They break down gigantic amounts of biomass and thus shape the carbon cycle in the oceans: Researchers have now found something amazing about the bacteria in seawater. Not the most common species, but the rare species break down a particularly large amount of organic material and release carbon dioxide in the process. Less than three percent of the bacteria in the oceans consume a third of all the oxygen. The findings shed new light on the mechanisms of the marine carbon cycle, which is of central importance for life on earth and climate change, say the scientists.
From blue whales to crabs - when it comes to marine life, the spotlight is usually on the enormous diversity of animals. But the diversity of an “inconspicuous world power” is much greater: there are hundreds of thousands of different types of bacteria in one liter of ocean water. These are tiny creatures of enormous importance: the microbes metabolize various organic substances, which are ultimately due to the build-up of biomass by photosynthetic organisms. Most of these bacteria breathe oxygen to get energy from the organic matter, producing carbon dioxide in the process. Due to their gigantic total mass, these microbes dominate the marine carbon cycle - they convert more organic matter than all other marine life combined.
Important tiny things in sight
Despite their importance, however, marine bacteria have been comparatively little researched. So far there has only been rough information about the metabolic rates of these microbes. Until now, researchers have simply recorded the sum of the total respiratory activity of the bacteria in a certain volume and divided it by the number of organisms present. This left it unclear to what extent there are species-specific differences in respiratory activity. In order to gain more detailed insights into this "black box", an international team of researchers has developed a method that makes it possible to determine the respiratory activity of individual types of bacteria.
They use fluorescent probes: the more a cell breathes, the more these markers begin to glow. The system captures the fluorescence of individual bacterial cells and then sorts them based on the strength of the signal. They are then subjected to a genetic analysis to find out which type of bacteria it is. For the study, the scientists examined bacterial communities from the Gulf of Maine, the Mediterranean Sea and from the open Atlantic and Pacific Oceans.
Rare species with enormous appetites
The team was able to show that the species-specific differences in breathability are sometimes huge. “The breathability of the individual types of bacteria in seawater can vary up to a thousandfold. We found out that precisely those bacteria that are less numerous in the ocean have the highest respiration activity, while very common bacteria have low respiration activity,” says co-author Gerhard Herndl from the University of Vienna. The scientists conclude that this means that the rare bacteria are more important for the carbon cycle in the oceans than the microorganisms that occur in large numbers in seawater. "It's a common misunderstanding in ecology and in the consideration of biogeochemical cycles: It is not those groups of organisms or nutrients that decompose in the highest concentration that are particularly important, but very often those that occur only in low concentrations," explains Herndl.
But what could be behind the pattern? “Apparently, the highly active bacteria are heavily grazed, i.e. eaten by other creatures, so that they only occur in low numbers. So high activity also means high loss rates,” says Herndl. "It is therefore becoming apparent that only a few types of bacteria ensure that we have a high carbon flux in the sea, while the majority of bacteria are rather inactive, grow slowly and are also grazed little," explains the scientist.
As he and his colleagues point out, this made it clear how crude the information about the metabolic activity of the planktonic bacteria was that had previously been incorporated into global biogeochemical models. With the help of her method, this “black box” can now be illuminated. "The new findings have major implications for the study of global nutrient cycles such as the carbon cycle, since the ocean is responsible for a large part of the global carbon cycle," concludes Herndl.
Source: University of Vienna, specialist article: Nature, doi: 10.1038/s41561-022-01081-3