Among diverse symbiotic relationships between two organisms, the ecophysiological features of, in particular, the cyanobacterial symbiont (cyanobiont)-dinoflagellate host consortia at the level of a unicellular organism remain poorly understood. We studied the genetic diversity, host or cyanobiont specificity, and cyanobionts seasonality of the dinoflagellate Ornithocercus cyanobionts.