Unveiling the Ecology of Legionella in a Stratified Freshwater Lake: Seasonal Dynamics, Host Interactions, and Persistence Under Oxygen Limited Conditions

AbstractO_ST_ABSBackgroundC_ST_ABSLegionella are predominantly recognized as aerobic pathogens in man-made water systems. However, their potential persistence in natural freshwater environments, particularly under oxygen limited conditions, remains poorly explored. In this study we investigated the spatio-temporal dynamics of Legionella occurrence in a seasonally stratified sub-tropical freshwater lake, with a focus on anaerobic conditions of the anoxic hypolimnion. ResultsOur study reveals significant seasonal variations in Legionella absolute abundance, with the highest concentrations occurring during and immediately following lake mixing events. Unexpectedly, high levels of Legionella were measured in the anaerobic hypolimnion layer of the lake. Utilizing genus specific amplicon-based sequencing, we found significant shifts in Legionella community composition, that are related to the sampling month. Several environmental factors were associated with the observed changes, including temperature, DO, chlorophyll and dinoflagellate biomass. Moreover, we identified Legionella genotypes unique to samples presenting hypoxic conditions - that were not closely related to known Legionella species. In addition, we noted genotypes present in anoxic samples, that were absent from the oxic layers of the corresponding sampling dates. These results were accompanied by changes in the interaction patterns between Legionella and their potential hosts, in oxic and anoxic conditions. ConclusionsThis study challenges the conventional view of Legionella as a strictly aerobic pathogen by demonstrating its persistence in anoxic freshwater environments. Our findings suggest that certain Legionella species may have adapted to low- oxygen conditions, potentially through alternate metabolic pathways or by residing within protozoan hosts. The identification of unique Legionella genotypes in the hypolimnion, along with shifts in occurrence, diversity, and host interactions, underscores the complexity of Legionella ecology. These results highlight the need for further research on Legionella in natural freshwater systems, which serve as reservoirs for the bacteria and potential sources for human infection. Further investigation into the mechanisms underlying Legionella persistence in anaerobic conditions and its interactions with environmental hosts is essential for a better understanding of the evolutionary forces shaping this family of human pathogens.