MICROORGANISMS FROM DEEP, HIGH-TEMPERATURE SANDSTONES - CONSTRAINTS ON MICROBIAL COLONIZATION

Cores were collected from Late Cretaceous and Early Tertiary rocks in the Piceance Basin of western Colorado, USA, to investigate the origin s of subsurface microorganisms under geological conditions likely to c onstrain microbial transport and survival. The sampled strata from 856 -862, 1996-1997 and 2091-2096 m recorded peak paleotemperatures of 120 -145 degrees C from 40-5 million years ago, while present temperatures range from 43 to 85 degrees C. Cores were analyzed for culturable ana erobic bacteria (Fe(III)- and Mn(IV)-reducing bacteria, fermenters, su lfate reducers, nitrate reducers and methanogens), ester-linked phosph olipid fatty acid and selected enzyme and physiological activities. Me asurable but low biomass (total phospholipid fatty acid) and anaerobic bacteria, primarily Fe(III) reducers and fermenters, were present in samples from the 856-862 m core; Cores from greater depths yielded onl y a single positive enrichment and lower biomass values. Methanogens a nd sulfate reducers were not detected in any of the samples nor were b acteria that could grow with methane and any added electron acceptors. I6S rRNA genes cloned from products of PCR amplification of DNA extra cted from an 858 m, 65 degrees C, Fe(III)-reducing enrichment were mos t closely related to bacteria in the genus Desulfotomaculum, Gram-posi tive, spore-forming sulfate-reducing bacteria. Assuming the maximum te mperatures would have eliminated any entrained bacteria, these anaerob ic microorganisms likely migrated into the shallower Wasatch formation within the last 5 million years. However, the deepest stratum sampled was hydrologically isolated and lacked any indication of microbial co lonization by all biological measures. Hydrologic connection to the su rface, high maximum temperatures and the presence of fractures are pro bably the primary factors that control distribution of microorganisms in these deep rock environments.