Selective heterotopic bacteria can selfishly process polysaccharides in freshwater lakes

Polysaccharides, one of the main components of dissolved organic matter, are utilized by bacteria through three foraging enzymatic mechanisms: scavenging, sharing, and selfish. Our research aimed to identify selfish polysaccharide utilization by bacteria in freshwater ecosystems by examining spatial and seasonal variations in two lakes. The results of our fluorescently labeled substrate incubations revealed selfish activity in both lakes, with a larger proportion of the community showing selfish uptake of pullulan. The mesotrophic lake showed greater microbial diversity and ability for selfish uptake, particularly during a phytoplankton bloom. On the other hand, the oligotrophic lake had higher selfish activity during periods of increased terrestrial influence. Fluorescence in situ hybridization (FISH) staining combined with microbial diversity analysis revealed the selfish activity of phyla Bacteroidota, Planctomycetota, and Verrucomicrobiota as well as class Gammaproteobacteria, with different genera active depending on lake, season, and substrate. This research provides a basis for future interpretation of nutrient cycling in freshwater ecosystems.