A Rhizobium bacterium and its population dynamics under different culture conditions of its associated toxic dinoflagellate Gambierdiscus balechii

Rhizobium bacteria are known as symbionts of legumes for developing nodules on plant roots and fixing N₂ for the host plants but unknown for associations with dinoflagellates. Here, we detected, isolated, and characterized a Rhizobium species from the marine toxic dinoflagellate Gambierdiscus culture. Its 16S rRNA gene (rDNA) is 99% identical to that of Rhizobium rosettiformans, and the affiliation is supported by the phylogenetic placement of its cell wall hydrolase -encoding gene (cwh). Using quantitative PCR of 16S rDNA and cwh, we found that the abundance of this bacterium increased during the late exponential growth phase of Gambierdiscus and under nitrogen limitation, suggesting potential physiological interactions between the dinoflagellate and the bacterium. This is the first report of dinoflagellate-associated Rhizobium bacterium, and its prevalence and ecological roles in dinoflagellate-Rhizobium relationships remain to be investigated in the future.