Climate Change Alters Phytoplankton’s Nutrient Composition

Phytoplankton are microscopic algae that float in the upper layers of the ocean. They obtain energy through photosynthesis, similar to land-based plants, and require sunlight, thriving in well-lit areas of oceans and lakes.

The term “phytoplankton” comes from ancient Greek: “phuton,” meaning “plant,” and “planktos,” meaning “drifter,” “roamer,” or “wanderer,” reflecting their buoyant, drifting nature in the water.

Phytoplankton rank among the most important organisms on Earth due to their many crucial ecosystem and environmental services. They are at the base of the food chain and the primary food source for zooplankton like krill, jellyfish, and snails, which are, in turn, consumed by larger animals such as fish, whales, squid, shellfish, and birds.

Besides providing food for the rest of the food web, phytoplankton play a key role in the global carbon cycle. They release oxygen as a byproduct, similar to terrestrial plants, and also absorb carbon and cycle key nutrients, thereby maintaining the health and stability of marine ecosystems and food webs while regulating Earth’s climate through carbon sequestration.

Phytoplankton’s growth, composition, and biomass act as indicators of water quality; however, these microscopic organisms are also affected by excess nutrients (eutrophication), ocean acidification, and warming sea temperatures.

The open study, “Biochemical remodelling of phytoplankton cell composition under climate change” published in Nature Climate Change, led by MIT scientists, is investigating how climate change, particularly rising ocean temperatures, will affect phytoplankton composition.

The scientists investigated how ocean conditions will affect the macromolecular composition of phytoplankton. Like all living organisms, phytoplankton are composed of a balance of macromolecules or large molecules, which include proteins, lipids, carbohydrates, and nucleic acids – the building blocks of DNA and RNA.

The study reveals that the macromolecular composition of phytoplankton shifts from protein-rich to carbohydrate-rich as water temperatures rise. The researchers simulated a scenario in which greenhouse gas emissions continue through the end of the century, leading to a 3°C increase in sea surface temperatures and a decline in sea ice coverage.

Additionally, warming ocean waters diminish ocean circulation and reduce the upwelling of cold, nutrient-rich water from the deep ocean to the surface.

Their experiment shows how environmental conditions influence the large molecules that make up phytoplankton. At present, phytoplankton in high latitude regions with cold, nutrient-rich, low-light regions contain more protein, while those in warm, nutrient-poor areas have more carbohydrates and fats.

Researchers predict that, under a high-carbon-emissions scenario, phytoplankton in high-latitude regions will produce more carbohydrates and fats but less protein by 2100. These changes are already being observed in Arctic phytoplankton, where the environment is rapidly evolving.

Alterations in phytoplankton composition could impact the availability of nutrients for marine animals and, eventually, for humans who rely on them.

The study underscores the importance of continuing to monitor phytoplankton composition, given its critical role in the marine environment and its influence on the climate.