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.”
Another good story during the week on Catalyst was a segment on another tiny ocean dweller, the pteropods.
http://www.abc.net.au/catalyst/stories/2886137.htm
Excerpts:
“Dr Donna Roberts
There are millions and billions of tonnes of pteropods in the Southern Ocean and in some regions, particularly the Ross Sea area, there is more pteropods per cubic metre than krill”
NARRATION
When CO2 dissolves, it reacts with water to form carbonic acid and carbonate ions, which decreases the pH. Aragonite is more soluble, so any change in pH will affect the growth of aragonite shells first.
Dr Donna Roberts
Now, aragonite is actually in the shells of pteropods and in corals. So corals and pteropods are going to be at risk first if the ocean health and the ocean chemistry changes and it’s changing because we’re putting carbon dioxide into the water.
NARRATION
That means the pteropods of the Southern Ocean, and the food chains they underpin, may be living on borrowed time. Donna can calculate the CO2 levels when they’ll basically run out of shell.
Dr Donna Roberts
The tipping point for pteropods in the southern ocean is four hundred and fifty parts per million and we are currently at three hundred and eighty eight so we’re heading there rapidly and we, we think from models that we’re going to get there in about the winter of 2030.
Also from the same episode, Antarctic Glaciers:
http://www.abc.net.au/catalyst/stories/2886106.htm
Interviews with various experts:
http://www.abc.net.au/catalyst/antarctica/