Photoautotrophic growth of a recently isolated N-2-fixing marine non-heterocystous filamentous cyanobacterium, Symploca sp (Cyanobacteria)

Photoautotrophic growth of a marine non-heterocystous filamentous cyanobact erium, Symploca sp. strain S84, was examined under nitrate-assimilating and N-2-fixing conditions. Under continuous light, photon flux density of 55 m u mol photons .m(-2).s(-1) was at a saturating level for growth, and light did not inhibit the growth rate under N-2-fixing conditions even when the p hoton flux density was doubled (110 mu mol photons .m(-2).s(-1)). Doubling times of the N-2-fixing cultures under 55 and 110 mu mol photons .m(-2).s(- 1) were about 30 and 31 h, respectively. Under 110 mu mol photons .m(-2).s( -1) during the light phase of an alternating 12:12-h light:dark (L:D) cycle , the doubling time of the N-2-fixing culture was also about 30 h. When gro wn diazotrophically under a 12:12-h L:D regime, C2H2 reduction activity was observed mainly during darkness. In continuous light, relatively large cyc lic fluctuations in C2H2 reduction were observed during growth. The short-t erm (<4 h) effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU; 5 muM) indicated that C2H2 reduction activity was not influenced by photosynthetic O-2 evolution. Long-term (24 h) effects of DCMU indicated that photosynthe sis and C2H2 reduction activity occur simultaneously. These results indicat e that strain S84 grows well under diazotrophic conditions when saturating light is supplied either continuously or under a 12:12-h L:D diel light reg ime.