It was in 1987 with a range of collaborators (Fig.10) which are continually growing, we started to use this integrated approach on deep sediments collected on The Ocean Drilling Program, with some surprising results. One of the most useful techniques was direct microscopy and this is a 1000 times magnification of sediment bacteria - the little yellow-shaped objects sitting on the red sediment (Fig.11). This is obtained by using a special dye which binds to the bacterial chromosome and with UV light we can illuminate the bacteria, like a light bulb in the dark, so we can actually count these organisms without having to grow them. If you look very carefully some of the bacteria are actually caught in the act of division so it gives us an indication that these bacteria are growing and active. As we're in Japan I thought I'd present one of the first examples of results of this work as it was obtained with Japan Sea sediments. This is one of the very first deep sequences we analysed (Fig.12) and this is a logarithmic depth plot, 1 metre to a kilometre, of bacterial populations in log numbers of cells. This line shows the total bacterial population and it rapidly decreases with depth. Although bacteria were present even in the deepest sample we obtained, 518 metres, and numbers were high, around 10 million ml, this represents a 99% decrease from the surface population. Dividing cells were present throughout the profile indicating that some portion of the population were active. In addition we, unlike Zobell, were also able to grow bacteria in the laboratory but notice there is a 10,000 fold lower concentration of cultural bacteria than total bacteria. These culturable bacteria also decrease with depth, presumably reflecting the increasing recalcitrance of buried organic matter, because the surface bacteria will use the most degradable organic components first leaving more and more recalcitrant organic matter in the deepest sediments. But to our surprise, around about 360 metres bacterial populations actually increased and also there was an increase in the total population. This coincided with an increase in thermogenic methane gas so it looked like we had our first indication of a geosphere might provide energy for the biosphere in a very direct manner. Sulfate-reducing bacteria are known to be able to oxidise methane and in this insert to the slide (Fig.13), which is a blown up section of the deeper zone, we can see that sulfate reduction rates, measured using radiotracers, indeed actually increased in this deeper zone. This slide also shows the numbers of culturable sulfate-reducing bacteria. We were also able to isolate these bacteria from great depths, initially 80 metres and then with further work at 500 metres, and these were unique bacteria called Desulfovibrio profundus, profundus because they grow very deep. And again these results verified that these deep bacterial populations were indeed active.
I jump ahead now several years and show the bacterial distributions with depth at an additional 8 sites (Fig.14).
You can see that surprisingly at sites very far away from each other and spread across the Pacific, Atlantic and even the Mediterranean Sea, bacteria population distributions on this long-long scale are very similar. The dotted lines are the 95% limits. Bacteria that fall outside these limits are associated with special oceanographic conditions. For example at the Peru Margin upwelling site populations are higher than expected and where populations are lower than expected these are low productivity and low organic carbon sediments in the East Equatorial Pacific. So bacterial distributions make sense. The average depth of sediments in the oceans is about 500 metres and when we add up the total bacterial biomass to 500 metres, we come to an amount of carbon that is equivalent to 10% of all surface life. So in these deep sediments there is an additional 10% of biomass on Earth. However, this figure is probably conservative because as you can see the decline in bacterial populations doesn't actually steepen as we go deeper (Fig.14) and, therefore, bacteria are likely to be present much deeper than 500m and deeper than we were able to obtain samples, which was at this time about 640 metres. In addition, bacterial population can also increase with depth as I've shown in the Japan Sea (Fig.12), in response to a deep methane source. Deep methane also occurs in special gas hydrate environments, for example, the Pacific ocean, this figure shows bacterial distributions with depth (Fig.15), at sites without (control) and with a discreet hydrate zone shown in the blown up section between 215 and 225 metres in the shaded area.
