Ubiquity and quantitative significance of bacterioplankton lineages inhabiting the oxygenated hypolimnion of deep freshwater lakes

Abstract

The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situ hybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (𝘊𝘩𝘭𝘰𝘳𝘰𝘧𝘭𝘦𝘹𝘪), CL500-3, CL500-37, CL500-15 (𝘗𝘭𝘢𝘯𝘤𝘵𝘰𝘮𝘺𝘤𝘦𝘵𝘦𝘴) and Marine Group I (𝘛𝘩𝘢𝘶𝘮𝘢𝘳𝘤𝘩𝘢𝘦𝘰𝘵𝘢), together accounted for 1.5–32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.