Evaluation of nitrogen fixation in the diatom genus Rhizosolenia Ehr. in the absence of its cyanobacterial symbiont Richelia intracellularis Schmidt

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Abstract
Aggregates of two species of Rhizosolenia (R. castracanei and R. imbricata var. shrubsolei) have been reported to contain endosymbiotic bacteria and to fix nitrogen. The general importance of this process to the genus is not known, although of great potential significance. Clonal cultures of five species of Rhizosolenia (R. alata, R. bergonii, R. calcar-avis, R. imbricata var. shrubsolei and R. setigera) were examined for nitrogen fixation using acetylene reduction and compared to cyanobacterial controls. Mecososm aggregates of R. imbricata var. shrubsolei were tested, as well as chains of R. debyana hand-collected using SCUBA. The Rhizosolenia species did not contain the endosymbiotic cyanobacterium Richelia intracellularis and were not examined for the presence of endosymbiotic bacteria. Nitrogen fixation was not found in the Rhizosolenia spp.; in contrast, the diazotrophic cyanobacterial controls Oscillatoria erythraea and Anabaena sp. reduced acetylene at significant rates. The absence of nitrogen fixation in the Rhizosolenia spp. suggests diazotrophy by endosymbiotic bacteria is not widespread in the genus Rhizosolenia, nor is it necessary for survival of these large diatom cells in nutrient-poor, near-shore waters.

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