A Deep Bacterial Biosphere In Marine Sediments
R. John PARKES*, Pete WELLSBURY, Barry A. CRAGG, and Kim GOODMAN
Dept of Geology, University of Bristol, Queens Road, Bristol, BS8 1RJ, UK
The presence of microbial populations in deep environments - marine, terrestrial and oil reservoirs - has recently been demonstrated (1-6). Depths extend to 2.8 km and even into basalt (3,7) and granites (8). The deep bacterial biosphere in marine sediments alone is estimated as ca. 10% of the surface biosphere. Bacteria can grow at temperatures up to 113℃ and possibly higher (9) and pressures over 1,000 atm. Hence, with an average thermal gradient of ca. 30℃/km in marine sediments temperatures would not be limiting for several kilometres and the "classic" separation between biologically catalysed alteration of organic matter to ca. 60℃ during diagenesis, followed by thermal alteration, catagenesis, (10) may no longer be appropriate. Conditions within deep environments would still be considered to become more hostile with depth, however, as buried organic matter becomes increasingly recalcitrant. Hence, the presence of high bacterial populations in deep sediments are enigmatic, especially as in some marine sediments bacterial activity can actually increase in deeper layers.
The distribution of bacterial populations and activity in a range of deep marine sediments will be described. The geochemical impact of this activity demonstrates that bacterial processes are substantial in deep sediments. In gas hydrate sediments bacterial activity is stimulated within the deep gas hydrate zone to such an extent that maximum activities can be greater than those close to the sediment surface (11). Below about 100 m in these sediments low molecular weight volatile fatty acids begin to increase and continue increasing with increasing depth (acetate > 15 mM at 691 m, 3 orders of magnitude increase, 12). This seems to be due to an increase in the bioavailability of organic matter during deep burial and heating, and results in a large stimulation in bacterial activity. If this processes is widespread it would provide a fuel for a deep bacterial biosphere which could extend to greater depths than has previously been considered.
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