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New Findings on Bacterial Dynamics in Freshwater Ecosystems

Bacteria play a fundamental role in breaking down organic matter across various ecosystems, including both soil and aquatic environments. While the majority of bacteria engage in extracellular digestion of large molecules, allowing a wider community to access the nutrients, a subset of these microorganisms adopts a more competitive strategy by internalizing entire molecules for digestion. A recent study, published on April 10 in the journal Cell Reports, provides the first evidence of this behavior, termed “selfish polysaccharide uptake,” within freshwater systems. Conducted in Croatia’s Kozjak and Crniševo Lakes, the research reveals how species that monopolize nutrient resources can outcompete others, impacting the entire food web of a lake—a dynamic that is likely to be influenced by climate change, particularly in colder lakes.

“The role of bacteria in nutrient cycling within lakes is critical, especially in their breakdown of polysaccharides. Their propensity for selfish consumption significantly affects the accessibility of carbon and other nutrients,” notes Andrea Čačković, the study’s first author, from the Ruder Bošković Institute.

The investigation focused on two distinct lakes in Croatia: Kozjak Lake, recognized as the largest and deepest lake within the Plitvice Lakes National Park, and Crniševo Lake, situated in a warmer Mediterranean climate. Kozjak Lake is characterized as oligotrophic, indicating low nutrient and algae levels and seasonal freezing, while Crniševo Lake is mesotrophic, featuring a higher abundance of nutrients and algal growth.

Researchers collected water samples during spring and summer of 2022 and winter of 2023. They quantified bacterial populations in each lake across the seasons. By incubating these bacteria with six different fluorescently tagged polysaccharides, the team aimed to determine whether bacteria preferred to digest these nutrients externally or via selfish internalization, and to identify any specific polysaccharides that were more frequently utilized in the latter manner. Additionally, genetic sequencing was employed to analyze bacterial community structures throughout the seasons.

Findings revealed that selfish bacteria were indeed present in both lakes, though the extent of selfish uptake varied with the seasons. In general, Crniševo Lake exhibited a higher abundance and diversity of bacteria alongside increased rates of selfish nutrient uptake when compared to the oligotrophic Kozjak Lake.

“It’s remarkable to discover a mechanism previously unknown until 2017 also exists in freshwater ecosystems,” comments Greta Reintjes, a senior author and microbial ecologist affiliated with the University of Bremen.

Intriguingly, the research team observed that in the mesotrophic Crniševo Lake, selfish bacterial activity surged after summertime phytoplankton blooms, a period marked by abundant nutrients. Conversely, the oligotrophic Kozjak Lake showed peaks in selfish activity during the winter months when nutrients were less plentiful, a pattern akin to behaviors documented in marine environments.

“In the case of Crniševo Lake, higher organic material present correlated with elevated selfish behavior, which was unexpected,” Reintjes remarks. “We currently lack an ecological explanation for this phenomenon and aim to conduct further research to identify the specific organisms exhibiting this unique behavior and the reasons behind it.”

The study also highlighted that not all polysaccharides are treated equally by bacteria. Pullulan—a sugar produced by fungi—was the most frequently hoarded polysaccharide in both lakes, being digested selfishly 12% of the time in the oligotrophic lake and 7% of the time in the mesotrophic lake.

“A notable finding from our research is that bacteria display selectivity in polysaccharide degradation, which could influence microbial community dynamics and the structure of the lake’s food web,” outlines Čačković.

Such interactions and dynamics are likely to be affected by climate change, especially as seen in the icy conditions of Kozjak Lake.

“The freezing of the lake in winter serves as a resetting mechanism for the ecosystem, a process that may be altered in the future as climate change leads to fewer freezing events than previously observed,” cautions coauthor Sandi Orlić, a microbiologist at the Ruđer Bošković Institute. “Understanding the mechanisms of sugar degradation by various bacteria is essential for grasping the broader implications for sugar and carbon cycling and its susceptibility to climate impacts.”

Looking ahead, the researchers intend to employ genomic techniques to identify the genes and enzymes facilitating this selfish uptake mechanism. They also aspire to investigate the occurrence of selfish bacteria in other ecosystem types.

“I eagerly anticipate examining additional systems to ascertain whether these selfish bacteria are pervasive,” Reintjes expresses. “Ultimately, I aim for a comprehensive understanding of the role these organisms play globally in carbon turnover.”

This research was made possible through various funding sources, including DAAD Research Grants, FEMS Research and Training Grant, the German Research Foundation, and the Croatian Science Foundation.

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