A deep dive into the epibiotic communities on aquacultured sugar kelp Saccharina latissima in Southern New England

Sugar kelp cultivation at the southern end of its range on the east coast of North America is being pursued commercially for human consumption, which demands a high-quality product. Blade quality can be compromised by attached organisms – epibionts. Biweekly examination of epibionts on sugar kelp was conducted April–May 2018, on a kelp farm in eastern Long Island Sound, CT, USA. Culturable Vibrio spp. were not present on kelp blades until May and were limited to only old sections. No Vibrio colonies were human pathogens V. parahaemolyticus or V. vulnificus, based upon ToxR-specific multiplex PCR assays. Neither epibenthic cyanobacteria Lyngbya spp. nor the dinoflagellate Prorocentrum lima, microbes of concern because of toxigenicity, were detected on kelp by microscopy or metabarcoding of partial rRNA genes. The lacy bryozoan was the only epibiotic animal observed that could cause damage to kelp, but its abundance was low. Summarizing most-common sequence reads, Gammaproteobacteria was the most abundant bacterial group on kelp blades (49%) and Alphaproteobacteria were the most abundant in seawater (39%). Bacillariophyta were the most abundant eukaryotes on kelp blades (36%) and Dinoflagellata were the most abundant eukaryotes in seawater (43%). Molecular operational taxonomic unit matrices were used for non-metric multidimensional scaling; the most prominent structure for both prokaryotic and eukaryotic communities was the separation between blade and seawater samples. This spatial separation explained 81% and 76% of the variation among prokaryotic and eukaryotic samples, respectively. Indicator Species Analysis identified Gammaproteobacteria (55%) and Bacillariophyta (56%) to be the most important blade indicator prokaryotes and eukaryotes, respectively. A closer examination of indicator species temporal patterns and their ecophysiology suggested that Aquimarina, Parcubacteria, and Peronosporomycetes are potential pathogens to sugar kelp. Ciliates may be the most important grazers that keep epiphytes (Bacillariophyta, Rhodophyta, and Phaeophyta) and Peronosporomycetes on kelp in check. •Kelp epibiotic microbial communities differ from planktonic microbial communities.•No pathogenic Vibrio or toxigenic epibenthic microalgae were detected.•Aquimarina, Parcubacteria, and Peronosporomycetes are potential kelp pathogens.•Gammaproteobacteria were abundant on kelp, including petroleum-degrading strains.•Benthic diatoms were most abundant microalgae on kelp.