Adaptation, Acclimation and Regulation in Algal Photosynthesis

Abstract

Relating the photosynthetic processes ofalgae to their environments requires that the responses are considered over time-scales ofseconds to minutes (regulation), hours to days (acclimation) and up to thousands ofmillions ofyears (adaptation). All ofthese responses are genetically determined, with so-called adaptations reflecting genetic changes which distinguish taxa from the strain (ecotype), i.e. infraspecific level, up to the Division (Phylum) level. Tempting as it is to assign the establishment ofthese genetie differences to natural selection, the genetic differences at the higher taxonomic levels should be related to the environments at the time at which they evolved. Genetic differences limit the responses that algal genotypes can make to their immediate environment (i.e. during a single generation). Using photosynthetic pigments as an example, the content per unit biomass of photosynthetic light-harvesting pigment-protein complexes, and, where they occur, of the energy-dissipating xanthophyll cycle pigments change with the photon flux density for growth; light-harvesting pigments decrease with increasing light, while xanthophyll cycle pigments increase. There are cost-benefit considerations not only in the extent ofsuch acclimation, but also ofthe rate at which acclimation occurs. Regulation involves allosteric or covalent modification ofpre-existing catalysts, e.g. ribulose bisphosphate carboxylase-oxygenase, xanthophyll cycle pigments, and the pigment-protein complexes involved in state transitions. Much remains to be done to not only understand adaptation, acclimation and regulation in algae, but also to understand how the three processes interact.