Study finds ocean acidification changes phytoplankton internal chemistry

A study published in Nature Geoscience has found that the carbon dioxide pumped into the atmosphere is changing the internal chemistry of phytoplankton, the microscopic photosynthetic organisms that form the basis of marine food webs and produce roughly half of the oxygen on Earth. A team led by the Yellow Sea Fisheries Research Institute and Harbin Institute of Technology in China grew a diatom species called Thalassiosira pseudonana in the laboratory. According to Phys.org, the researchers exposed the diatoms to high carbon dioxide and low phosphate to see how they would respond.

The team cultured the diatoms for 880 days, spanning more than 1,000 generations, and tested them in four setups: a control group mimicking current ocean conditions, an environment with low phosphate, an environment with high carbon dioxide, and a combined environment with both high carbon dioxide and low phosphate. The team monitored the cells over the course of the experiment, profiling shifts in growth, gene expression and internal chemistry. To test whether any changes were inherited or were temporary stress reactions, they moved diatoms that had evolved under high carbon dioxide back to normal carbon dioxide levels after 1,000 generations.

When carbon dioxide levels were high, diatoms grew faster and maintained higher rates of photosynthesis even when they were placed back under normal carbon dioxide levels, which the study authors said suggests the changes had become inherited rather than being temporary responses to their environment. Under low-phosphorus conditions, diatoms exposed to high carbon dioxide levels contained about 26% less organic phosphorus relative to carbon over time. That drop in phosphorus content reversed when carbon dioxide levels changed, meaning internal phosphorus allocation remains flexible rather than permanently fixed.

To manage this, cells drained their backup phosphorus stores and switched to phosphorus-free membrane lipids, allowing them to keep growing on a leaner phosphorus budget. "As a result, carbon accumulation becomes progressively decoupled from phosphorus storage," the scientists wrote.

The researchers also compared their results against wild plankton samples and experiments with natural coastal plankton communities, analyzed global gene-expression data, and used Earth system models to forecast global changes through 2100. Natural coastal plankton communities mirrored the laboratory results, particularly where phosphate was scarce. However, the researchers noted that "the direction of OA-linked [ocean acidification] changes in P metabolism is unlikely to be universal across taxa."

The findings suggest that as the oceans become more acidic, phytoplankton could keep growing while using less phosphorus, potentially changing the balance of nutrients across marine ecosystems. Phys.org reported that exactly what those changes will mean for marine life is uncertain.