Comparison of the microbiota recovered from surface and deep subsurface rock, water, and soil along an elevational gradient

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Abstract
The isolation of viable microorganisms from deep volcanic rock formations (50–450 m) within Rainier Mesa, Nevada Test Site has posed questions regarding the origin of in situ bacteria. One hypothesis under investigation is that microbial transport to depth has been facilitated via recent natural water flow from surface bacterial populations. Recoverable microbiota from surface topsail, paleosol, rock, and outflow spring water were compared to those from subsurface rock and fracture flow water to determine if relationships existed between microbial communities along an elevational gradient. Direct counts were higher than culturable counts in all samples. Top‐soil samples had higher culturable counts and numbers of nitrogen‐fixing and sulfur‐reducing bacteria than samples from the subsurface rock. Heterogeneity in microbial communities from the different samples (measured by diversity and evenness indices and by comparison of morphological and physiological tests of representative isolates) was demonstrated and provided evidence against the hypothesis for recent bacterial transport from the surface. An alternative hypothesis, that bacterial communities in deep subsurface rock have been isolated from surface environments for long periods of time, is discussed.