Phytoplankton are microscopic plants that live in the ocean and produce oxygen through photosynthesis. They also play a vital role in the global carbon cycle, as they capture and transport carbon to the deep ocean. But how do they affect the climate? A new study reveals that the physiology of phytoplankton, especially their nutrient uptake, can influence the chemical composition of the ocean and the atmosphere.
The Redfield Ratio and Its Implications
In the 1930s, an American oceanographer named Alfred C. Redfield discovered that the concentrations of carbon, nitrogen, and phosphorus in phytoplankton and seawater follow a fixed ratio of approximately 106:16:1. This ratio, now known as the Redfield ratio, indicates a strong connection between the particulate and dissolved nutrient pools in the ocean. The question of whether the dissolved pool controls the ratio in phytoplankton, or vice versa, has long puzzled the marine science community.
“It’s a chicken-and-egg question,” says Dr Chia-Te Chien, a researcher at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, who is investigating the role of variable stoichiometry of phytoplankton in marine biogeochemistry. He and his colleagues have developed a new model that can simulate how phytoplankton adjust their nutrient uptake according to environmental conditions. The model shows that phytoplankton can deviate from the Redfield ratio depending on factors such as light, temperature, and nutrient availability.
The Feedbacks Between Phytoplankton and Climate
The variations in phytoplankton stoichiometry have important implications for the global climate. For example, when phytoplankton take up more nitrogen than phosphorus, they release excess phosphorus back to the water, which can stimulate more phytoplankton growth and carbon export. On the other hand, when phytoplankton take up more phosphorus than nitrogen, they release excess nitrogen as ammonium or nitrate, which can affect the nitrogen cycle and the production of nitrous oxide, a potent greenhouse gas.
The researchers also found that phytoplankton stoichiometry can respond to changes in atmospheric carbon dioxide concentration and temperature. Under higher carbon dioxide levels, phytoplankton tend to take up more carbon than nitrogen and phosphorus, which can enhance their role as a carbon sink. Under higher temperatures, phytoplankton tend to take up less carbon than nitrogen and phosphorus, which can reduce their carbon sequestration capacity.
The Implications for Future Ocean and Climate Research
The study provides new insights into how phytoplankton physiology can affect the ocean chemistry and climate. It also highlights the need for more observations and experiments to validate and improve the model. “Our model is a first step to explore the complex interactions between phytoplankton stoichiometry and climate,” says Dr Chien. “We hope that our study will stimulate more research on this topic and help us better understand and predict the future of our ocean and climate.”
