‘Black box’ plankton and carbon fixation in the open ocean

National Oceanography Centre (UK): Carbon fixation by phytoplankton in the open ocean plays a key role in the global carbon cycle but remains to be fully understood. Scientists at the University of Warwick (UoW) and the National Oceanography Centre (NOC) in Southampton have now identified the dominant smallest algae and determined their contributions to carbon fixation in the subtropical and tropical northeast Atlantic Ocean.

Blue-green algae, or cyanobacteria, grow in vast numbers in the sunlit surface waters of the oceans, the photic zone. They use sunlight to ‘fix’ carbon by converting carbon dioxide into sugars and other organic compounds through photosynthesis.

Cyanobacteria belong to the ‘picophytoplankton’, the tiniest phytoplankton. They are considered to dominate carbon fixation in the open ocean, with species belonging to the genera Prochlorococcus and Synechococcus being particularly abundant.

Like all bacteria, cyanobacteria are prokaryotes, distinguished from eukaryotes by the absence of a cell nucleus. However, although much less abundant than cyanobacteria, the photic zone also has a high biomass of small eukaryotic phytoplankton capable of carbon fixation.

“The eukaryotic phytoplankton community has been a ‘black box’ in terms of its composition as well as contribution to carbon fixation,” says Professor Dave Scanlan of UoW; “Determining how much carbon different groups fix into biomass is required for a full understanding of the Earth’s carbon cycle,” adds Professor Mikhail Zubkov of the NOC…..

…Zubkov recently showed that small eukaryotic phytoplankton can obtain carbon by feeding on bacteria, supplementing carbon fixed through photosynthesis. It is likely that some of the organic carbon of prymnesiophytes and other eukaryotic phytoplankton is eventually exported from the photic zone to the deep ocean, rather than being returned to the atmosphere in the form of carbon dioxide….

Image of plankton from the NOC website