We thought that age might be a controlling factor for bacteria at depth because the age of the sediment would probably determine the amount of degradable organic matter. As the most degradable compounds will be preferentially utilized resulting in organic matter becoming more recalcitrant during burial. But as the correlation's are not so strong this indicates that factors other than age or porosity are controlling bacterial depth distribution. Also by 10 million years of burial, you would have thought that any organic matter that was degradable would have already been removed. So what component of depth actually is controlling bacterial distributions ? One feature that is constant and not related to either age or porosity, is temperature, which increases with depth due to the thermal gradient of the Earth. And possibly heating the sediments during burial might make organic material more bioavailable and hence control bacterial depth distributions.
However, before I explore that possibility, I would like to summarise the global significance of the deep biosphere generally, and also in subsurface marine environments specifically. In the introduction I mentioned that recent estimates indicate that bacterial biomass on Earth is equivalent to between 60 and 100% of plant biomass (Fig.22). That itself is remarkable but when we look how that bacterial biomass is distributed in the open ocean, soil, ocean subsurface and terrestrial subsurface we get even more surprises. It looks like over 90% of total global bacterial biomass is actually in the subsurface, and of that some 67% is in the oceanic subsurface. Hence, surprisingly, this means a majority of bacteria on our planet are living an upside-down existence. They are not near the surface fed by photosynthetically derived energy, they are actually much deeper than the surface manifestation of life and we know very little about these types of organisms. However, if you look at how quickly these bacteria divide, it gives some clues as to the life strategy of these deep bacteria. Although bacteria can divide very rapidly in the laboratory, as quickly as once every ten minutes, in natural environments, they grow much slower mainly due to restriction by energy limitation. However, even a six days doubling time for the open ocean is pretty fast and even in oligotropohic, nutrient poor marine environments division still takes only about 0.8 years. When we come to the soil, however, division takes even longer, something like 2.5 years, this presumably reflects the recalcitrance of the plant polymers the bacteria have to degrade, such as cellulose and lignin. But when we come to the subsurface, although it's very difficult to obtain accurate estimates of division of time we come up with the figure, of around one division every 1,000 to 2,000 years, which is really remarkable.
Hence, although the majority of bacterial biomass is in the deep subsurface they are only growing very slowly and that might reflect the high recalcitrance of sedimentary organic matter which allows only slow growth but hence also enables them to survive on buried organic matter for very long periods of time. It would be fascinating to find out how they can grow so slowly. And my guess is that at any one moment in time, only a small percentage of the total bacterial population is actively dividing and some others are just maintaining themselves giving, overall, very long division times. We have, for example, some estimates of generation times in excess of 100,000 years. So all this is commensurate with a geosphere habitat where energy flow is relatively low, although organic matter reservoirs are large, and this supports a large bacterial biomass which grows very slowly. Except when there is new energy supply as we saw in the Japan Sea and gas hydrate sediments where bacteria at depth can respond very rapidly to a new energy source.
However exploring this deep subsurface is anything but stagnant. This is a photograph (Fig.23) of deep sediment samples and myself being transferred in the middle of the Japan Sea from the ODP ship followed by three days in a Japanese tugboat to get the samples back as rapidly as possible.
