ULTRASTRUCTURAL CHARACTERIZATION OF CELLS SPECIALIZED FOR NITROGEN-FIXATION IN A NONHETEROCYSTOUS CYANOBACTERIUM, TRICHODESMIUM SPP

Trichodesmium is the first described example of a filamentous cyanobac terium without heterocysts that contains cells specialised for nitroge n fixation. The ultrastructure of cells with and without nitrogenase w ere compared using primarily Trichodesmium tenue Wille, but also T. th iebautii Gomont and T. erythraeum Ehrenberg et Gomont. Immunohistochem istry demonstrated that the cytoplasm of certain cells was densely lab elled with antibodies against Fe-protein (dinitrogenase reductase). Co mparative TEM-image analysis revealed that these cells were also disti nguished by a denser thylakoid network. dividing the vacuole-like spac e into smaller units. The nitrogenase-containing cells also exhibited less extensive gas vacuoles as well as fewer and smaller cyanophycin g ranules compared to cells which lacked nitrogenase. Carboxysomes were present in both cell types in equal proportion. Longitudinal sections showed that cells with nitrogenase were arranged adjacent to each othe r, and that groups of cells with and without nitrogenase may coexist i n the same trichome. The correlation between modifications in ultrastr ucture and the presence of nitrogenase suggests a new type of cyanobac terial cell specialisation related to nitrogen fixation. The results o btained also question the systematic affiliation of the genus Trichode smium.