Almost half of the ocean’s carbon fixation is done by eukaryotic phytoplankton, despite the fact that their presence is significantly less than the more abundant blue-green algae known as cyanobacteria.
Cyanobacteria, that grow in vast numbers in the sunlit surface waters of the oceans (the photic zone), 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. Until now they have been thought to dominate carbon fixation in the open ocean, with species belonging to the genera Prochlorococcus and Synechococcus being particularly abundant.
“The eukaryotic phytoplankton community has long been a ‘black box’ in terms of its composition as well as contribution to carbon fixation,” says professor Dave Scanlan of the University of Warwick.
“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 National Oceanography Centre in Southampton.
Details of the research are published in the April 15 issue of theJournal of the International Society for Microbial Ecology.
Using samples collected from surface waters, scientists measured carbon fixation by dominant phytoplankton groups in the subtropical and tropical northeast Atlantic Ocean. They discovered that eukaryotic phytoplankton actually fix significant amounts of carbon, contributing up to 44 percent of the total, despite being considerably less abundant than cyanobacteria.
“This is most likely because eukaryotic phytoplankton cells, although small, are bigger than cyanobacteria, allowing them to assimilate more fixed carbon,” explains Zubkov “This suggests that they play a key role in oceanic carbon fixation, but this needs to be confirmed by widespread sampling from the world’s oceans.”