A community of microbes has been found living in 86 million year old deep sea clay, although they’re not exactly thriving – they seem to be surviving at the minimum energy requirement needed to qualify as “alive”.
Along with colleagues in Germany and the USA, Hans Røy from Aarhus University in Denmark collected long cylinders of mud known as sediment cores from deep under the Pacific Ocean sea bed during a cruise of the current system known as the North Pacific Gyre. The sea bed beneath the Gyre is particularly interesting, as oxygen can penetrate many metres below the surface, unlike other ocean areas where oxygen penetrates just a few centimetres. This makes them a good place to look for unusual forms of microbial life, which could in turn help our search for life outside our planet.
Writing in the journal Science, the team measured oxygen and carbon concentrations at different depths and, by making some assumptions about factors like the degradability of organic matter, were able to calculate how oxygen consumption changed with depth.
They then measured the numbers of microbial cells, which falls off rapidly with increasing depth. At the surface they found 108 (100,000,000) cells per cubic centimetre, but by 20m below the surface, in sediments laid down over 66 million years ago, they were finding just 1000 cells per cubic centimetre. Further down the cells were so sparsely distributed as to be uncountable, but evidence of their presence was still seen in oxygen consumption. One interesting finding was that the drop in density of microorganisms lagged behind the drop in oxygen concentration – suggesting a gradual change in the way oxygen is consumed.
The oxygen consumption rate did eventually reach a stable minimum, with the deepest microorganisms consuming 3 orders of magnitude less oxygen than similar cells in culture. This incredibly low respiration rate suggests these cells may be existing in a long-term stationary state, subsisting on the bare minimum to survive and unlikely to be growing or dividing.
Our present understanding of microorganism respiration is built mainly on our experience of culturing fast-growing microbes at the surface, so it’s no surprise that what we discover deep under the sea bed doesn’t match our expectations – even less so when you consider that this layer of sediment has seen no new food since the dinosaurs!