Climate change will alter functioning of marine microbial communities, study shows

Plankton are the foundation of life in the oceans and produce half the world’s oxygen. These marine microorganisms will suffer from rising temperatures at the poles, rising salinity in the tropics, and a reduction in the flow of nutrients in the temperate zone, according to simulations based on data collected from all oceans around the globe.

By André Julião  |  Agência FAPESP – A study by an international group of researchers shows that interaction between communities of plankton – microorganisms that live at the bottom of the food chain in the oceans and supply most of the planet’s oxygen – will be affected by climate change in different ways depending on location.

Computer simulations suggested that plankton communities at the poles will be particularly badly damaged by the rise in temperature, while in temperate zones they will suffer from a reduced flow of nutrients and in the tropics from increased salinity.

The findings, published in the journal Science Advances, resulted from mathematical modeling based on the largest-ever inventory of marine plankton, carried out by Tara Ocean expeditions between 2009 and 2013. The Tara is a research schooner that sailed around the world collecting samples of marine plankton from every ocean during the period.

The first results of the expedition were published in 2015 in a special number of the journal Science (more at: agencia.fapesp.br/21347/). 

“That publication provided a snapshot of all the microorganisms in the oceans – the species, and the abundance of each one. It was one of the largest genetic sequencing projects executed to date,” said Hugo Sarmento, a professor in the Hydrobiology Department of the Federal University of São Carlos (UFSCar), in the state of São Paulo, Brazil, and one of two authors affiliated with Brazilian institutions who participated in the latest study, with FAPESP’s support. 

“However, when we analyzed the data, we realized that these organisms depend on each other to survive and form complex microbial assemblages. They interact in many more ways than we imagined, and climate change will have a significant impact on them.”

Sarmento joined the expedition in 2009. The Tara returned to Brazil on September 19, 2021, stopping first at Belém, for a new project called Mission Microbiomes, part of the AtlantECO project funded by the European Union. Focusing on the South Atlantic, it has researchers on board from 13 European countries, Brazil and South Africa. The laboratory schooner will travel 70,000 km along the South American and African coasts as far as Antarctica. 

After collecting samples in the ocean off North Brazil, it is scheduled to dock at Salvador in October, and at Rio de Janeiro and Florianópolis in November, after which it will sail to Antarctica and the African coast. On returning to its homeport in France, which it left in December 2020, it will have been at sea for two years.

In Brazil, the project also involves scientific expeditions led by the research vessel Alpha Crucis operated by the University of São Paulo’s Oceanographic Institute (IO-USP), and the research vessel Veleiro ECO developed by the Federal University of Santa Catarina (UFSC). The other Brazilian institutions involved are UFSCar, the Federal University of Bahia (UFBA), and the Federal University of Rio Grande (FURG).

Microbial networks

Using statistical tools to analyze data from the 2009-13 circumnavigation, the researchers reconstructed the networks of interactions among the microorganisms. They identified 20,810 nodes corresponding to operational taxonomic units (OTUs, mainly genera and species) and 86,026 edges corresponding to potential interactions, which may be positive (symbiosis or mutualism, for example) or negative (e.g. predation or parasitism).

“If we identified two species that always appeared together, there could be a positive interaction, with one depending on the other. If one increases every time the other decreases, there may be a negative interaction, with one feeding on the other, for example,” Sarmento said. “We compared the more than 20,000 species pair by pair, and obtained this number of potential interactions, where each species or OTU is a node in a complex network.”

The number of ubiquitous species, which occur worldwide, were a minority. The majority were more or less abundant depending on latitude, forming distinct networks at the poles, in temperate regions, and the tropics.

Based on this information, in conjunction with environmental factors such as temperature, salinity, and availability of nutrients, the researchers simulated the effects of climate change on each of these communities.

It is known, for example, that most species can survive only in a certain temperature range. With the global average temperature set to rise more than 3 °C by the end of the century, some species may cease to exist in their present homes, and the functioning of communities that now include them will change profoundly in the future.

“We ran the simulations for several stressors,” Sarmento said. “In the temperate zone, nutrient regime changes appeared to be most important, whereas in the tropics the main stressors for plankton networks were temperature, albeit less so than at the poles, and salinity.”

At the poles, temperature is an even more critical factor. “Given that the largest increases will occur precisely in polar regions, we can foresee major changes in the functioning of these communities, with important consequences for the equilibrium of the system,” he said.

The researchers warn that these changes may result in lower oxygen production since marine microorganisms account for about half the oxygen produced on Earth. They may also affect the oceans’ capacity to capture and store atmospheric carbon. Currently, they absorb about a quarter of the greenhouse gases emitted by human activities, such as fossil fuel burning. Changes in plankton activity could make the situation worse.

The changes could also affect plankton biomass, the foundation for the marine food chain. As a result, there could be changes in the distribution and quantity of fish stocks.

The other Brazilian co-author of the study was Pedro Ciarlini Junger Soares, a PhD candidate at UFSCar supervised by Sarmento and currently working as a research intern at Spain’s Institute of Marine Sciences (ICM), in both cases with FAPESP’s support.

The article “Environmental vulnerability of the global ocean epipelagic plankton community interactome” is at: www.science.org/doi/10.1126/sciadv.abg1921.