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Invisible carbon pumps
A group of oceanic micro-organisms just might prove a surprising ally in the fight against climate change
UNDERSTANDING how the oceans absorb carbon dioxide is crucial to understanding the role of that gas in the climate. It is rather worrying, then, that something profound may be missing from that understanding. But if Jiao Nianzhi of Xiamen University in China is right, it is. For he suggests there is a lot of carbon floating in the oceans that has not previously been noticed. It is in the form of what is known as refractory dissolved organic matter and it has been put there by a hitherto little-regarded group of creatures called aerobic anoxygenic photoheterotrophic bacteria (AAPB). If Dr Jiao is right, a whole new “sink” for carbon dioxide from the atmosphere has been discovered.
The main way that carbon dioxide is absorbed by the ocean is through photosynthesis by planktonic algae. These algae are the basis of most food chains in the sea—being eaten by tiny animals that are, in turn, eaten by larger ones. When all these creatures die, their remains (those bits that are not immediately eaten, anyway) sink to the sea floor, where some are eaten and some are buried indefinitely. These remains are known in the jargon as particulate organic matter.
Some of the organic compounds the dead creatures contain, though, dissolve out of them and into the water. This dissolved organic matter was not, until recently, thought to be an important component of the total. But Dr Jiao noticed something odd about its distribution in the sea. It would be expected to correlate with the distribution of planktonic algae—the ultimate drivers of biological productivity. But it does not.
The reason, it turned out, was that previous researchers had been tracking only a small fraction of the total—the portion composed of molecules such as sugars and L-amino acids that can be metabolised easily by living things. They had missed a whole host of molecules that cannot easily be metabolised. These included D-amino acids, which are mirror images of the L-variety normally found in living things, and compounds called porins, lipopolysaccharides and humic acids. Because they are not metabolised, these molecules are referred to as “refractory”. Only when this refractory material is taken into account does the chemical map match the planktonic one.
