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article imageOil-dwelling bacteria demonstrate how life gets everywhere

By Robert Myles     Dec 14, 2014 in Science
Oslo - Deep below Earth's surface are scattered vast lakes of oil. These hydrocarbon reservoirs are dispersed randomly much like remote islands in Earth’s oceans.
But despite the hostile environment that oil reserves present, like pretty much everywhere else on Earth, life exists there. Not only that but the wide dispersal of oil reserves appears to make little difference to the species that have made hydrocarbons their home, according to a new study led by New Hampshire-based Dartmouth College in conjunction with researchers from the University of Oslo in Norway.
The researchers found that the underground microbes that inhabit such apparently inhospitable and diverse realms are social creatures: they’ve exchanged genes for aons leading to greater similarities between species types that reside there than might otherwise be supposed. Their findings cast new light on Earth’s “deep biosphere,” a largely unexplored habitat far below Earth’s crust and ocean floors.
The inhabitants of these hydrocarbon habitats comprise single-celled residents whose numbers and diversity are estimated to be roughly equal to the totality of microbes discovered to populate what’s usually regarded as Earth’s biosphere, namely the surface of the land, the seas and the air.
Such deep microbial research has implications for gaining a better understanding of the early evolution of life on Earth. It has implications, too, in the search for life elsewhere in the solar system. On Mars, NASA’s Curiosity Rover has provided strong evidence that rivers once flowed over the Martian surface while on Titan, one of Saturn’s moons, NASA’s Cassini-Huygens mission has confirmed the existence of vast hydrocarbon lakes.
Previously, some scientists held the view that bacteria found to live in oil reservoirs were descended from isolated bacterial communities buried millions of years ago in the sediments of organic material that would mature into what we know today as the world’s oil reserves — the so-called "burial and isolation" scenario.
But the new research suggests a far more complex picture where sub-surface and marine bacteria have been continuously migrating into these lakes of oil and colonizing from the most ancient of times to the present day. In the process, such bacteria have influenced the genetic composition of species found there, according to the study’s co-author Olga Zhaxybayeva, an assistant professor at Dartmouth.
An increasing number of microbial life-forms have been found in some of Earth’s most inhospitable places since the 1980’s. In the case of species living deep below Earth’s crust much remains to be discovered about when and how such organisms came to reside in habitats which, on the face of it, appear to be as inhospitable to life as it’s possible to be. Quite apart from the scarcity of nutrients and the complete lack of sunlight, a source of energy, extremes of both temperature and pressure would appear to make such habitats a Hell on Earth for living things.
Microorganisms, however, are the oldest form of life on Earth and, despite these unpromising conditions, bacteria living deep below ground manage to survive in a habitat where it isn’t the Sun that’s the giver of life but Earth itself. Such bacteria survive on a combination of inner heat from Earth’s molten core and Earth’s intrinsic chemicals and nutrients.
Precedent to the new study, the researchers posed a number of questions:
1. Do buried bacteria adapt to living in oil reservoirs as they form from sediments?
2. Do bacteria evolve in isolation?
3. Or, do they migrate to oil reservoirs, exchanging genes with surrounding bacteria, including surface ones introduced through, for example, drilling fluids used in oil production?
In a wide-ranging survey of bacteria, the scientists analyzed 11 genomes of Thermotoga, an ancient lineage of heat-loving bacteria. Samples of bacteria were drawn from oil reservoirs as far apart as the North Sea in Europe and from hot water vents on the ocean floor near the Kuril Islands north of Japan. Bacteria were also sourced from Italy and the Azores island chain west of Portugal. In addition, the researchers analyzed Thermotoga community DNA from the environment — so-called metagenomes — from North America and Australia, already available in public databases.
The analytical results disclosed extensive gene flow across all the sampled environments. This suggested the bacteria don’t stay isolated in the oil reservoirs. Instead, over a long period of time they have migrated and colonized oil reservoirs, in the process contributing to their genetic make-up.
"The pathway of the gene flow remains to be explained, but we hypothesize that a lot of the gene flow may happen within the subsurface," said co-author Camilla Nesbø, a researcher at the University of Oslo’s Centre for Ecological and Evolutionary Synthesis.
In previous research, Zhaxybayeva and Nesbø demonstrated that Thermotoga and its close relatives have exchanged small pieces of genome with Archaea, an ancient single-celled life form different from bacteria, and with another distant group of bacteria, Firmicutes.
Details of the most recent study were published last week in the ISME journal under the title “Evidence for extensive gene flow and Thermotoga subpopulations in subsurface and marine environments”.
More about Bacteria, oildwelling bacteria, oil reservoirs, hydrocarbon bacteria, life on earth
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