Sea biology – shrinkage at the base of the food nets

Blue algae multiply particularly in warm water temperatures, with some types of toxic substances being able to secrete. Scientifically correctly mean the often unicellular tiny bacteria living in the water and there are thousands of different types of it. Despite the unpleasant properties of some of their representatives, they are a basis for life on earth due to their ability to photosynthesis and the fast formation of biomass. One of the most common cyanobacteria types in the sea is Prochlorococcus. It stands in the tropics and subtropics at the base of the marine food network and is estimated to be responsible for a fifth of global oxygen production.

A research team under the direction of François Ribalet has now found that there is a threshold for the heat -loving unicellular organism from which it apparently gets too warm. “For a long time, scientists assumed that Prochlorococcus would play a major role in the future, but in the warmest regions it is not so good, which means that there will be less carbon – less food – for the rest of the marine food network,” says the research professor for oceanography at the University of Washington about the results of the journal on Monday in nature »Nature» Microbiology «published. The research team had taken rehearsals in tropical and subtropical Pacific in three to eight meters for over ten years and examined the division rates of the Prochlorococcus contained therein. The surprising finding: the division rates sank abruptly when temperatures of over 28 degrees Celsius prevailed on the surface of the sea.

Based on this observation, the researchers modeled the population development of microorganism under various climate scenarios. In the case of persistently high greenhouse gas emissions, the inventory of unicellular organisms in the tropics could therefore decrease by 51 percent, and still 17 percent with a medium emission scenario.

Cyanobacteria represent a basis of life on earth.

Above all, these are bad news for the marine food networks in the tropics. There, the water is now so bright blue in places because it contains hardly anything other than Prochlorococcus, explains Ribalet. The researchers are not sure whether the cyanobacterium Synechococcus could take its place in a decline, which is more heat -tolerant, but also needs more nutrients. And if that were, whether it could replace Prochlorococcus in the food network. Prochlorococcus, on the other hand, would probably continue to hike further polar.

In addition to the question of whether another cyanobacterium can take on the role of Prochlorococcus in tropical sea ecosystems, it remains unclear whether it could not adapt to higher temperatures. His short genome speaks against this. “Prochlorococcus went through a genome lift and lost many stress response genes to minimize the resource requirement and at the same time optimize performance within a narrower environmental area,” the study said.

Bernhard Fuchs from the Max Planck Institute for Marine Microbiology, which was not involved in the study, also sees methodical weaknesses. This included the sampling and the measurement method used to the top layers of water. With the polarity spread of the cyanobacteria, there could be some surprises that may at least compensate for a good part of the negative effects, “said the microbiologist.

Miriam Seifert from the Alfred Wegener Institute, Helmholtz Center for Polar and Sea Research (AWI), which was also not involved in the study, would have no major influence on the oxygen concentration in the atmosphere, which was also not involved in the study. The Oceans’ CO2 recording capacity is also more influenced by physical factors. “The most important thing is therefore for projections of the future marine food network, since all organisms up to fish and marine mugs depend on primary production.”