Nitrogen Fixation and Microbial Communities Associated with Decomposing Seagrass Leaves in Temperate Coastal Waters.

Seagrass meadows play pivotal roles in coastal biochemical cycles, with nitrogen fixation being a well-established process associated with living seagrass. Here, we tested the hypothesis that nitrogen fixation is also associated with seagrass debris in Danish coastal waters. We conducted a 52-day in situ experiment to investigate nitrogen fixation (proxied by acetylene reduction) and dynamics of the microbial community (16S rRNA gene amplicon sequencing) and the nitrogen fixing community (nifH DNA/RNA amplicon sequencing) associated with decomposing Zostera marina leaves. The leaves harboured distinct microbial communities, including distinct nitrogen fixers, relative to the surrounding seawater and sediment throughout the experiment. Nitrogen fixation rates were measurable on most days, but highest on days 3 (dark, 334.8 nmol N g-1 dw h-1) and 15 (light, 194.6 nmol N g-1 dw h-1). Nitrogen fixation rates were not correlated with the concentration of inorganic nutrients in the surrounding seawater or with carbon:nitrogen ratios in the leaves. The composition of nitrogen fixers shifted from cyanobacterial Sphaerospermopsis to heterotrophic genera like Desulfopila over the decomposition period. On the days with highest fixation, nifH RNA gene transcripts were mainly accounted for by cyanobacteria, in particular by Sphaerospermopsis and an unknown taxon (order Nostocales), alongside Proteobacteria. Our study shows that seagrass debris in temperate coastal waters harbours substantial nitrogen fixation carried out by cyanobacteria and heterotrophic bacteria that are distinct relative to the surrounding seawater and sediments. This suggests that seagrass debris constitutes a selective environment where degradation is affected by the import of nitrogen via nitrogen fixation.

Temperature, sediment resuspension, and salinity drive the prevalence of Vibrio vulnificus in the coastal Baltic Sea.

The number of Vibrio-related infections in humans, e.g., by Vibrio vulnificus, has increased along the coasts of the Baltic Sea. Due to climate change, vibriosis risk is expected to increase. It is, therefore, pertinent to design a strategy for mitigation of the vibriosis threat in the Baltic Sea area, but a prerequisite is to identify the environmental conditions promoting the occurrence of pathogenic Vibrio spp., like V. vulnificus. To address this, we sampled three coastal Baltic sites in Finland, Germany, and Denmark with salinities between 6 and 21 from May to October 2022. The absolute and relative abundances of Vibrio spp. and V. vulnificus in water were compared to environmental conditions, including the presence of the eelgrass Zostera marina, which has been suggested to reduce pathogenic Vibrio species abundance. In the water column, V. vulnificus only occurred at the German station between July and August at salinity 8.1-11.2. Temperature and phosphate (PO43-) were identified as the most influencing factors for Vibrio spp. and V. vulnificus. The accumulation of Vibrio spp. in the sediment and the co-occurrence with sediment bacteria in the water column indicate that sediment resuspension contributed to V. vulnificus abundance. Interestingly, V. vulnificus co-occurred with specific cyanobacteria taxa, as well as specific bacteria associated with cyanobacteria. Although we found no reduction in Vibrio spp. or V. vulnificus associated with eelgrass beds, our study underscores the importance of extended heatwaves and sediment resuspension, which may elevate the availability of PO43-, for Vibrio species levels at intermediate salinities in the Baltic Sea. IMPORTANCE: Elevated sea surface temperatures are increasing the prevalence of pathogenic Vibrio at higher latitudes. The recent increase in Vibrio-related wound infections and deaths along the Baltic coasts is, therefore, of serious health concern. We used culture-independent data generated from three Baltic coastal sites in Denmark, Germany, and Finland from May to October (2022), with a special focus on Vibrio vulnificus, and combined it with environmental data. Our temporal model shows that temperature, combined with sediment resuspension, drives the prevalence of V. vulnificus at intermediate salinities in the coastal Baltic Sea.