Light-Harvesting Systems in Algae

Abstract

Light harvesting in algae is very diverse and reflects the broad spectrum of organisms involved and their long history of evolution compared to higher plants. This chapter concentrates on the chlorophylls (Chls) and their binding proteins, as they are the major photosynthetic pigments. Three chlorophylls occur in the algae, Chl a, Chl b and Chl c, attached to light-harvesting proteins, mainly in the CAB/CAC family. In classical Cyanobacteria typical CAB/CAC proteins are not found and the only member of the Chl c family present is Mg divinyl-2,4- pheoporphyrin methyl ester (MgDVP). In Cyanobacteria the light-harvesting proteins are typically phycobiliproteins (under nitrogen sufficient conditions) and isiA proteins (under nitrogen limiting conditions). In prochlorophytes and Acaryochoris marina, which are Cyanobacteria, a prochlorophyte chlorophyll binding protein (pcb protein) binds Chl a and Chl b, and sometimes MgDVP, or Chl d. The binding of Chl in these proteins and in other antenna proteins is discussed. These proteins serve to optimize energy distribution to the two photosystems, with controls at several levels of organization. A major problem in all oxygenic photosynthetic organisms (Cyanobacteria, algae and higher plants) is the generation of oxygen free radicals, particularly by Photosystem II. This leads to photoinhibitory damage, which is partially offset by mechanisms which down- regulate photosynthesis, particularly Photosystem II, and dissipate incoming energy as heat. The xanthophyll cycle is found in all algae, with the possible exception of red algae and cryptophytes, and, by processes which are only partially known, diverts light energy to heat energy when switched on. Algae can control their uptake of light energy in a variety of ways: by physiological mechanisms and by regulation of transcription and translation of proteins. These responses can be to both light quality and light quantity. Algae show a wide range of rearrangements of the light harvesting apparatus in relation to the photosystems, known as state transitions, which alter the optical cross-sectional areas of PS I and PS II.