Climate change could turn polar phytoplankton into fast food

We are what we eat. In the ocean, much of life gets its food from phytoplankton, microscopic plant-like algae that drift through the sunlit upper layers of the sea. These organisms are the main food source for krill, marine snails, some small fish and jellyfish. Those animals, in turn, feed larger marine species that become prey for the ocean’s top predators, including humans.

A study by MIT scientists, published in Nature Climate Change, finds that warming oceans could significantly alter the nutritional foundation of that food web. As sea-surface temperatures rise over the next century, phytoplankton in polar regions may become lower in protein, higher in carbohydrates and, overall, lower in nutrients.

Shlomit Sharoni, the study’s lead author and an MIT postdoctoral researcher, describes the trend in striking terms: "We are moving toward a sort of fast-food ocean in the poles." Based on the projection, she says, the nutritional composition of the ocean surface will be very different by the end of the century.

A shift of about 20%

The team modeled a scenario in which humans continue emitting greenhouse gases through 2100. Under those conditions, changing ocean conditions, particularly in polar regions, would alter the balance of phytoplankton proteins, carbohydrates and lipids by approximately 20%.

The model combines laboratory experiments with an established ocean circulation and dynamics model previously developed at MIT. It simulates phytoplankton composition under different combinations of temperature, circulation, light, nutrients and sea-ice cover across the world’s oceans and under different climate scenarios.

When the researchers modeled present-day conditions, the results indicated that slightly more than half of an average phytoplankton cell is made of protein. The remainder is a mixture of carbohydrates and lipids. In polar regions, phytoplankton is slightly richer in protein, possibly because sea ice limits available sunlight and favors the production of more light-harvesting proteins.

The study’s figures show modeled, depth-integrated preindustrial mean cellular allocation, measured in mol C per mol C, for proteins and combined carbohydrates plus lipids. They also show cell energy content, in kcal per gram of carbon, and the C:N org, C:P org and N:P org elemental ratios in organic matter comprising phytoplankton, herbivores and POM. White areas indicate regions where sea-ice cover exceeds 50%.

Less ice, more carbohydrates and lipids

In the future climate scenario, ocean-surface temperature would rise by 3 degrees Celsius, while sea-ice cover would decline substantially. Warmer temperatures would also weaken ocean circulation and reduce the amount of nutrients that can move upward from the deep ocean.

With less ice, polar phytoplankton would receive more sunlight and would not need as much protein for light capture. The model projects that polar phytoplankton populations will increase significantly, consistent with previous studies, but that their composition will shift from protein-rich to carbohydrate- and lipid-rich.

Total protein levels in polar phytoplankton would fall by as much as 30%, with a corresponding increase in carbohydrates and lipids. The simulated trends compare 2070-2100 with 1870-1900 and include depth-integrated cellular allocation for total proteins and for combined carbohydrates and lipids.

The researchers also modeled photosynthetic and biosynthetic protein allocation, along with surface biomass from 0 to 115 meters, expressed in mmol C per cubic meter. A separate projection covers depth-integrated phytoplankton cell energy content, in kcal per gram of carbon, as well as the C:N org, C:P org and N:P org ratios in organic matter containing phytoplankton, herbivores and POM.

A different pattern in subtropical waters

The response would not be the same everywhere. In subtropical regions at higher latitudes, phytoplankton populations are expected to decline by 50%. As the ocean warms, weaker circulation would limit the supply of nutrients rising from depth, potentially forcing phytoplankton to live deeper to balance access to sunlight and nutrients.

Under those conditions, subtropical phytoplankton could become slightly richer in protein. It may rely on the same photosynthetic proteins that polar organisms will need less of as sea ice retreats. Across the world as a whole, the average phytoplankton composition would shift toward more carbohydrates and lower nutrient content.

Phytoplankton are photosynthetic and depend on sunlight, atmospheric carbon dioxide and nutrients such as nitrogen and iron that rise from the deep ocean. They generate half of the oxygen in our atmosphere and play a fundamental role in exporting and storing carbon in the deep ocean. The new study also helps project changes in the Redfield ratio.

Macromolecules at the base of the food web

Macromolecules are large molecules essential to life. The major types include proteins, lipids, carbohydrates and nucleic acids, the building blocks of DNA and RNA. Every form of life, including phytoplankton, contains a balance of macromolecules that helps it survive in its particular environment.

"Almost all the material in a living organism is in these broad molecular forms, each with a specific physiological function depending on the circumstances in which the organism finds itself," says Michael J. Follows, a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. Sharoni says there is growing awareness that phytoplankton’s nutritional value may change with climate change, but very little work has directly addressed the question.

The researchers first examined how present-day ocean conditions influence phytoplankton macromolecular composition. They used data from laboratory experiments conducted by collaborators at Dalhousie University. Those experiments showed how the balance of phytoplankton macromolecules, including proteins and carbohydrates, changes with water temperature and the availability of light and nutrients.

Using those laboratory data, the group developed a quantitative model that simulates how laboratory plankton would adjust its protein and carbohydrate balance under different light and nutrient conditions. Sharoni and Keisuke Inomura then combined the new model with MIT’s established ocean circulation and dynamics model.

The MIT coauthors are Mick Follows, Stephanie Dutkiewicz and Oliver Jahn. Other coauthors are Keisuke Inomura of the University of Rhode Island; Zoe Finkel, Andrew Irwin and Mohammad Amirian of Dalhousie University in Halifax, Canada; and Erwan Monier of the University of California, Davis.

Evidence from the Arctic and Antarctic

The team compared its projections with a small set of real phytoplankton samples previously collected by other scientists in the Arctic and Antarctic. The Arctic data included 307 protein observations and 302 observations of combined carbohydrates and lipids. The sources were Irwin et al., 1980, 1984, 1983a and 1983b; Kim et al., 2015 and 2020; Yun et al., 2015; Ahn et al., 2019; and Choe et al., 2021.

The Antarctic data included 85 protein observations and 93 observations of combined carbohydrates and lipids. Those data came from Handa and Tanoue, 1983; Fabiano et al., 1993 and 1996; Fabiano and Pusceddu, 1998; Pusceddu et al., 1999; Kim et al., 2016 and 2018; Misic et al., 2024; and Jo et al., 2021.

Box plots for the Arctic and Antarctic data show the median, or 50th percentile, as the central line. The box limits represent the 25th and 75th percentiles, while the whiskers show the 5th and 95th percentiles. Only data obtained with colorimetric methods were included.

The samples show that phytoplankton became richer in carbohydrates and lipids over recent decades, matching what the model projected under a warming climate scenario. "In these regions, you can already see climate change happening, because sea ice is already melting," Sharoni explains. "And our model shows that proteins in polar plankton are decreasing, while carbohydrates and lipids are increasing."

Follows adds that climate change is accelerating in the Arctic and that data already show a response in phytoplankton composition. The consequences for the rest of the marine food web remain unclear. Some organisms may suffer from reduced protein, while others that store lipids to survive the winter may benefit.

The calorie content at the base of the marine food web is already changing. The unresolved question is how that shift will move through the rest of the food web.

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Reporting: Anne Silva / Amazonia Mag