The effect of simultaneous N2 fixation and light limitation
on the growth of two strains of Anabaena sp. Bory
de St. Vincent and Aphanizomenon flos-aquae (L.) Ralfs was
investigated using continuous cultures. Under
severely light-limited conditions, Aphanizomenon showed a
broader absorption spectrum (due to the presence of
phycoerythrin), a higher maximum efficiency of photosynthesis, a higher
steady-state N2 fixation activity and a
higher growth affinity for light than did Anabaena. On the other
hand, under light saturation, Anabaena showed
a higher maximum rate of O2 production and a higher maximum
specific growth rate 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 difference
between the two species in their
acclimatory response is discussed in terms of a species-specific alteration
of the PSI[ratio ]PSII stoichiometry. Besides
the species-specific modulation of the accessory pigments, such an
acclimation would provide a biochemical basis
for the observed physiological differences. The monoculture results
were used to differentiate the niches of the two
species and suggested that Aphanizomenon would competitively
displace Anabaena under N2-fixing, light-limited
conditions. However, when both species were grown together, Anabaena
became dominant and seemed to be the
superior competitor for light. In order to explain this finding, the possible
effects of release of allelopathic
compounds, or dynamic aspects of light supply, are discussed.