Depending on the temperature of the sea water, the plankton in the oceans is composed of different species. A new study now suggests that the unicellular algae in the polar seas could be displaced by bacteria when the water temperature rises. This would collapse the base of the food web in the polar seas. Since the algae also bind a lot of carbon dioxide from the atmosphere, their disappearance could further exacerbate climate change.
The phytoplankton forms the basis for large food webs in the oceans. The tiny algae and cyanobacteria carry out photosynthesis and, with the help of sunlight and carbon dioxide bound from the atmosphere, form organic material that forms the basis of food for marine animals – from tiny krill to whales. The phytoplankton communities in the oceans differ depending on the latitude. The main reason for this are differences in the water temperature and the associated nutrient content of the upper water layers.
Life communities from pole to pole
A team led by Kara Martin from the University of East Anglia in Norwich has now recorded the phytoplankton communities of the world’s oceans for the first time in a comprehensive research project from pole to pole. As part of four research expeditions, Martin and her colleagues took samples from the Arctic Ocean, the North and South Atlantic and the Southern Ocean at different geographical latitudes. They analyzed the genetic material contained in the samples as well as the proteins produced by the phytoplankton and were able to use this to draw conclusions about which species were present in which proportions at the respective locations.
The phytoplankton communities in all oceans are based on an interaction between different groups of unicellular organisms: prokaryotes, i.e. microorganisms without a nucleus such as cyanobacteria; and eukaryotes, living things with a cell nucleus, which include algae.
Invisible division
The study now shows: How these communities are composed does not gradually shift with latitude. Instead, they split into two broad geographic groups – those in colder, polar waters and those in warmer, non-polar waters. “Although oceans appear like a homogeneous medium, there are differences in nutrients, temperatures and other physico-chemical properties,” explains co-author Igor Gregoriev of the Lawrence Berkeley National Laboratory in California. “Even if there are no boundaries in the ocean, we have found that there is an invisible division of the phytoplankton communities.”
Eukaryotes predominate in the colder waters near the poles, while prokaryotes dominate in warmer waters. According to the researchers, the change in species composition is defined by the temperature of the surface water. Up to an average surface temperature of 14 degrees Celsius, polar communities predominate, which are rich in eukaryotes, while in warmer waters non-polar communities with a higher proportion of prokaryotes.
Far-reaching consequences for food webs
Using a model from the fifth IPCC climate report, the authors modeled how this temperature limit in the oceans will shift in the course of the coming decades in view of climate change. Accordingly, with increasing global warming, the polar phytoplankton communities are being pushed further and further towards the poles and replaced by microbes that are adapted to warmer water temperatures. According to the analysis, areas between the 40th and 60th degrees north latitude, i.e. between Rome and Oslo, would be particularly affected.
“That would have a significant impact on the entire food web and thus on the ecosystem services on which we all depend,” says Martin’s colleague Thomas Mock. Animals such as whales, seals and also many commercially used fish would lose their food source. “An important part of our food comes from fisheries in the North Atlantic, North Pacific and South Pacific, thanks to the eukaryotic phytoplankton – not the prokaryotes. Prokaryotes are unable to produce all of the proteins and lipids that eukaryotes produce. “
Effects on the climate
This change in the composition of phytoplankton could also have a significant impact on the global climate. “Eukaryotic phytoplankton is responsible for at least 20 percent of the annual global carbon fixation,” the authors write. If it is increasingly displaced, the global carbon cycle could change. “I think this study can serve to make policymakers aware of the need to mitigate the effects of climate change on ecosystems,” says Mock. “We now have a fresh look at how warming affects marine communities, and therefore humans too. The most important thing we have to do now is to reduce CO2 emissions. “
Source: Kara Martin (University of East Anglia, Norwich, UK) et al., Nature Communications, doi: 10.1038 / s41467-021-25646-9