Division cycles in the nitrogen-fixingRhizosolenia(Bacillariophyceae)-Richelia(Nostocaceae) symbiosis

Abstract

The oceanic nitrogen-fixing Rhizosolenia clevei-Richelia intracellularis symbiosis was studied in laboratory culture. Division cycles of the host and symbiont were documented, and the temporal sequence of host and symbiont division recorded. Rhizosolenia division was phased, with cytokinesis occurring during a restricted portion of the late dark and early light period. Richelia division, as measured by frequency of migrating trichomes, occurred at a constant rate throughout the light-dark cycle. Asymbiotic Rhizosolenia appeared to be the inevitable consequence of this uncoupled division periodicity in the relatively small host cells studied. Larger Rhizosolenia cells containing more symbionts would probably be less likely to give rise to asymbiotic cells; however, the factors regulating symbiont distribution appear to be more complex than a simple size-growth rate relationship. Trichomes escaped from moribund Rhizosolenia by direct penetration through the frustule. Colonization of asymbiotic Rhizosolenia was not observed, but direct transfer of the symbiont during vegetative cell enlargment or auxosporulation appears to be the most likely process for propagating the symbiosis.