BioEd Online

As a child, you might have jumped in a pool with your brother or sister, each seeing how long you could go without air. Well, there’s taking a deep breath and then there’s what a recently discovered community of deep biosphere microbes beneath the Pacific seabed can do.

Using so little oxygen that they barely qualify as life, microbial communities deep below the Pacific sea floor discovered by Hans Roy and his colleagues of the Centre for Geomicrobiology at Aarhus University, Denmark, languidly enjoy low rates of metabolism where biomass turnover (replacement of the stuff organisms are made of), occurs once every few hundred or even every few thousand years.

Microbes require energy to maintain an electric potential across their membrane and to keep their enzymes and DNA ticking over, so these critters may be living at the absolute minimum energy requirement needed to subsist, the researchers believe. This 86-million-year-old community of microbial couch potatoes is basically enjoying the maximum laziness possible.

For years, scientists thought that the radically ascetic conditions in the deep, sub-seabed - high pressure, lack of oxygen, and low supply of nutrients and energy – made such environments uninhabitable for any form of life. But the discovery of living microbial communities buried deep in marine sediments has sparked investigation of life confronted by such extreme energy limitations.

The North Pacific subtropical gyre - a system of high-powered rotating ocean currents – together with its South Pacific counterpart is the most nutrient-poor region of the oceans. Yet here, researchers found oxygen penetrating as deep as 30 metres into the sediment.

“There wasn’t a good explanation for this,” Roy says. “The old way of thinking about it was that there couldn’t be anything deeper than about half an inch, so it was obvious that we were missing something in our understanding.”

Then Roy’s team found in mud samples collected from sediment columns oxygen penetration depths of over 30 metres.

“And it’s become clear that this missing element is the low sedimentation rate. In this area, the sediment is accumulating at an incredibly slow rate.”

The sediment accumulation is so low that if a grain lands on the seabed, it will take 1000 years before another one sits on top of it. In essence, the food that falls from above sits at the surface for thousands of years while all the good parts get eaten. The material that then gradually becomes buried offers a very poor diet even for microbes, so is just eaten very slowly. When there is nothing to eat, the bacteria do not breathe and the oxygen just hangs around.

The discovery of this ‘slo-mo life’ prompted the Centre and Nasa’s Astrobiology Division at its Ames Research Centre to organise a conference geomicrobiologists and computer modelers in this young field last week in Aarhus to explore the limits of microbial life and the biological demand for energy.

“The limits of life are so much further beyond what we have imagined,” explains co-organiser Bo Jorgensen. “Here we appear to see the lower energy limit for life under our current understanding. But we have to say now that we do not know where the lower limit is anymore.”

Which is why Nasa is fascinated by the research. The surface of Mars may be very inhospitable, but subsurface, “there may be conditions that are reminiscent of the deep subsurface on Earth in terms of energy flux available. These studies show that life is able to exist under such conditions.”

While Jorgensen stresses that he is not saying this means there is life on Mars, “It’s now really challenging to show where there is no life.”