During Leg 146, bacterial numbers and activity rates were found to be stimulated by the presence of a discrete gas hydrate zone in samples from Site 889/890 (Cragg et al., 1995). The data presented here from Leg 164 confirm that the presence of gas hydrate stimulates bacterial populations and activity. These increases within the gas hydrate-stability zone seem to be associated with the presence of free gas within the hydrate deposits rather than the solid hydrate. Bacterial populations in solid hydrate are much lower than in the surrounding sediment. In the free-gas zone immediately below the BSR, there are increases of an order of magnitude in bacterial numbers. Similarly, a range of potential bacterial activities increase around and below the BSR, suggesting that the free gas associated with the hydrate is an important factor in this stimulation.
Microbiological data from the Blake Ridge sites, and those collected during Leg 146 (Cragg et al., 1995), clearly demonstrate that gas hydrate systems are more biogeochemically dynamic than previously thought. In particular, the high acetate concentrations occurring through and beneath the base of the gas hydrate zone provide an unexpected source of methane gas for the formation of hydrates. The thousand-fold increase in potential rates of acetate methanogenesis (rather than H2:CO2) associated with high interstitial water acetate concentrations emphasizes this and, when combined with intense rates of methane oxidation associated with, and beneath the BSR, highlights the role of carbon cycling in these sediments. This, in turn, fuels a thriving bacterial population at depth, reflected by increases in bacterial numbers, numbers of dividing and divided cells, and productivity.
Thus, there is the exciting potential for an increasing bacterial biosphere at depth in gas hydrate-bearing sediments. Both bacterial activity and populations actually increase around and beneath the base of the gas hydrate-stability zone. Previous estimates of the extent of a deep bacterial biosphere in marine sediments, 10% of the surface biosphere in terms of organic carbon (Parkes et al., 1994), were based on a model in which bacterial numbers continued to decrease exponentially with increasing depth. Increasing bacterial activity and populations at depth such as found at Blake Ridge and Cascadia Margin hydrate sediments will increase significantly this estimate of deep bacterial biomass because hydrate-containing sediments are widespread (Kvenvolden, 1988) and contain a significant amount of global fossil fuel carbon.