unique, compressed sediments, which can be a useful asset in preparation for future IODP sampling procedures and analyses as well as investigations in deep subsurface microbiology around the world.