Experts reconsider the processes of the carbon cycle in the ocean

Madrid. A study with coccolithophores, a common type of plankton, forces scientists to reconsider the processes that drive carbon cycling in the ocean.

It was shown that they can survive in low light conditions using organic forms of carbon. This is the first natural evidence of the osmotrophy of coccolithophores, which play an essential role in cycling carbon between the ocean and the atmosphere.

The results of this research from the Bigelow Laboratory of Ocean Sciences were published in the journal Science Advances.

The ability to extract carbon from the direct absorption of dissolved organic carbon is known as osmotrophy.

Although scientists have already observed this behavior of coccolithophores in laboratory cultures, this is the first evidence of this phenomenon in nature.

The team, led by William Balch, conducted their experiments on populations of coccolithophores in the northwestern Atlantic Ocean.

He measured the rate at which the phytoplankton fed on three different organic compounds, each tagged with chemical markers for tracking.

The dissolved compounds were used by the coccolithophores as a carbon source both for the organic tissues that make up their cells and for the inorganic mineral plates, called coccoliths, that they secrete around them.

The uptake of organic compounds was slow compared to the rate at which phytoplankton can absorb carbon through photosynthesis, but it was not negligible.

“Coccolithophores are not winning any ‘growth race’ by taking up these dissolved organic materials,” Balch said. They are just making a living, but they can still grow, albeit slowly.”

Plants, like coccolithophores, typically acquire the carbon necessary for their growth from inorganic forms of carbon removed from the atmosphere, such as carbon dioxide and bicarbonate, through photosynthesis.

When coccolithophores die, they sink and take all that carbon to the bottom of the ocean, where it can be remineralized or buried, sequestering it for millions of years. This process is called a biological carbon pump.

As part of a parallel process called the alkalinity pump, coccolithophores also convert bicarbonate molecules in surface water to calcium carbonate—essentially limestone—which forms their protective coccoliths. Once again, when they die and sink, all that dense inorganic carbon is deposited on the seafloor. Some of it dissolves back into bicarbonate, pumping thus the alkalinity from the surface to the depths.

biological pumps

But the new evidence suggests that coccolithophores don’t just use these inorganic forms of carbon near the surface. They also absorb dissolved organic carbon, the largest reserve of carbon in the sea, and fix part of it in their coccoliths, which eventually sink to the ocean depths.

This suggests that the uptake of these floating organic compounds is another step in the alkalinity and biological pumps that drive the transport of carbon from the ocean surface to the depths.

“There is a large source of dissolved organic carbon in the ocean that we have always assumed was not related to the carbonate cycle in the sea,” Balch added. Now we say that some fraction of the carbon that goes to the depths comes from that huge pool of dissolved organic carbon.”

This is the third and final article published in a three-year project funded by the National Science Foundation.

The global effort was inspired by a decades-old thesis by William Blankley, a graduate student at Scripps Institution of Oceanography in San Diego, California, alma mater de Balch.

In the 1960s, Blankley managed to grow coccolithophores in the dark for 60 days by feeding them glycerol, one of the organic compounds used in the present study. He died before his research could be published.

According to Balch, the fact that Blankley’s findings could be reproduced all these years later with new technology is a credit to the quality of the early work. The real challenge of the most recent study, however, was carrying it out outside of a controlled laboratory setting.