Cyanobacterial connectivity from freshwater to estuaries: insights into genotypes and cyanotoxins flow.

Cyanobacterial blooms are detrimental events that affect the quality of water and the normal functioning of ecosystems, especially when dominated by toxin-producing species. Although cyanobacteria and cyanotoxins have been reported in the land-sea interface since the late 80s, genetic evidence on how inland freshwaters influence the cyanobacterial communities in these systems is very scarce to date. This study aims to investigate the relationship between the cyanobacterial communities of an inland freshwater reservoir and an estuary located in an aquaculture-rich coastal area of NW Spain. During 2022 and 2023, a total of six sampling campaigns were carried out surrounding the blooming seasons. Cyanobacterial communities and their potential toxicity were analyzed through metabarcoding of the 16S rRNA gene and PCR-based screening of genes involved in the biosynthesis of cyanotoxins. Results demonstrate that the reservoir likely contributed significantly to the presence of potentially toxic cyanobacteria in the estuary, with more than 80 % of reservoir shared cyanobacterial ASVs (Amplicon Sequence Variants) belonging to potentially toxic genera. Genes related to microcystins and anatoxins were detected in both systems, accompanied by low toxin concentrations. Most key cyanobacterial genera were consistently dominated by specific ASVs, suggesting the presence of a few genotypes resistant to environmental gradients during transport. These findings highlight the influence of freshwater systems on estuarine cyanobacterial communities and demonstrate the potential of genetic tools for their high-resolution monitoring, useful in water management of ecological and economic risks. Further research is recommended to understand the global impacts of cyanobacterial bloom dispersion towards land-sea interface systems, particularly in economically relevant areas.