Bacterial communities associated to the toxic dinoflagellate Ostreopsis cf. ovata during bloom development

Diversity of bacterial communities associated with the toxic benthic dinoflagellate Ostreopsis cf. ovata were investigated at a site named Passetto located along the Ancona coast (43°30’ N and 13°37’ E; Northwestern Adriatic Sea; Italy) and characterized by rocky bottom and shallow waters. Ambient seawater samples (< 0.5 m depth) and dinoflagellate-colonized macrophytes were collected at two different stages of the O. cf ovata bloom. Different bacterial assemblages were separated by samples’ fractionation to observe subtle changes within each partition. Among others, free-living seawater community, aggregates/mucilage associated community, and single O. cf ovata cell associated community were considered. Abundances of O. cf. ovata cell were two fold higher at the peak sampling (Oct 2, 2012; 206 × 103 ± 28.5 × 103 cells g-1 of fresh macrophyte) than at the beginning of the bloom (Sep 18, 2012; 94.9 × 103 ± 5.88 × 103 cells g-1 fw). Bacterial cell abundance of the aggregate fractions (range: 3.06 × 106 to 6.62 × 106 cells ml-1 ; 1st and 2nd sampling respectively) and aggregate virus-like particle concentrations (range: 1.10 × 108 and 1.23 × 108 VLP ml-1) showed significant correlations with microalgae cell densities. The number of bacterial cells attached to the single O. cf. ovata cells was generally low (mean: < 2 bacteria, with a range from 0 to 5 bacteria). Bacterial communities associated to the different fractions were compared using automated rRNA intergenic spacer analysis (ARISA) and results were summarized by similarity based analysis and ANOSIM test. Pyrosequencing analysis was performed for taxonomic affiliation. Among the 178 operational taxonomic units (OTUs) detected, only 11 were present in all samples. Unique OTUs were mainly observed in relation with the aggregate fraction and with single algal cells. Large dissimilarity was found between seawater free-living and aggregate-associated bacterial community structures, either within or between samplings. Interestingly, microbial consortia associated with single O. cf. ovata cells were significant different from communities associated with all the other partitions. On the whole, a total of 409 taxonomic affiliations bacterial genera distributed over 21 phyla were found.