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Nostocacean cyanobacteria typically produce gliding filaments termed hormogonia at a low frequency as part of their life cycle. We report here that allNostoc spp. competent in establishing a symbiotic association with the hornwortAnthoceros punctatus formed hormogonial filaments at a high frequency in the presence ofA. punctatus . The hormogonia-inducing activity was produced byA. punctatus under nitrogen-limited culture conditions. The hormogonia of the symbiotically competentNostoc spp. were characterized as motile (gliding) filaments lacking heterocysts and with distinctly smaller cells than those of vegetative filaments; the small cells resulted from a continuation of cell division uncoupled from biomass increase. An essentially complete conversion of vegetative filaments to hormogonia occurred within 12 h of exposure ofNostoc sp. strain 7801 toA. punctatus growth-conditioned medium. Hormogonia formation was accompanied by loss of nitrogen fixation (acetylene reduction) and by decreases in photosynthetic CO2 fixation and in vivo NH4 + assimilation of 30% and approximately 40%, respectively. The rates of acetylene reduction and CO2 fixation returned to approximately the control rates within 72 to 96 h after hormogonia induction, as the cultures ofNostoc sp. strain 7801 differentiated heterocysts and reverted to the vegetative growth state. The relationship between hormogonia formation and symbiotic competence is discussed.

applied and environmental microbiology

Characteristics of Hormogonia Formation by Symbiotic <i>Nostoc</i> spp. in Response to the Presence of <i>Anthoceros punctatus</i> or Its Extracellular Products

Characteristics of Hormogonia Formation by Symbiotic Nostoc spp. in Response to the Presence of Anthoceros punctatus or Its Extracellular Products