Photoacclimation of photosynthesis irradiance response curves and photosynthetic pigments in microalgae and cyanobacteria

The photosynthesis-irradiance response (PE) curve, in which mass-specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light-limited rate to chl a yields the chl. a-specific initial slope (alpha(chl)). This is proportional to the light absorption coefficient (alpha(chl)), the proportionality factor being the photon efficiency of photosynthesis (phi(m)). Thus, alpha(chl) is the product of alpha(chl) and phi(m). In microalgae achl typically shows little (<20%) phenotypic variability because declines of phi(m) under conditions of high-light stress are accompanied by increases of achl. The variation of alpha(chl) among species is dominated by changes in achl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, alpha(chl) declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light-saturated photosynthesis (P-m) is limited by a factor other than the rate of light absorption. Normalizing P-m to organic carbon concentration to obtain P-m(C) allows a direct comparison with growth rates. Within species, P-m(C) is independent of growth irradiance. Among species, P-m(C) covaries with the resource-saturated growth rate. The chl a:C ratio is a key physiological variable because the appropriate currencies for normalizing light-limited and light-saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a:C is reduced to a out 40% of its maximum value at an irradiance that supports 50% of the species-specific maximum growth rate and light-harvesting accessory pigments show similar or greater declines. In the steady state, this down-regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light-limited region for growth. The reason for down-regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.