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, photosynthetic performance, organic carbon release and growth. Light absorption showed a wavelength-specific increase, especially in the acclimation to the blue part of the spectrum. The total amount of absorbed photons was similar for blue and white light and much higher than red light treatment at the end of the experiment. The optimal quantum yield (Fv/Fm) was not affected by the treatments but under blue light photosynthetic race (measured as O2 evolution) was always lower in comparison with white and red light, despite the increase in absorption and photosynthesis observed after 14 days of acclimation to blue light. The operational quantum requirement for O2 evolution (QR′) was 41 mol absorbed photons mol−1 O2 in blue light, almost double that of the QR′ values for white and red light. The uncoupling between absorption and oxygen production seems to be located in the water hydrolysis step, since only 28% of the excitons reaching the reaction centres promoted water hydrolysis. This is probably caused by a little overlap of photosystems II and I under blue light. However, differences in photosynthesis 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 efficiency in the investment of the assimilated carbon into new biomass is pointed out, since blue light-grown thalli showed high rates of organic carbon release to the external medium (up to 63% of assimilated C, disregarding respiration), while for white and red light-grown thalli the organic carbon release accounted for about 30%. The total amount of heterosides (floridoside, L-isofloridoside and D-isofloridoside) accumulated in the cell was 122 μmol g−1 dry wt. under blue light, and 385 and 447 under white and red light, respectively. These concentrations correlated with growth race. As observed with the results, the effect of white light was a combination of blue 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 band, and is mediated by changes in the accumulation of the main photosynthetic products, namely the heterosides.