The epiphytic microbiota of the globally widespread macroalga Cladophora glomerata (Chlorophyta, Cladophorales)

• Premise of the study: The filamentous chlorophyte Cladophora produces abundant nearshore populations in marine and freshwaters worldwide, often dominating periphyton communities and producing nuisance growths under eutrophic conditions. High surface area and environmental persistence foster such high functional and taxonomic diversity of epiphytic microfauna and microalgae that Cladophora has been labeled an ecological engineer. We tested the hypotheses that (1) Cladophora supports a structurally and functionally diverse epiphytic prokaryotic microbiota that influences materials cycling and (2) mutualistic host–microbe interactions occur. Because previous molecular sequencing‐based analyses of the microbiota of C. glomerata found as western Lake Michigan beach drift had identified pathogenic associates such as Escherichia coli, we also asked if actively growing lentic C. glomerata harbors known pathogens.• Methods: We used 16S rRNA gene amplicon pyrosequencing to examine the microbiota of C. glomerata of Lake Mendota, Dane, Wisconsin, United States, during the growing season of 2011, at the genus‐ or species‐level to infer functional phenotypes. We used correlative scanning electron and fluorescence microscopy to describe major prokaryotic morphotypes.• Key results: We found microscopic evidence for diverse bacterial morphotypes, and molecular evidence for ca. 100 distinct sequence types classifiable to genus at the 80% confidence level or species at the 96–97% level within nine bacterial phyla, but not E. coli or related human pathogens.• Conclusions: We inferred that bacterial epiphytes of lentic C. glomerata have diverse functions in materials cycling, with traits that indicate the occurrence of mutualistic interactions with the algal host.