In this gas hydrate zone bacterial populations actually increase presumably again using deep methane. Methane gas is in the form of ice crystals (Fig.16) which contain an amount of carbon estimated to be twice the amount in all other fossils fuels, and thus they represent a globally significant source of energy for a deep bacterial biosphere.
But if there is plenty of deep energy around in some environments what would limit the depth of bacteria in the sediments? Well, as temperature increases with depth, perhaps temperature might start to limit bacteria. To investigate this we went to a high temperature environment, Juan de Fuca Ridge (Fig.17), where we would be able to get very high temperatures at fairly shallow depths and thus obtain an indication of the upper temperature limit for life in sediments and therefore an idea of how deep bacteria might go to in normal marine sediments.
This is a depth plot of the bacterial distributions measured (Fig.18), also shown are the bacterial distributions in normal, low temperature sediments, dotted line. You can see at this site, where the temperature gradients are quite steep bacteria distributions decreases are steeper than in non-hydrothermal sites. However, they do follow and fit in with our understanding of different temperature types of bacteria. Starting with Mesophiles at the surface which grow at medium temperatures up to about 40℃ and as we go deeper there are bacteria in the thermophilic region which can grow up to about 80℃ and even deeper still as temperatures start to get very high, we go into the region of hyperthermophiles, bacteria that can grow at temperatures over 80℃. So it looks like the temperature characteristics of bacteria at this site reflects the current upper temperature limit for life and, therefore, bacteria should be present at least at temperatures equivalent to 113℃ which would be equivalent to about 4km deep in non-hydrothermal sediments.
However, when we got nearer to a 276°hydrothermal vent we got some even more interesting results. This graph shows bacterial distributions plotted against temperature, rather than depth (Fig.19) as temperature varied considerably with depth at the four sites. The temperature increased very quickly with depth and you can see by about 100℃ although we could detect bacteria, they were not statistically significantly different from zero. Again consistent with the 113℃ upper temperature limit for life. But at all four sites from two different ODP legs, to our great surprise, around 165℃ bacterial populations increased, and become significant again, before decreasing at even higher temperatures deeper in the sediment. This zone of high temperature bacteria corresponds with hydrothermal fluid flow beneath the surface and this hydrothermal fluid could contain organisms at high temperature, around 160℃, than we've currently been able to isolate. It is also possible that this hydrothermal flow brings in bacteria from shallower depths or even from sea water and they just survive long enough for us to detect. However, the residence time, the amount of time in this high temperature fluid, would have to be relatively short for them to remain as intact cells. Clearly more research is needed to confirm the existence of these very high temperature organisms. But, if they exist this would push the potential depth limit of the biosphere even deeper.
If elevated temperature, alone, doesn't control bacterial populations, what might? If we now plot just the total bacterial populations at non-hydrothermal sites with depth, we can see a very good significant correlation of bacteria with depth (Fig.20). However, there are several aspects of depth which might control bacterial populations, such as age and porosity (space between sediment particles for bacteria to grow). If we plot bacteria against age and also porosity (Fig.21) we see that even though there is a reasonable correlation, the correlation is much better with depth than either with age or porosity.
