Understanding filamentous cyanobacteria survival strategies through nutrient cycling patterns and population genomics

Every summer, the Baltic Sea is affected by extensive blooms of filamentous cyanobacteria, which is often noticed in the media as they may form toxic and thick accumulations on beaches. These can be dangerous for animals, negatively affect tourism and contribute to the increasing lack of oxygen in the bottom water. However, we have shown that their abundance increases in some regions of the Baltic Sea while not in others. For example, the abundance of cyanobacteria has increased in the Bothnian Sea during recent years, as a result of increased temperature and decreased salinity with climate change, their abundance has not changed in the southern Bothnian Sea. This suggests that there are local adaptations, so-called ecotypes, but we still lack knowledge on genetic population structures of filamentous cyanobacteria in the Baltic Sea and their adaptations to the varying environments. With this knowledge, we could increase the understanding of how they will be affected in the future and what causes them to increase in some places but not in others. My research therefore aims to understand these regional differences, their ecological impacts, and the underlying mechanisms along two main research paths; one focusing on nitrogen cycling and one on genetic population structures.

Firstly, to study nitrogen cycling in natural cyanobacteria populations I am using a single-cell approach (secondary ion mass spectrometry; SIMS), which enables quantification of assimilation and fixation rates and reveals each taxa’s relative contribution. In the Baltic Proper, filamentous cyanobacteria are responsible for about half of all new nitrogen added to the environment via nitrogen fixation. In contrast, the Gulf of Bothnia has never been explored in terms of rate measurements, but we predict the internal nitrogen loads to be substantial, which would be important to quantify in management purposes. Therefore, we recently conducted a field survey along a transect from north to south, with decreasing levels of bioavailable nitrogen and increasing salinities. This will help us understand this changing environment and predict how these cyanobacteria may affect this ecosystem in the future climate.

My second research path aims to investigate how the genetic population patterns of filamentous cyanobacteria differs between regions within the Baltic Sea and adjacent freshwater areas. The summer of 2022 I organized a sampling campaign around the Baltic Sea where we isolated cyanobacterial genomes from more than 40 different locations. From this library of genomes, we may reveal the genomic structures specific to the environments, with variation in salinity, temperature, and nutrient availability. Here, we will use powerful methods that can distinguish sequenced genomes down to individual nucleotides and look for patterns that are unique to each region and thus define different ecotypes of the species. We will also use sediment archives to reveal adaptation to present state environments to gain understanding in survival strategies of the taxa during the Baltic Sea formation through historic events.