Source: AGU Advances
Single-celled algae in the ocean known as coccolithophores play an important role in the marine carbon cycle when they take up bicarbonate from seawater to build their shells. Coccolithophore numbers have been increasing globally in recent years, meaning their influence is growing, even as scientists still don’t fully understand the factors driving their explosive growth. One explanation could be changes to the alkalinity of ocean water, specifically, greater amounts of bicarbonate available for the tiny creatures to use.
For more information on how coccolithophores grow and flourish, Zhang et al. looked to the last time the phytoplankton surged in number, between 300,000 and 500,000 years ago. Using fossilized coccolithophore morphology and examining carbon isotope ratios, the authors constructed models that allowed them to pick apart the ingredients for coccolithophore success.
Comparisons of inorganic to organic carbon ratios in the shells, as well as comparisons of photosynthesis and calcification rates revealed by carbon isotope ratios, showed a large increase in calcification linked to greater bicarbonate uptake. Though increasing alkalinity was likely a factor in the coccolithophores’ increased growth, it doesn’t explain all of it, the authors say. Instead, greater nutrient availability allowed coccolithophore populations to swell, both by giving them more food to use and by allowing them to move to shallower depths where there was more sunlight for photosynthesis.
The findings have implications for the present day, as we see marine phytoplankton numbers shifting alongside changes in ocean chemistry. Previous works focused on the change in seawater alkalinity and pH. But more information on how nutrient availability influences coccolithophore growth is needed, the authors conclude, especially in light of proposed geoengineering schemes that could shift the types of nutrients available. (AGU Advances, https://doi.org/10.1029/2024AV001609, 2025)
—Nathaniel Scharping (@nathanielscharp), Science Writer
