BIOSPHERE:GEOSPHERE INTERACTIONS IN THE SUB-SEAFLOOR BIOSPHERE

It has been estimated that total global prokaryotic carbon is equivalent to 60-100% of total plant carbon. As green plant photosynthesis is considered to be the primary production mechanism on Earth, prokaryotic biomass seems surprisingly large, particularly considering that ~60% of this is in sub-seafloor sediments; remote from direct photosynthetic carbon input. However, this may point to novel use of photosynthetic carbon and geosphere, “dark energy” sources in deep sediments. Certainly, significant cell numbers are consistently present in deep marine sediments, including all the way to the rock basement, when this has been sampled (e.g. 16.5 mya sediments). In addition, deep prokaryotic populations and activities are also stimulated in the subsurface (e.g. brine incursions and CH4:SO4 interfaces, lithological changes, subsurface fluid flow, gas hydrate formations, sapropels and other discrete zones of high organic matter [e.g. diatom layers]). Slow heating during burial can gradually increase the reactivity of buried recalcitrant organic matter, potentially providing a slow but continuous energy supply. Reactions lead to increased maturation of the organic matter and release of oxygenated compounds (e.g. volatile fatty acids), H2 (from aromatization) and hydrocarbons (e.g. CH4) which can be used as prokaryotic substrates. These organic matter maturation reactions can be microbially catalysed at temperatures too low for abiological reactions (~80°C) but at higher temperatures there is evidence for abiological processes directly providing substrates for prokaryotes. In addition, inorganic reactions, particularly those involving iron minerals, are also stimulated by increased temperatures and can be an additional source of H2. Therefore, the deep sub-seafloor may not be as energy limited as previously thought and may account for the large biomass in this “geosphere” habitat.