Bacterial Communities under Heatwave Stress in Mediterranean Freshwater and Coastal Systems

Mediterranean freshwater and coastal ecosystems are highly vulnerable to increasingly frequent heatwaves, yet their impact on microbial communities remains poorly understood. In this study, we investigated bacterial community responses under natural and simulated thermal stress in two diverse aquatic ecosystems in Sardinia (Western Mediterranean): the artificial Bidighinzu Lake and the coastal Cabras Lagoon. Seasonal field samplings were conducted from January to December 2024, while a controlled laboratory experiment was performed in July 2024 to simulate a summer heatwave (+5 °C, 14 days). Water samples collected at specific depths and stations during field activities were fractionated by sequential filtration (20 µm to 0.22 µm), while samples collected in the laboratory at different experimental times were 0.22 µm filtered. DNA was extracted from each planktonic fraction for 16S rRNA gene amplicon sequencing. Sequences were processed with DADA2 and taxonomic assignment performed using the SILVA database. Analyses are ongoing, but we expect that elevated temperatures will lead to a decrease in bacterial diversity and a shift toward potentially pathogenic and/or faecal bacteria. This integrated molecular and ecological approach, combining long-term monitoring with experimental simulation, will enhance our understanding of how climate change may reshape microbial assemblages and affect aquatic ecosystems and public health in Mediterranean waters.