| Abstract: | The effects of photon flux density (PFD) and spectral quality on biomass, pigment content and composition, and the photosynthetic activity of Oscillatoria agardhii Gomont were investigated in steady-state populations. For alterations of PFD, chemostat populations were exposed to 50, 130 and 230 μmol photons·m?2·s?1 of photosynthetic active radiation (PAR). Decreases in biomass, chlorophyll a (Chl a) and c-phycocyanin (CPC) contents, and CPC: Chl a and CPC: carotenoid content was not altered. Increases in the relative abundances of myxoxanthophyll and zeaxanthin and deceases in the relative abundances of echinenone and β-carotene within the carotenoid pigments coincided with increasing PFD. Increases in Chl a-specific photosynthetic rates and maxima and decreases in biomass-specific photosynthetic rates and maxima with increasing PFD were attributed to increased light harvesting by carotenoids per unit Chl a and reduction in total pigment content, respectively. Responses to spectral quality were tested by exposing chemostat populations to a gradient of spectral transmissions at 50 μmol photons·m?2·s?1 PAR. Biomass differences among populations were likely attributable to the distinct absorption of the PAR spectrum by Chl a, CPC, and carotenoids. Although pigment contents were not altered by spectral quality, relative abundances of zeaxanthin and echinenone in the carotenoid pigments increased in populations exposed to high-wavelength PAR. The population adapted to green light possessed a greater photosynthetic maximum than populations adapted to other spectral qualities. |
