PHOTOAUTOTROPHIC GROWTH OF A RECENTLY ISOLATED N2‐FIXING MARINE NON‐HETEROCYSTOUS FILAMENTOUS CYANOBACTERIUM, SYMPLOCA SP. (CYANOBACTERIA)

Abstract

Photoautotrophic growth of a marine non‐heterocystous filamentous cyanobacterium, Symploca sp. strain S84, was examined under nitrate‐assimilating and N2‐fixing conditions. Under continuous light, photon flux density of 55 μmol photons·m−2 ·s−1 was at a saturating level for growth, and light did not inhibit the growth rate under N2‐fixing conditions even when the photon flux density was doubled (110 μmol photons·m−2 ·s−1). Doubling times of the N2‐fixing cultures under 55 and 110 μmol photons·m−2 ·s−1 were about 30 and 31 h, respectively. Under 110 μmol 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 N2‐fixing culture was also about 30 h. When grown diazotrophically under a 12:12‐h L:D regime, C2H2 reduction activity was observed mainly during darkness. In continuous light, relatively large cyclic fluctuations in C2H2 reduction were observed during growth. The short‐term (<4 h) effect of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU; 5 μM) indicated that C2H2 reduction activity was not influenced by photosynthetic O2 evolution. Long‐term (24 h) effects of DCMU indicated that photosynthesis and C2H2 reduction activity occur simultaneously. These results indicate 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 regime.