Light quality effect on photosynthesis and efficiency of carbon assimilation in the red alga Porphyra leucosticta

The long-term effects of white, blue and red light on carbon metabolism of Porphyra leucosticta have been studied in relation to light absorption, pho tosynthetic performance, organic carbon release and growth. Light absorptio n showed a wavelength-specific increase, especially in the acclimation to t he blue parr of the spectrum. The total amount of absorbed photons was simi lar for blue and white light and much higher than red light treatment at th e end of the experiment. The optimal quantum yield (F-v/F-m) was not affect ed by the treatments but under blue light photosynthetic rate (measured as O-2 evolution) was always lower in comparison with white and red light, des pite the increase in absorption and photosynthesis observed after 14 days o f acclimation to blue light. The operational quantum requirement for O-2 ev olution (QR') was 41 mol absorbed photons mol(-1) O-2 in blue light, almost double chat of the QR' values for white and red light. The uncoupling betw een absorption and oxygen production seems to be located in the water hydro lysis step, since only 28 % of the excitons reaching the reaction centres p romoted water hydrolysis. This is probably caused by a little overlap of ph otosystems II and I under blue light. However, differences in photosynthesi s were not enough to explain the observed growth rates. Biomass production under blue light was much lower than under white and red light. A low effic iency in the investment of the assimilated carbon into new biomass is point ed out, since blue light-grown thalli showed high rates of organic carbon r elease to the external medium (up to 63 % of assimilated C, disregarding re spiration), while for white and red light-grown thalli the organic carbon r elease accounted for about 30 %. The total amount of heterosides (floridosi de, L-isofloridoside and D-isofloridoside) accumulated in the cell was 122 mu mol g(-1) dry wt. under blue light, and 385 and 447 under white and red light, respectively These concentrations correlated with growth rate. As ob served with the results, the effect of white light was a combination of blu e and red light. We suggest that the effect of light quality on the growth rate of P. leucosticta involves both the photosynthetic performance and the capability to maintain assimilated C inside the cell under each spectral b and, and is mediated by changes in the accumulation of the main photosynthe tic products, namely the heterosides.