Microbes deep beneath the ocean floor

A study revealed life in the plutonic rocks of the lower oceanic crust. Shown here is a thin section photomicrograph mosaic of one of the samples.

In a study published in a recent issue of “Nature”, scientists from the Woods Hole Oceanographic Institution (WHOI) describe how micro-organisms survive in rocks nestled thousands of feet beneath the ocean floor in the lower oceanic crust. The first analysis of messenger RNA, the genetic material containing instructions for making different proteins, from this region, coupled with measurements of enzyme activities, microscopy, cultures and biomarker analyses, has provided evidence of a low biomass but diverse community of microbes that includes heterotrophs that obtain their carbon from other living (or dead) organisms.

“Organisms eking out an existence far beneath the sea floor live in a hostile environment,” says Paraskevi (Vivian) Mara, a WHOI biochemist and one of the lead authors of the paper. Very little resources find their way into the seabed through seawater and subsurface fluids, which circulate through fractures in the rock and carry inorganic and organic compounds.

The researchers collected rock samples from the lower oceanic crust, spending over three months aboard the International Ocean Discovery Program Expedition 360. The research vessel travelled to Atlantis Bank, an underwater ridge that cuts across the southern Indian Ocean. Tectonic activity there exposes the lower oceanic crust at the sea floor, “providing convenient access to an otherwise largely inaccessible realm”, the authors write.

“We applied a completely new cocktail of methods to explore these precious samples,” says Virginia Edgcomb, a microbiologist at WHOI and the principal investigator of the project. By isolating messenger RNA and analysing the expression of genes, the researchers found evidence that micro-organisms under the ocean floor express genes for various survival strategies. Some microbes appeared to have the ability to store carbon in their cells for use in times of shortage. Others showed indications they could process nitrogen and sulphur to generate energy, produce Vitamin E and B12, recycle amino acids, and pluck out carbon from the hard-to-break-down compounds called polyaromatic hydrocarbons.

The findings provide a complete picture of carbon cycling by illuminating biological activity deep below the oceans.