New findings from the Bigelow Laboratory for Ocean Sciences have revealed that coccolithophores, a type of phytoplankton found globally, can survive in low-light conditions by utilizing dissolved organic forms of carbon. This discovery challenges previous assumptions about the mechanisms driving carbon cycling in the ocean. The study, published in Science Advances, provides the first evidence of osmotrophy (the ability to extract carbon from dissolved organic carbon) in coccolithophores in their natural environment.
Led by Senior Research Scientist William Balch, the team conducted experiments on coccolithophore populations in the northwest Atlantic Ocean. They measured the rate at which these phytoplankton species consumed three different organic compounds, each marked with chemical labels for tracking. The coccolithophores utilized these dissolved compounds as a carbon source for both their organic cellular components and the inorganic mineral plates, known as coccoliths, which they secrete around themselves. Although the uptake of organic compounds was slower compared to carbon acquisition through photosynthesis, it was not insignificant.
Balch explained, “The coccolithophores aren’t outpacing other organisms in terms of growth by taking up these dissolved organic materials. They are simply managing to survive and grow, albeit at a slow rate.”
Typically, plants, including coccolithophores, obtain carbon for growth through photosynthesis, utilizing inorganic forms of carbon such as carbon dioxide and bicarbonate extracted from the atmosphere. When coccolithophores die, they sink to the ocean floor, carrying the carbon with them. This process effectively sequesters the carbon for millions of years through remineralization or burial, constituting the biological carbon pump.
Furthermore, coccolithophores contribute to the alkalinity pump. They convert bicarbonate molecules in surface water into calcium carbonate, which forms their protective coccoliths. Upon the death and sinking of coccolithophores, the dense inorganic carbon in their coccoliths is transported to the seafloor. Some of it subsequently dissolves back into bicarbonate, facilitating the “pumping” of alkalinity from the surface to deeper ocean regions.
The latest evidence indicates that coccolithophores are not solely reliant on inorganic forms of carbon near the ocean’s surface. They also uptake dissolved organic carbon, which constitutes the largest pool of organic carbon in the sea. Remarkably, these phytoplankton species incorporate some of this organic carbon into their coccoliths, which eventually sink into the deep ocean. This discovery suggests that the uptake of free-floating organic compounds plays a role in both the biological and alkalinity pumps, which contribute to the transport of carbon from the ocean’s surface to its depths.
William Balch, the senior research scientist leading the team, emphasized the significance of this finding: “There’s this substantial source of dissolved organic carbon in the ocean that we previously assumed was not directly linked to the carbonate cycle. Now we’re recognizing that a portion of the carbon transported to deeper regions originates from this immense pool of dissolved organic carbon.”
This study is the final publication resulting from a three-year project. The inspiration for the research stemmed from the dissertation of William Blankley, a graduate student at Scripps Institution of Oceanography, who managed to sustain coccolithophores in the dark for 60 days by feeding them glycerol—an organic compound also utilized in the recent study. Unfortunately, Blankley passed away before his findings could be published. Balch noted that being able to reproduce Blankley’s work decades later with advanced technology speaks to the quality of his early research.
The recent study’s primary challenge was conducting the research outside of a controlled laboratory setting. The team had to develop a method for measuring these organic compounds in seawater, considering their ambient concentrations, which were orders of magnitude lower than in Blankley’s experiments. Subsequently, they tracked how these compounds were being taken up by wild coccolithophores.
Balch explained the difficulties encountered during the research: “When you culture phytoplankton in the lab, you can grow as much as you want. But in the ocean, you take what you get. The challenge was finding a clear signal amidst the noise, providing definitive proof that coccolithophores were indeed incorporating these organic molecules into their coccoliths.”
Although the current project has concluded, Balch mentioned that the next step is to investigate whether coccolithophores can uptake other organic compounds present in seawater at the same rate as the three compounds tested thus far. While the coccolithophores exhibited slow rates of uptake for the three dissolved compounds in these experiments, there are thousands of other organic molecules in seawater that they could potentially absorb. If they are capable of utilizing a broader range of organic compounds, this finding may have even greater implications for understanding the global carbon cycle.