Today the activity of aerobes rapidly remove oxygen and in stagnant environments such as lakes and seas and also subsurface sediments, oxygen, which is shown in this profile (Fig.7), is rapidly removed in a few millimetres producing a black anaerobic zone just below the sediment surface. In these environments, including environments within us, for example our digestive systems, anaerobic bacteria dominate.
I apologise for the complexity of the slide (Fig.8) but I need to introduce some organisms I'm going to talk about, and basically if you just concentrate on the right hand side of the slide these show anaerobic bacteria that use sulfate rather than oxygen in respiration and they produce hydrogen sulfide, the rotten egg smell of decaying matter, which interacts with iron in the sediment forming the black layer shown in the previous slide (Fig.7), these are called sulfate-reducing bacteria. Also involved are the methanogenic bacteria that use CO2 as a respiratory compound and produce methane, biogenic gas. Both of these bacterial types use either hydrogen or acetate as some of their energy sources and therefore compete against each other. In marine conditions, because the supply of sulfate is quite high, sulfate-reducing bacteria will predominate as soon as oxygen is removed and then subsequently when sulfate is removed methanogens will dominate, a process which will go on deep into the sediment. Bacteria in anaerobic systems act as an interacting team, another member of that team are acetogens that focus organic carbon flow through acetate, either by using organic substrates and producing acetate, the heterotrophic acetogens, or by using hydrogen and CO2 to produce acetate (Fig.8). Therefore, you can immediately see that compounds like hydrogen and acetate are very important energy sources for anaerobic systems. Which itself produces an interesting proposition, because we know that when organic matter is buried very deep, high temperature processes called thermogenic, breakdown organic material to produce low molecular compounds as part of fossil fuel formation, including hydrogen, acetate and methane. So this deep thermogenic energy source could provide energy for a deep biosphere, fuelling both sulfate reduction and methanogenesis. I will come back to this again in a later part of my talk.
So as anaerobes don't require oxygen, and some are even killed by it, they are not limited to the surface of our planet. So how deep might they go? And are there some very unique and ancient organisms waiting to be discovered in the deep subsurface? However, for a long time the deep subsurface was considered too extreme for life, due to a combination of factors. But we now know that some bacteria actually grow better under these extreme conditions (Fig.6). One of the limiting conditions is high temperature because the deeper you go, the hotter the sediments become. So because of that assumption it was thought that life, basically, was restricted to a very thin veneer on the planet and deeper thermogenic processes dominating below. However, recently the presence of bacterial populations in a range of deep environments has been demonstrated, including deep marine sediments, Cretaceous shales (approximately 100 myo), terrestrial basalts and oil reservoirs. This was a surprise particularly for marine sediments as the early findings in the 1950's by Claude Zobell, a very famous marine microbiologist, lead him to pronounce the end of the marine biosphere at 7.47 metres deep. This was because he couldn't culture any organisms at that depth or greater depths. These findings stopped a lot of research in this area. Subsequently it was demonstrated that we could only culture a very small percentage of the total bacterial population, often much less than 1 per cent, and hence this early research would have dramatically underestimated the numbers of bacteria present, especially in deeper layers. It is now recognised that you need to use a suite of complementary techniques to measure the presence and activity of bacteria in the environment (Fig.9), including total numbers of bacteria by direct microscopy, the range of difference types of bacteria present, by culturing and also by molecular genetic techniques, as well as measuring their activity using radioisotopes and measuring the production or consumption of the microbial metabolic products. At the same time integrating this data with geochemical and stable isotope analysis to confirm the presence of bacteria and to indicate their impact on the sediment.
