Is evolution possible without reproduction? Bacteria living hundreds of metres below the seafloor carry more genetic changes than their peers nearer the surface – even though the deep microbes are unlikely to reproduce and undergo natural selection in its traditional sense.

Investigations over the last 30 years have revealed that life exists several kilometres below both the land surface and the seafloor. Or, perhaps more accurately, life endures: nutrient levels drop off so rapidly with depth that the microbes can barely function at all. In fact, the cells show so few signs of life that it wasn’t until 2011 that researchers confirmed that microbes in sediments below the seafloor are, indeed, living.

To find out more about how they manage to survive, Brandon Briggs at Miami University in Oxford, Ohio, and Frederick Colwell at Oregon State University in Corvallis have sequenced and compared genomes belonging to one particular class of deep life – Firmicutes bacteria – sampled 21, 40 and 554 metres below the floor of the Andaman Sea, west of Thailand. The sediments from 554 metres were dated to 8.76 million years old.

The results show – for the first time, Briggs thinks – that the bacterial genomes change with depth: the micro-organisms at 554 metres carry more mutations in genes that code for energy-related processes like cell division and biosynthesis of amino acids than are seen in their shallower counterparts. He presented the findings at the American Geophysical Union Fall Meeting in San Francisco last month.


Mysterious mutations

The results raise a bunch of questions that, right now, are difficult to answer. Do those mutations help the bacteria cope with the scarceness of food? Or are they harmful mutations brought on by the extreme conditions? Either way, exactly how did the deeper Firmicutes population accumulate more mutations?

“In the classic sense of evolution, the organisms with the beneficial mutations would have to pass these down to their offspring,” says Briggs. But there is so little energy in these deep environments that many researchers question whether the microbes can afford to have offspring. “It is hard to believe that these microbes are dividing, based on the amount of energy that has been calculated that they get,” he says. “If they are not dividing, then can there be evolution?”

“There is simply too little energy per cell to allow growth,” says Hans Røy at Aarhus University in Denmark. “But genome evolution, as observed by Briggs and Colwell, could indicate that the bacteria go through many generations within the deep biosphere. So things seem not to match up.”

“It is very difficult to know what might happen in the subsurface,” says Jennifer Biddle at the University of Delaware in Newark. Her group has previously found signs that cells might be able to grow at depth. “But so far no one has really shown duplication of a cell, hence it’s hard to know if evolution would happen,” she adds.

Slow growers

But if you take evolution in its broader sense to mean genetic changes across the population, then it might be occurring even without cell division, says Briggs. That’s because in theory, bacteria in these environments grow so slowly that they may survive for hundreds of thousands of years.

Individual bacteria might have begun life at the seafloor before being gradually buried, over a period of thousands of years, as more sediment accumulated at the bottom of the sea. If so, perhaps the bacteria now at 554 metres were rare cells in the initial population that have now come to dominate because the other cells, which didn’t carry their genetic mutations, have all died.

No matter how the new results are interpreted, they clearly show that there is still plenty to learn about Earth’s “intraterrestrials”. “My hope is that by understanding these genetic mutations we can better understand the nature of life in these ecosystems,” says Briggs.

“It’s another piece of the puzzle,” says Røy. “It’s most useful in our quest to find out where we go wrong with our measurements, our assumptions, or our prejudgements about how life works.”