Although recalcitrant deep organic matter could explain slow bacterial growth rates, it's still very difficult to understand how after ten million years, in contact with bacteria, there's any degradable organic matter left. I previously suggested that perhaps temperature, increasing with depth, slightly but consistently increases the degradability of buried organic matter. Now this may sound far-fetched but we know a similar process happens already, but this time in terms of oil and gas generation (Fig.24). We know that when deeply buried organic material gets heated and pressurized, that kerogen becomes rearranged and low molecular weight compounds are produced. As you go deeper hydrocarbons start being produced within the oil window, between 100 and 150℃, and then at higher temperature methane gas. As well as hydrocarbons, low molecular weight organic acids and hydrogen are produced which are key energy sources for anaerobic bacteria (Fig.25). So it might be that this process provides a deep thermogenic energy source similar to that previously shown with deep methane in the Japan Sea (Fig.12) and which fuels a deep biosphere. And just possibly some of the deep kerogen alteration might also be due to bacterial activity also.
To test this hypothesis we took a surface marine sediment and simulated burial by heating it in a thermal gradient system between 0 and 100℃ (Fig.26). This would be equivalent to a sediment depth of approximately 3km. To our surprise, very high concentrations of the organic acid, acetate, was produced. The solid line on the graph represents the experimental results with respect to temperature and it looks like this organic acid production was bacterial as thermogenic production of acetate doesn't start until much higher temperatures, 80℃, and results follow a typical bacterial temperature distribution. The solid symbols are actually in situ acetate concentrations from a gas hydrate site plotted at their in situ temperatures. There is a remarkable similarity between the acetate concentration we can produce in bacterial simulation experiments and what occurs in this gas hydrate deposit. So perhaps deep acetate in the gas hydrate site is also being bacterial generated in situ.
If I plot all these acetate concentrations versus temperature but now include oil source rocks from Kimmeridge Clay which has been heated to much higher temperatures in the laboratory (Fig.27), it looks like there is an intriguing relationship between bacterially generated acetate and acetate that can be generated thermogenically. Perhaps bacteria at low temperature are catalysing the same types of macro-molecular rearrangement of kerogen that temperature does on its own at very high temperatures and which ultimately leads to oil and gas formation. Possibly if we go even deeper than we did in Blake Ridge, the gas hydrate site, which was 750 metres, we might find bacteria are catalysing this process at even higher temperatures than currently demonstrated (Fig.26).
Therefore, the deeper we go, we might find a hot deep biosphere fuelled by what was thought to be purely thermogenic processes. In the gas hydrate site, the acetate that we produced as a response to temperature increases was then utilized to produce methane just below the gas hydrate zone. This might explain why in this particular environment, gas hydrates themselves were actually formed. Bacteria may be involved in both the production and consumption of methane in these gas hydrate sites.
In more normal low organic matter sediments (approximately ＜1%) for example Woodlark Basin, Pacific Ocean, acetate concentrations actually don't increase with depth (Fig.28), which in itself is quite remarkable as bacteria are present (Fig.29) which should consume acetate. Hence, as deep acetate concentrations are relatively constant with depth this may indicate that acetate is slowly being produced and consumed and this maintains deep bacterial populations even to 842m, our deepest samples to date.
