Freshwater zooplankton microbiome composition is highly flexible and strongly influenced by the environment

The association with microbes in plants and animals is known to be beneficial for the host's survival and fitness, but the generality of the effect of the microbiome is still debated. For some animals, similarities in microbiome composition reflect taxonomic relatedness of the hosts, a pattern termed phylosymbiosis. The mechanisms behind the pattern could be due to coevolution and/or to correlated ecological constraints. General conclusions are hampered by the fact that the available knowledge is highly dominated by microbiomes from model species. We addressed the issue of the generality of phylosymbiosis by analysing the species-specificity of microbiomes across different species of freshwater zooplankton, including rotifers, cladocerans, and copepods, coupling field surveys and experimental manipulations. We found that no signal of phylosymbiosis is present, and that the proportion of 'core' microbial taxa, stable and consistent within each species, is very low. Changes in food and temperature under laboratory experimental settings revealed that the microbiome of freshwater zooplankton is highly flexible and can be influenced by the external environment. Thus, the role of coevolution, strict association, and interaction with microbes within the holobiont concept highlighted for vertebrates, corals, sponges, and other animals does not seem to be supported for all animals, at least not for freshwater zooplankton. Zooplankton floats in the environment where both food and bacteria that can provide help in digesting such food are available. In addition, there is likely redundancy for beneficial bacterial functions in the environment, not allowing a strict host-microbiome association to originate and persist.