Deep beneath the mirror-like surface of Croatian lakes, in a space where the human eye rarely ventures, an invisible battle unfolds that shapes the lives of numerous organisms and maintains the balance of entire ecosystems. The latest discoveries of microbiologists are opening a new chapter in the understanding of bacteria which, although microscopically small, have a huge impact on the circulation of nutrients and the resilience of lake systems to climate change.

Unexpected role of bacteria in lake ecology

Until now, it was believed that bacteria living in freshwater lakes break down complex sugars, such as polysaccharides, in a way that benefits the wider community – by breaking them down outside the cell, they release nutrients that can then be used by other microorganisms. However, new research by domestic and international scientists reveals that certain bacteria behave extremely selfishly: they retain valuable nutrients for themselves, within their cells, limiting the availability to other organisms.

This discovery changes the current picture of microbial life in lakes and raises numerous questions about how this "selfish" strategy affects the stability of food webs, especially in conditions of increasingly pronounced climate fluctuations.

Study on Croatian lakes – Kozjak and Crniševo

The research was conducted on two very different lakes: the cold and oligotrophic Lake Kozjak, located in the heart of Plitvice Lakes National Park, and Lake Crniševo, which is located in the southern regions and is known for its warmer temperatures and larger amount of algae. This diversity allowed scientists to compare how microorganisms function in contrasting conditions.

Over several seasons – spring and summer 2022 and winter 2023 – water samples were taken, and in the laboratory, the bacteria were exposed to different types of fluorescently labeled polysaccharides. Analysis revealed that certain types of bacteria specifically recognize and internally process certain sugars, which confirms their selectivity and "selfishness".

Seasonal variations and climate challenges

In Lake Kozjak, selfish degradation was most pronounced during winter, when resources were scarce, while in Crniševo this activity peaked during summer, when nutrients were abundant due to algal blooms. This behavior has so far only been observed in ocean ecosystems, so its discovery in freshwater environments caused great surprise in scientific circles.

These changes would not be so important if bacteria did not form the basis of ecological chains – their way of using nutrients can directly affect the availability of food for phytoplankton, zooplankton, and all the way up to fish and birds that depend on them. All this is further complicated by climate change, which is changing water temperature, the length of seasons, and the amount of available nutrients.

Selective degradation and microbial pickiness

One of the key findings of the research was that bacteria do not react equally to all types of polysaccharides. For example, pullulan – a sugar synthesized by certain fungi – was identified as the most common target of selfish degradation. Such specificity indicates a high level of adaptation of bacterial communities to environmental conditions and a potential change in the dynamics of food chains if ecological conditions change.

Given that lakes like Kozjak freeze over in winter, there is a kind of "reset" of the ecosystem, but warmer winters and shorter periods of freezing could disrupt this natural renewal in the future. Changes in such seasonal patterns could have long-term consequences for the balance of microbial communities and the circulation of substances such as carbon and nitrogen.

Location and value of the site

Lake Kozjak, the jewel of Plitvice Lakes, is not only a tourist pearl, but also a natural laboratory for studying climate change. Its location within the UNESCO World Heritage Site provides additional context to the importance of such research. On the other hand, Lake Crniševo, although less known to the general public, offers exceptional value as a representative of Mediterranean freshwater systems.

The location of both lakes in different climate zones allowed scientists to understand how microorganisms react to changing conditions – from the cold winter of Kozjak to the warm summers of Crniševo. It is this contrasting nature that provided valuable insights into the adaptability and behavior of bacteria.

Future research – what's next?

The research team now plans to expand the research to new lakes and expand the genetic analysis to discover the specific genes and enzymes responsible for the selfish behavior of bacteria. They are asking themselves how widespread such strategies are and what their role is in the global carbon cycle – a question that has a direct link to environmental protection policies and sustainable development.

All these findings emphasize the need for further study of the microscopic world that lives beneath the surface of our lakes. Because at a time when there is increasing talk about reducing emissions and green policies, we often forget that the answers may lie precisely in the smallest organisms – bacteria that quietly but relentlessly manage vital processes in nature.