Investigating the core microbiome concept: Daphnia as a case study.

BACKGROUND: Host-associated microbiomes play an important role in the ecology and fitness of organisms. Given their significance, it is much debated to what extent these associations are widespread and even obligatory. Such frequent associations are captured by the concept of the core microbiome. The cladoceran Daphnia is a pivotal genus in freshwater ecosystems occupying a central position in the food webs of standing waters. With its unique standing in pelagic waters, Daphnia serves as a key grazer, regulating algal populations and nutrient cycling, making its microbiome essential to understanding ecosystem function and stability. In recent years, Daphnia has become an increasingly popular study system for exploring host‒microbiota interactions. There is, however, limited knowledge on the baseline taxa that consistently inhabit this host and potentially contribute to its fitness. Identifying whether such a host-associated "core microbiome" exists for Daphnia and, if so, which microbial taxa it comprises is important both for enhancing our ecological understanding of this genus and its ecosystem function and for interpreting future experiments. RESULTS: We compiled a dataset on Daphnia magna microbiome based on 12 published studies, comprising gut and whole microbiome samples of both laboratory-cultured and field-grown animals across five countries spanning three continents. To identify core taxa, we employ quantification metrics based on prevalence and a combination of prevalence and relative abundance. Our analysis demonstrates that the D. magna microbiome is highly variable, yet, a consistent association with specific taxa, notably Limnohabitans planktonicus, is observed especially under laboratory conditions. However, this pattern is tempered by the observation that field-grown animals exhibit a more diverse microbiome with a weaker presence of L. planktonicus, challenging its status as a core member. CONCLUSIONS: Our analysis suggests that the D. magna microbiome is defined by its high variability and few conserved associations, with L. planktonicus being the most stable taxon in laboratory settings but not necessarily a core member in natural environments. These findings underscore the need for caution when using laboratory results to interpret natural microbiome compositions and emphasize the need for further research on field-grown animals to better understand the structuring of microbial communities under natural settings.