engleski

Microbial community response to salinity variation in a coastal Mediterranean lake

Fresh surface water resources, as karst lakes in the coastal region of the Mediterranean, represent a fragile hydrological system. Rising sea levels make previously freshwater coastal ecosystem partially saline. Seasonal changes in salinity and water temperature greatly influence the biological and nutrient structure of the lake. In freshwater lakes, bacteria are a key player in organic carbon processing and nutrient re-mineralization, forming the base of the microbial food web. Therefore, an increase of ionic content and water temperature potentially affect the nutrient fluctuation, which may lead to changes in the diversity of aquatic microbial communities in food webs and result in a shift of the trophic state. To evaluate the shifts in microbial community composition driven by salinity and temperature variation, we sampled the largest natural freshwater lake in Croatia, Vrana Lake Biograd na Moru, in the period from April to September in 2017. The lake is a karst cryptodepression connected to the nearby Adriatic Sea through the karst subsoil and a channel. Current trends suggest higher salinity and seawater intrusions into the lake which places this lake as a model system to explore changes of microbial community structure along with dissolved organic matter (DOM) molecular composition. To track changes during the seawater intrusion, the lake was sampled at two sites: nearby the channel connected to the sea and in the inner part of the lake. We used molecular approaches of Nextgeneration sequencing (Illumina) amplifying 16S and 18S rRNA genes to evaluate prokaryotic and eukaryotic composition, and ultrahigh resolution mass spectrometry for molecular DOM composition. Species richness (α diversity) of the microbial community and DOM molecular composition showed a high correlation between two sampling sites. Cluster analysis showed a higher difference between the sites based on eukaryotic composition. Through the season, from lower to higher ionic content, the chemoorganotrophic bacteria - Actinobacteria, followed by Gammaproteobacteria and Verrucomicrobiae - were dominant. This may indicate their ability to persist completely independent of salt concentration and ionic ratios. We found evident seasonal changes of the eukaryotic community with the highest relative abundance in September of Cryptophyta and Stramenopiles (Bacillarophyta), when the concentration of sodium, potassium and chloride was the highest in the season. This study highlights the changes in the microbial community and DOM composition in response to seawater intrusion and high-water temperature.

Bacteria ; Eukaryotes ; DOM ; NGS

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