Further evidence for the effect of heating on acetate generation is provided by heating the ODP Blake Ridge gas hydrate samples at higher temperatures than they experience in situ as even more acetate is generated. This shows that the near surface sediment heating experiments to generate acetate can also be reproduced in deep sediments.
In addition, we can also isolate acetogenic bacteria at very high temperature from these experiments and from a range of depths (0.13, 365 and 691m). These cultures produced reasonable concentrations of acetate in the laboratory within about six weeks (Fig.30), with the highest concentrations of acetate produced at the highest temperature, 94℃. These results reinforce our hypotheses that bacteria can generate acetate at high temperatures. Unfortunately the enrichment at 94℃ refused to subculture, but we had enough biomass to conduct molecular genetic analysis and of the resulting clones was most closely matched to an uncultured marine bacteria from the Mariana's Trench (Fig.31). This sequence was thought to be from a barophilic bacterium which therefore likes high pressure. This is consistent with the deep origin of our acetogenic isolate at about 3.5km, including the water depth. The enrichment at 30℃ contained a mixture of bacteria and some of these were related to the sulfate-reducing bacteria we had previously isolated from the Japan Sea. These can also ferment and produce acetate, in the absence of sulfate. The cells of this bacterium were very distinct, very small almost square-shaped cells (Fig.32), which we have not seen before and which do not look very similar to Desulfovibrio profondus. There are also other bacterial types in this culture. The 4℃ culture was a mixture of filaments and small rods, and also spores (Fig.33) which is consistent with the molecular genetic data which indicated the presence of spore forming Clostridia (Fig.31). 4℃ is a very important temperature for marine environments because it approximates average seawater temperature of 2℃. We tend to be fascinated by very high temperatures, but most of the marine environments are actually cold, so acetate generation at 4℃ itself it is very important.
We in fact have a lot to learn about subsurface marine microbiology. Although my talk has concentrated on sediments, other researchers have been looking at what happens within basaltic rock that lies beneath these sediments. Bacteria are present in weathering cracks in the basalt (34) and are associated with turning the basaltic glass into clay and if you look at square D you can see that by the black arrow there are branching filaments and shown by the red arrows there are actually what looks like bacterial cells in those filaments. So not only have we got 15km of sediments as a potential deep bacterial habitat, we have also the basalts beneath.
Well, I hope I've been able to demonstrate that the deep biosphere in marine sediments is a significant component of intra-terrestrial life and equally fascinating and important as extra-terrestrial life.
What I've talked about is summarised in this schematic diagram (Fig.35)： a) bacterial populations are very high near the sediment surface, b) they decease very rapidly with depth, although at some sites they can increase in the subsurface, for example in methane hydrate sites, where there are brine seeps or hydrothermal flow, c) then bacterial populations can increase again as response to thermogenic energy sources. d) it may well be that bacteria themselves have a role to play in this fossil fuel formation process. This would explain why when we recover our fossil fuels that in the production fluid of some oil reservoirs, hyperthermophilic high temperature bacteria are being produced in enormous concentrations. Hence, in this situation the deep biosphere is inoculating the surface biosphere and this process may have happened continuously throughout the origin of life on Earth. e) It may well be that life even originated deep and inoculated the surface as soon as the surface environment was suitable for life to take hold.
