Adaptation, Acclimation and Regulation in Algal Photosynthesis

Summary

Relating the photosynthetic processes of algae to their environments requires that the responses are considered over time-scales of seconds to minutes (regulation), hours to days (acclimation) and up to thousands of millions of years (adaptation). All of these 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 of these genetic 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 of such acclimation, but also of the rate at which acclimation occurs. Regulation involves allosteric or covalent modification of pre-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.