ABOVE: Amorphous carbon (black) in cultures of anaerobic methanotrophs (amber) GUNTER WEGENER

An international team of scientists claims to have identified two deep-sea microbe groups that produce amorphous carbon, a kind of elemental carbon. “This is the first report of amorphous carbon being produced by any organism on Earth,” says Virginia Tech biochemist and paper coauthor Robert White in a press release, adding that the team is “very interested in the possible implications it may have for the carbon cycle.”

“We never thought that amorphous carbon could be produced by living organisms because of the normally extreme chemical reactions that are needed to form it,” he adds.

Elemental carbon contains only carbon atoms, in contrast to molecules associated with life, in which carbon is linked to other elements such as hydrogen and oxygen. It exists in many different forms, from crystalline diamonds formed under high pressure and temperature to the black, powdery amorphous carbon in soot. In all previously known cases, its synthesis is abiotic, reports Chemistry World. But a paper published on October 27 in Science Advances describes elemental carbon that was produced by microbes cultured from deep sea sediments. 

The microbes were initially collected between 2003 and 2009 and cultured for several years to gain a deeper understanding of the biochemical processes of organisms living in the depths of the sea. Among the organisms studied were members of two microbial groups: methanogens, which produce methane instead of carbon dioxide during respiration, and anaerobic methanotrophs, which consume methane as a carbon source. Both thrive in low oxygen environments such as hydrothermal vents in the Gulf of California and deep-sea mud volcanoes in the Mediterranean Sea where the researchers sourced their study samples.

When they cultured the microbes, they noticed the organisms produced black specks, which they initially thought might be metal deposits. But Raman spectroscopy revealed the material to be pure carbon, a finding that follow-up analyses confirmed.

Lead author and Virginia Tech biochemist Kylie Allen notes in the university’s announcement that it’s unclear why the organisms are making amorphous carbon. They could be synthesizing it directly for some purpose or as a byproduct of a vital process; “we don’t know which one it is yet. This is something that we really want to figure out in the future,” she says.

The authors of the paper suggest that amorphous carbon could play a role in protecting the microbes against toxins or serve as a conductive element for electron transfer. “Elemental carbon is a good electrical conductor. Carbon may be the key to the symbiosis between the archaea and their partners," hypothesizes coauthor and marine microbiologist Gunter Wegener in a press release by the University of Bremen.

See “The Hunt for Electrically Active Microbes” 

“It is also completely unclear how much elemental carbon is formed by microorganisms in nature and where it resides," he notes in the Virginia Tech release.

Answering such questions about the nature and volume of carbon production could help illuminate whether these organisms have a marked impact on the global carbon cycle. It is possible, given amorphous carbon’s relatively unreactive nature and these microbes’ location at the bottom of the sea, that much of the carbon they produce stays deep in the ocean, thus acting as a carbon sink. But, Allen stresses, any conclusions of that nature are still a long way off.