Laboratory Culture and Preliminary Characterization of the Nitrogen‐Fixing Rhizosolenia‐Richelia Symbiosis

Abstract.The oceanic diatomRhizosolenia cleveiand its cyanobactcrial symbiontRichelia intracel‐luluriswere isolated into laboratory culture and their biology and growth dynamics examined. Nitrogen‐fixation by the symbiont could sustain the symbiosis. Growth and nitrogenase activity demonstrated light saturation kinetics, with no photoinhibition up to 315 μE m‐1s‐1for growth and 780μE m‐1s‐1for nitrogen fixation. The symbiosis is not obligate forRhizosolenia, which is capable of growth independent of the symbiont if a nitrogen source is available. The symbiont is contained in the pcriplasmic space between theRhizosoleniaplasmalcmma and frustulc, and preliminary evidence suggests excretion of fixed nitrogen into the medium may be occurring.