Seasonal variation in bacterial community composition in the brackish meromictic Lake Suigetsu.

Lake Suigetsu, a deep meromictic lake connected to Wakasa Bay, receives freshwater inflow and exchange surface water with intruding seawater. To investigate bacterial community dynamics and substrate transport, we conducted seasonal 16S rRNA gene sequencing across three stratified layers (mixolimnion, chemocline, and monimolimnion) and two size fractions (0.2-0.8 µm for free-living; 0.8-3.0 µm for particle-attached). Community composition differed by depth, fraction, and season. Winter communities were dominated by Actinobacteria, Gammaproteobacteria, Alphaproteobacteria, and Bacteroidia, forming a potential "seed" community. In the monimolimnion, Bacteroidia, Chlorobia, and Cloacimonadia prevalent in free-living fraction, while Desulfobacterales and Desulfatiglandales and Anaerolineae were in particle-attached fraction. In autumn, a community shift toward Pseudomonadales and Enterobacteriales corresponded with lowered salinity and temperature in the chemocline, suggesting episodic entrainment driven by mixing of freshwater and monimolimnetic water. These findings highlight seasonal and vertical microbial dynamics and suggest that hydrological mixing may facilitate microbial connectivity between surface and deep waters.