This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Tiny photosynthetic organisms in the Baltic Sea can contribute substantially to the transport of carbon from surface waters to deeper parts of the sea. A new study from the Department of Ecology, Environment and Plant Sciences at Stockholm University shows that their ability to form colonies and aggregates is an important factor determining how much of their biomass sinks.
"In this study, we have shown that in situ aggregation properties vary among picocyanobacterial strains and that the degree of aggregation directly determines the magnitude of export. Given that picophytoplankton are expected to increase in abundance in a warmer and more nutrient-poor (stratified) ocean, understanding mechanisms allowing their biomass to be exported to the deep ocean is important for predicting future atmospheric CO2 levels," says lead author Martin Ekman, a former researcher at the Department of Ecology, Environment and Plant Sciences at Stockholm University (at the time of the study). The work is published in The ISME Journal.
Phytoplankton play a central role in the ocean's carbon cycle by taking up carbon dioxide (CO2) through photosynthesis—approximately half of global photosynthesis is attributed to phytoplankton. When this biomass sinks from surface waters toward the depths below, carbon is transported with it. This process, known as the biological pump, is an important part of the global carbon cycle and has a major role in regulating atmospheric CO2 levels.
In addition, remineralization of sinking phytoplankton is the direct cause of anoxic bottom waters in coastal seas such as the Baltic Sea. Among the smallest but also most common phytoplankton are picocyanobacteria, which are less than 2 micrometers in size (one millionth of a meter, 10-6 m). Because individual cells are so small, they are not expected to sink efficiently on their own.
Previous research has shown that these organisms can make a significant contribution to carbon export, but the mechanism by which the biomass of these tiny organisms is exported has remained unknown. In collaboration with the Marine Ecological Lab (MEL), the researchers measured abundances of picocyanobacteria in the Baltic Sea during spring and summer, sampling surface waters as well as water at depths of 25 meters (82 feet) and 75 meters (246 feet). Additionally, they took samples using sediment traps, which were moored below the sunlit surface zone to collect sinking particulates.
They combined microscopy and flow cytometry with genetic analyses of water samples and material collected in sediment traps. Key to their study was the sampling strategy of separating the particulates in the water samples by size fractionation. Larger cells (>10 µm, large fraction) were collected on the first filter, while the small cells passed through and were collected on a second in-line filter (0.2 µm, free fraction).
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