Characterization of a non-heterocystous subtropical marine cyanobacterium that produces a unique multicellular structure and facilitates dinitrogen fixation
A previously un-described filamentous non-heterocystous cyanobacterium was isolated from the South China Sea. Molecular phylogenetic analyses together with morphological observations suggested that this organism should be assigned a new specific epithet. It was designated as Leptolyngbya nodulosa Li et Brand. Filaments were enclosed in a sheath, and were unusual in their flattened appearance as seen in cross section and in the frequent occurrence of a void space (as seen by transmission electron microscopy) between the trichome and the sheath. A distinctive feature of L. nodulosa was the presence, under low light intensities, of previously un-described multicellular structures (nodules). A L. nodulosa nodule consists of a portion of a filament folded and twisted into a distinct unit surrounded by a firm continuous sheath. Nodules were highly variable in size and shape, and occurred at irregular intervals along the filament. They disappeared from filaments of cultures grown at relatively high light intensities. L. nodulosa cultures could be grown indefinitely in media devoid of any source of combined nitrogen. Acetylene reduction assays showed that L. nodulosa cultures fix dinitrogen in the dark period of a diurnal cycle under microoxygenic conditions. The addition of DCMU ([3-(3,4-dichlorophenyl)-1,1-dimethylurea], an inhibitor of Photosystem II) to a culture of L. nodulosa induced much higher rates of dinitrogenase activity and altered the cycle of activity such that most acetylene reduction occurred during the light. Measurements of dinitrogenase activity in the presence of chloramphenicol (an inhibitor of protein synthesis) indicated that dinitrogenase is synthesized in darkness and destroyed in the subsequent light period. In the presence of DCMU, a much higher dinitrogenase activity is measured, but in this case only in the light. Neither the dark-mediated dinitrogenase in the absence of DCMU nor the light-mediated activity in the presence of DCMU could be sustained for more than two days without a photoperiodic light/dark cycle. Dinitrogenase activity occurred only in non-axenic cultures of L. nodulosa. A single nifH gene, with an identical sequence in axenic and non-axenic cultures, was isolated. The requirement of heterotrophic bacteria for dinitrogenase activity in L. nodulosa is not yet understood.