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The perfoliate pondweed, Potamogeton perfoliatus, is a common macrophyte in freshwater and subarctic coastal areas. This species builds extensive meadows that play a role as a filter removing nutrients traversing from land to sea and maintain essential ecosystem functions. Here, we investigated the function of perfoliate pondweed as a filter of potentially pathogenic bacteria by combining culture-dependent and 16S rRNA metabarcoding approaches. Our results suggest no significant nutrient reduction in the meadow region but the enrichment of potentially pathogenic bacteria, such as Vibrio, Legionella and Leptospira, particularly attached to macrophyte leaves. The bacterial community composition differed between seawater and macrophyte habitats, with higher relative abundances of Cyanobacteriia attached to macrophytes, without affecting alpha-diversity. The metabolic pathways of bacteria for aromatic and polymer compound degradation were enriched in the macrophytes, attributed to members of the genera Pseudorhodobacter, Novosphingobium and Erythrobacter. Functions related to such degradation suggest that the bacteria may be able to remove complex organic compounds coming from land. We argue that the macrophyte meadows may be relevant to both animal and human health, as these habitats can be hot spots for potentially pathogenic bacteria.

Bacteria are major consumers of dissolved organic matter (DOM) in aquatic systems. In coastal zones, bacteria are exposed to a variety of DOM types originating from land and open sea. Climate change is expected to cause increased inflows of freshwater to the northern coastal zones, which may lead either to eutrophication or to increased inputs of refractory terrestrial compounds. The compositional and functional response of bacterial communities to such changes is not well understood. We performed a 2-day microcosm experiment in two bays in the coastal northern Baltic Sea, where we added plankton extract to simulate eutrophication and soil extract to simulate increased inputs of refractory terrestrial compounds. Our results showed that the bacterial communities responded differently to the two types of food substrates but responded in a similar compositional and functional way in both bays. Plankton extract addition induced a change of bacterial community composition, while no significant changes occurred in soil extract treatments. Gammaproteobacteria were promoted by plankton extract, while Alphaproteobacteria dominated in soil extract addition and in the non-amended controls. Carbohydrate metabolism genes, such as aminoglycan and chitin degradation, were enriched by plankton extract, but not soil extract. In conclusion, the coastal bacterial communities rapidly responded to highly bioavailable substrates, while terrestrial matter had minor influence and degraded slowly. Thus, in the northern Baltic Sea, if climate change leads to eutrophication, large changes of the bacterial community composition and function can be expected, while if climate change leads to increased inflow of refractory terrestrial organic matter the bacterial communities will not show fast compositional and functional changes. Degradation of terrestrial organic matter may instead occur over longer periods of time, e.g. years. These findings help to better understand the ability of bacterial communities to utilize different carbon sources and their role in the ecosystem.

In northern marginal seas, like the northern Baltic Sea, climate change will lead to many alterations, for example increased inflows of nutrients and dissolved organic matter (DOM). Nutrients and DOM are fundamental drivers shaping marine microbial communities, including both bacterial and phytoplankton populations. Potentially microbial communities and their functions can be used as descriptors of environmental change in marine systems. 

Changed nutrient availability will affect the phytoplankton communities, which is widely used as descriptor of environmental state in aquatic systems. Traditionally phytoplankton is analyzed using microscopy in monitoring, while molecular methods holds a great potential for future development. I performed a comparative study between metabarcoding and microscopy. Metabarcoding and microscopy displayed relatively similar distribution pattern at the group level. The results showed that the relative abundances of the 18S rRNA amplicon at the group level best fitted the microscopy carbon biomass metric. The results are promising for implementing DNA metabarcoding as a complement to microscopy in phytoplankton monitoring, especially if databases would be improved and group level indexes could be applied to classify environmental state of water bodies.

