Bacterial community associated with the toxic dinoflagellate Ostreopsis cf. ovata blooms

Diversity of bacterial community associated with the toxic benthic dinoflagellate Ostreopsis cf. ovata was investigated at one site characterized by rocky bottom and shallow water along the Ancona coast, northern Adriatic Sea, Italy. Ambient (< 0.5 m depth) seawaters and dinoflagellate-colonized macrophytes were collected at two different stages of the bloom. The number of O. cf. ovata cells were higher at the peak sampling, Oct 2, 2012 (206 ×103 ± 28.5 × 103 cells g-1 of fresh macrophyte) than during the initiation of the bloom (Sep 18, 2012, 94.9 × 103 ± 5.88 × 103 cells g-1 fw). Bacterial concentration showed a strong correlation with microalgaes cell density ( r = 0.84; p < 0.02). A similar trend was observed for virus-like particle concentration. Bacterial colonization of single O. cf. ovata cells was generally low (mean < 2 bacteria, with a range from 0 to 5 bacteria), whereas higher rates of colonization were observed for detrital particles. The bacterial community was fractionated to observe more subtle changes within each partition. Briefly, seawater samples were first pre-filtered using 11 µm nylon filters, which retained suspended particles and attached microbes; then free-living bacteria in the flow through were collected on 0.2 µm filters. Macrophytes detached aggregates were gently centrifuged. Pellets containing bacteria attached to organic matter and microalgaes were harvested, while bacteria in the supernatant was partitioned as described for seawaters. Bacterial community structure was compared using automated rRNA intergenic spacer analysis (ARISA). Among the 178 operational taxonomic units (OTUs) detected, only 11 were present in all samples. Unique OTUs were mainly observed in relation with the solid fraction of aggregates and algal single cells. Jaccard similarity based analysis showed large dissimilarity between free-living bacteria in seawater and pellet-associated bacteria, either within or between samplings. Low similarity values were also reported among OTUs detected in isolated single O.cf. ovata cells, even within each sampling (mean 1st set = 0.15; mean 2nd set = 0.13, mean between sets = 0.12). While differences in specific OTUs were observed between the free-living bacteria community and that associated with organic matter, the two partitions were not significantly different by analysis of similarities (ANOSIM). However, our analysis shows significant differences in the microbial consortia associated with single O.cf. ovata cells than community associated with other sample partitions.