COMPARISON OF THE LIGHT-LIMITED GROWTH OF THE NITROGEN-FIXING CYANOBACTERIA ANABAENA AND APHANIZOMENON

The effect of simultaneous N-2 fixation and light limitation on the gr owth of two strains of Anabaena sp. Bory de St. Vincent and Aphanizome non flos-aquae (L.) Ralfs was investigated using continuous cultures. Under severely light-limited conditions, Aphanizomenon showed a broade r absorption spectrum (due to the presence of phycoerythrin), a higher maximum efficiency of photosynthesis, a higher steady-state N-2 fixat ion activity and a higher growth affinity for light than did Anabaena. On the other hand, under light saturation, Anabaena showed a higher m aximum rate of O-2 production and a higher maximum specific growth rat e than Aphanizomenon. These monoculture results characterize Anabaena and Aphanizomenon, in relative terms, as a 'sun' and a 'shade' species respectively, and are in accordance with field observations. The diff erence between the two species in their acclimatory response is discus sed in terms of a species-specific alteration of the PSI:PSII stoichio metry. Besides the species-specific modulation of the accessory pigmen ts, such an acclimation would provide a biochemical basis for the obse rved physiological differences. The monoculture results were used to d ifferentiate the niches of the two species and suggested that Aphanizo menon would competitively displace Anabaena under N-2-fixing, light-li mited conditions. However, when both species were grown together, Anab aena became dominant and seemed to be the superior competitor for ligh t. In order to explain this finding, the possible effects of release o f allelopathic compounds, or dynamic aspects of light supply, are disc ussed.