Bacterial diversity of Gymnodinium catenatum and its relationship to dinoflagellate toxicity

Gymnodinium catenatum Graham (Dinophyceae) is one of several marine dinoflagellates responsible for outbreaks of paralytic shellfish poisoning (PSP), a problem that is considered to be increasing globally. Bacteria associated with these dinoflagellates have been implicated as potentially involved with the production of PSP toxins, and this study sought to identify whether there was a link between the toxicity of G. catenatum laboratory cultures and the diversity of the associated bacterial community. Bacterial 16S rRNA gene clone libraries were constructed and sequenced to identify the bacterial diversity of 7 G. catenatum cultures of 2 contrasting toxicity levels. Phylogenetic membership and community structure were examined, including the use of UniFrac, FST and LIBCOMPARE. No statistically significant differences that distinguished between toxic and low-toxicity G. catenatum cultures were identified in the bacterial community membership or structure. Furthermore, no coherent phylogenetic group of bacteria was observed to co-associate with culture toxicity. However, observed variation in bacterial diversity and community structure was based on the geographic origin of the G. catenatum cultures. Overall, while it was not possible to identify an apparent link between bacterial diversity and the toxicity of G. catenatum cultures, we suggest, on the balance of this study and others, that bacterial influence on PSP toxin production may be indirect and mediated by the effects of the bacterial community on algal physiology.