GROWTH,PHYSIOLOGY, AND ULTRASTRUCTURE OF A DIAZOTROPHIC CYANOBACTERIUM,CYANOTHECE SP. STRAIN ATCC 51142, IN MIXOTROPHIC AND CHEMOHETEROTROPHIC CULTURES1

The growth, physiology, and ultrastructure of the marine, unicellular, diazotrophic cyanobacterium, Cyanothece sp. strain ATCC 51142, was examined under mixotrophic and chemoheterotrophic conditions. Several organic substrates were tested for the capacity to support heterotrophic growth. Glycerol was the only substrate capable of enhancing mixotrophic growth in the light and supporting chemoheterotrophic growth in the dark. Dextrose enhanced mixotrophic growth but could not support chemoheterotrophic growth. Chemoheterotrophic cultures in continuous darkness grew faster and to higher densities than photoautotrophic cultures, thus demonstrating the great respiratory capacity of this cyanobacterial strain. Only small differences in the pigment content and ultrastructure of the heterotrophic strains were observed in comparison to photoautotrophic control strains. The chemoheterotrophic strain grown in continuous darkness and the mixotrophic strain grown in light/dark cycles exhibited daily metabolic oscillations in N₂ fixation and glycogen accumulation similar to those manifested in photoautotrophic cultures grown in light/dark cycles or continuous light. This “temporal separation” helps protect O₂-sensitive N₂ fixation from photosynthetic O₂ evolution. The rationale for cyclic glycogen accumulation in cultures with an ample source of organic carbon substrate is unclear, but the observation of similar daily rhythmicities in cultures grown in light/dark cycles, continuous light, and continuous dark suggests an underlying circadian mechanism.     

GROWTH AND NITROGEN FIXATION OF THE DIAZOTROPHIC FILAMENTOUS NONHETEROCYSTOUS CYANOBACTERIUM TRICHODESMIUM SP. IMS 101 IN DEFINED MEDIA: EVIDENCE FOR A CIRCADIAN RHYTHM1

Trichodesmium sp. IMS 101, originally isolated from coastal western Atlantic waters by Prufert-Bebout and colleagues and maintained in seawater-based media, was successfully cultivated in two artificial media. Its characteristics of growth, nitrogen fixation, and regulation of nitrogen fixation were compared to those of natural populations and Trichodesmium sp. NIBB 1067. Results indicate that the culture grown in artificial media had nitrogen fixation characteristics similar to those when the culture is grown in seawater-based medium and to those of Trichodesmium sp. in the natural habitat. The study provides practical artificial media to facilitate the physiological studies of these important diazotrophic cyanobacteria, as well as the cultivation of other Trichodesmium species in future studies. Manipulations of the light/dark cycle were performed to determine whether or not the daily cycle of nitrogen fixation is a circadian rhythm. Cultures grown under continuous light maintained the cycle for up to 6 days. We demonstrated that the daily cycle of nitrogen fixation in Trichodesmium sp. IMS 101 was at least partially under the control of a circardian rhythm.