Chloroplast Ion and Metabolite Transport in Algae

In this chapter we review the current knowledge about channels and transporters in the envelope and thylakoid membranes of chloroplasts from algae. These proteins mediate the exchange of ions and metabolites between the cytosol and the chloroplast stroma and between the different chloroplast sub-compartments. In secondarily-evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Most chloroplast transporters have been identified in the green alga Chlamydomonas reinhardtii, residing in the envelope membranes, and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play roles in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and their roles in the chloroplast are proposed. A greater knowledge in this important field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.