SUB-SEAFLOOR PROKARYOTES ACTIVE ON GEOLOGICAL TIME SCALES?

Although sub-seafloor sediments contain the largest global reservoir of organic matter they have not, until recently, been considered to be a significant habitat for prokaryotes. Sedimentary organic matter was considered to survive because it was recalcitrant to degradation by near-surface microorganisms, and hence, it would not be utilized under the increasingly less optimum conditions during burial (e.g. less efficient electron acceptors, decreasing porosity, increasing pressure, decreasing nutrients). However, this material, requiring very long timescales for degradation, may be capable of sustaining micoorganisms over geological time. Certainly, significant cell numbers are consistently present in deep marine sediments, including all the way to the rock basement, when this has been sampled (e.g. 16.5 mya sediments). Stimulation of prokaryotic populations and activities due to a range of deep subsurface changes (e.g. brine incursions, lithology, subsurface mixing of hydrothermal fluid and seawater, gas hydrate formations, sapropels and other discrete zones of high organic matter [e.g. diatom layers]) demonstrates the presence of viable cells and this is confirmed by direct molecular genetic analysis (Q-PCR, FISH). Prokaryotic diversity also changes at geochemical (CH4:SO4) and lithological (volcanic ash:pelagic clay layers) interfaces demonstrating that microorganisms are adapted to their deep sediment habitat and are not dormant or dying slowly. Although growth rates are very difficult to estimate and there are contradictory results, data from sapropel layers of increasing age suggests that in situ growth rates are extremely slow (division time ~100 ky) and very different from the fast growing near surface isolates that we are familiar with. Understanding life in the geosphere is a major challenge for future research.