The cold polar oceans give rise to some of the largest food webs on Earth. And at their base are microscopic and photosynthetic algae. But man-made climate change, These large communities of cold-water algae are being replaced by communities of heat-adapted algae, a trend that threatens to destabilize the delicate marine food web and change the oceans as we know them, according to a new study.
At the base of marine food webs are photosynthetic microscopic organisms called phytoplankton (from the Greek phyto for “plant” and plankton for “wandering”). But they vary across the world ocean. Phytoplankton communities in warmer waters, including the tropics, tend to be dominated by prokaryotes (microorganisms without a defined nucleus).
Cooler waters closer to the poles, however, tend to favor eukaryotes (microorganisms with a core). These photosynthetic eukaryotes, or algae, form the basis of productive food webs in cold but fertile polar waters.
“Much of our food comes from the fisheries of the North Atlantic, North Pacific and South Pacific, because of eukaryotic phytoplankton – not prokaryotes,” said Thomas Mock, marine microbiologist at the University of East Anglia. (UEA, UK) and lead author of the study. “Prokaryotes are not able to produce all of the juicy proteins and lipids that eukaryotes are.”
But according to a new study published in Nature Communication, warmer waters and prokaryotic-dominated communities could replace those of eukaryotes much more easily than previously thought.
“This would have significant consequences for the entire food chain, and therefore for the ecosystem services we all depend on,” Mock said.
Mock and the other lead scientists had embarked on the study – a collaboration of eight institutions led by the UEA and including the Joint Genome Institute (JGI) of the United States Department of Energy (DOE), a user facility. from the DOE Office of Science located at the Lawrence Berkeley National Laboratory – with the desire to understand the nuance and gradation of evolution of eukaryotic phytoplankton communities with latitude.
An invisible border
The team embarked on a Lewis and Clark-style expedition to explore, collect and catalog samples, and search for patterns in algal communities, including the microbiomes associated with algae that influence algal diversity and l gene expression. Navigating from pole to pole on four research cruises, they dipped their self-closing containers in seawater to sample communities of algae along transects in the Arctic Ocean, North Atlantic Ocean, l the South Atlantic Ocean and the Southern Ocean.
After isolating the algal communities on filters, they sequenced DNA “marker” gene sequences to identify the microbes. And in order to determine which genes the algae were expressing, the team sequenced their RNA transcripts. All sequencing was carried out under the JGI community science program.
Using an ecological metric called beta diversity, the team observed that algae communities were not changing gradually across the global ocean. Instead, they sharply demarcated into two major geographic groups: those in colder polar waters and those in warmer non-polar waters.
In other words, some like it hot; some don’t.
“We can think of the ocean, naively, as a sort of homogeneous environment. In reality, this is not the case – there is variation in nutrients, temperatures and other physicochemical properties, ”said study co-author Igor Grigoriev, head of the JGI Fungal & program. Algal. “But still, there are no borders in the ocean. Yet what has been found here is that there is this invisible partition of algae communities.
The team found that the limit, or “breaking point” of biodiversity between these algal communities occurs in moderate waters that have an average surface temperature of around 58 degrees Fahrenheit – a cool intermediate to extremes. of the ocean about 28 and 97 degrees Fahrenheit.
“The authors of the study highlight this fundamental observation of cold and hot microbial networks, and how clear and sharp the biogeographic boundary is between them. The data is somewhat good in that regard, ”said Andy Allen, a biological oceanographer at the University of California at San Diego and the Scripps Institution of Oceanography, which was not affiliated with the study.
“But the results also suggest a certain level of vulnerability that we may not have been aware of,” he added. “If the system is disrupted it could be very difficult to get back to the baseline. “
Climate change is indeed seriously affecting sea ice and water temperature in polar climates, putting these polar communities at risk.
“We know so little about these algae communities; they might have beneficial results, like antibiotics, pharmaceuticals, and new enzymes that work at low temperatures. said Katrin Schmidt, co-lead author of the study with Kara Martin. “But these ecosystems are literally melting. “
Driven by climate change
The team used a model from the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) to predict where and how fast the 14 degree Celsius limit is moving.
“It’s determined by the climate: hot water replaces cold water communities. And that changes everything, ”Mock said.
The steady advance of warmer waters towards the poles could have disastrous consequences for marine organisms in these food webs, Schmidt said. Several species of whales, including gray whales and humpback whales, migrate for food in the polar regions. And the shrimp feed on the algae that clings to the bottom of the pack ice.
Krill is a major algae eater that could be affected by warming waters and changing algae communities, an organism that thrives in the Southern Ocean, resembles shrimp and serves as food for larger organisms such as whales, penguins and seals. “The biomass of krill is at least equal to the biomass of all humans on the planet,” Mock said. “It gives you an idea of the importance of these organizations. And now imagine that the base of the ecosystem changes from cold water, eukaryotic phytoplankton communities, to warm water, prokaryotic phytoplankton communities. ”
A change in the base would reverberate throughout the food web, like bringing a jackhammer to the foundation of a cathedral. Additionally, because phytoplankton (eukaryotic and prokaryotic combined) contribute about 50 percent of the world’s fixed carbon, altering the balance of eukaryotic and prokaryotic communities could alter the global carbon cycle, the rates at which carbon is fixed and metabolized globally.
What’s more, these changes – brought on by climate change – could threaten marine food industries and other ecosystem services, such as tourism and recreation, on which coastal and island nations, like the United Kingdom, depend, Mock said. .
“I think this document is going to be used to advise policymakers to mitigate the effects of climate change on ecosystems, because we now have a new perspective on the impact of warming on these marine communities,” Mock said. The greenhouse gas carbon dioxide (CO2), produced from the combustion of fossil fuels, is responsible for the increase in the surface temperature of the oceans. “What you have to do is reduce the production of CO2 – this is the first and most important thing that we have to do.