Stenostomum leucops (Catenulida, Platyhelminthes) has a flexible microbiome in time and space

Multicellular organisms and their microbiomes can have a restricted and enduring relationship, sometimes reflected in their phylogenies, called phylosymbiosis. However, in some organisms, such as freshwater zooplankton, these relationships appear to be more flexible and more easily influenced by the environment. Here, we analysed the microbiome of a freshwater flatworm, Stenostomum leucops and sequenced the 16S RNA gene of the microbiota of a strain that was maintained in the laboratory for 12 years. This strain underwent four different cultivation conditions over the past 6 years, and the microbiome was characterised for each of these conditions. In addition, the microbiome of a wild S. leucops population was analysed. The microbiomes were highly variable between populations (Shannon index ranging from 0.26 to 2.06). It was not possible to determine a core microbiome, although Bacillus, Pseudomonas and Ralstonia were the predominant bacteria in populations under stable conditions. Under culture conditions where the water was contaminated with iron, Rhodoferax ferrireducens, a bacterium involved in iron reduction, was the predominant species. Our results are consistent with other studies on freshwater zooplankton. The microbiomes were very flexible and were influenced by the environment.