Bacterioplankton are the main DOM processors in the marine food web. A shift in the quality and quantity of the DOM pool could affect the microbial community structure and alter their functioning. Presently it is poorly known how coastal bacteria respond compositionally and functionally to quality and quantity changes in DOM supply. To comprehensively address this question, there is a critical need for microcosm experimental studies as well as field studies. Thus, I used approaches from single species laboratory experiment to community levels in situ incubation experiments and community levels spatiotemporal field surveys to evaluate the impacts of climate shifts on microbial community. Intricate relationships between environmental factors and microbial communities, for example, links between key microbial functional genes and DOM conditions were identified. Results showed that bacteria isolated from coastal area harbor genes for the sequestration of autochthonously produced carbon substrates, while bacteria isolated from a river contained genes for the degradation of relatively refractive terrestrial organic matter. A field experiment showed that Gammaproteobacteria was promoted by plankton extract addition and the genes for chitin and cellulose catabolism are enriched by addition of autochthonous carbon sources. The field survey with comprehensive metagenomic investigation of microbial community composition indicated that the temporal variation is larger compare to spatial changes. Bacteroidia, Actinomycetia, Gammaproteobacteria, Acidimicrobiia, and Alphaproteobacteria were the dominant bacterial classes, with Bacteroidia being more abundant in inshore stations compared to offshore locations. The seasonal shift in the relative abundance of these bacterial classes suggests that environmental factors and ecological processes drive changes in the abundance of different bacterial classes over time. Overall, these studies strengthen our understanding of the relationships between microbial composition and biogeochemical processes in coastal areas.

Bacteria are major utilizers of dissolved organic matter in aquatic systems. In coastal areas bacteria are supplied with a mixture of food sources, spanning from refractory terrestrial dissolved organic matter to labile marine autochthonous organic matter. Climate scenarios indicate that in northern coastal areas, the inflow of terrestrial organic matter will increase, and autochthonous production will decrease, thus bacteria will experience a change in the food source composition. How bacteria will cope with such changes is not known. Here, we tested the ability of an isolated bacterium from the northern Baltic Sea coast, Pseudomonas sp., to adapt to varying substrates.

We performed a 7-months chemostat experiment, where three different substrates were provided: glucose, representing labile autochthonous organic carbon, sodium benzoate representing refractory organic matter, and acetate – a labile but low energy food source. Growth rate has been pointed out as a key factor for fast adaptation, and since protozoan grazers speed-up the growth rate we added a ciliate to half of the incubations. The results show that the isolated Pseudomonas is adapted to utilize both labile and ring-structured refractive substrates. The growth rate was the highest on the benzoate substrate, and the production increased over time indicating that adaptation did occur. Further, our findings indicate that predation can cause Pseudomonas to change their phenotype to resist and promote survival in various carbon substrates. Genome sequencing reveals different mutations in the genome of adapted populations compared to the native populations, suggesting the adaptation of Pseudomonas sp. to changing environment.

Global warming scenarios indicate that in subarctic regions, the precipitation will increase in the future. Coastal bacteria will thus receive increasing organic carbon sources from land runoff. How such changes will affect the function and taxonomic composition of coastal bacteria is poorly known. We performed a 10-day experiment with two isolated bacteria: Shewanella baltica from a seaside location and Duganella sp. from a river mouth, and provided them with a plankton and a river extract as food substrate. The bacterial growth and carbon consumption were monitored over the experimental period. Shewanella and Duganella consumed 40% and 30% of the plankton extract, respectively, while the consumption of the river extract was low for both bacteria, ∌1%. Shewanella showed the highest bacterial growth efficiency (BGE) (12%) when grown on plankton extract, while when grown on river extract, the BGE was only 1%. Duganella showed low BGE when grown on plankton extract (< 1%) and slightly higher BGE when grown on river extract (2%). The cell growth yield of Duganella was higher than that of Shewanella when grown on river extract. These results indicate that Duganella is more adapted to terrestrial organic substrates with low nutritional availability, while Shewanella is adapted to eutrophied conditions. The different growth performance of the bacteria could be traced to genomic variations. A closely related genome of Shewanella was shown to harbor genes for the sequestration of autochthonously produced carbon substrates, while Duganella contained genes for the degradation of relatively refractive terrestrial organic matter. The results may reflect the influence of environmental drivers on bacterial community composition in natural aquatic environments. Elevated inflows of terrestrial organic matter to coastal areas in subarctic regions would lead to increased occurrence of bacteria adapted to the degradation of complex terrestrial compounds with a low bioavailability.