Carboxysomes and peptidoglycan walls of cyanelles: possible physiological functions

Abstract

The cyanelles of glaucocystophyte (= glaucophyte) algae and the thecate amoeba Paulinella were previously thought to be endosymbiotic cyanobacteria or intermediates in the evolution of plastids from endosymbiotic cyanobacteria. While the more recent determinations of the number and nature of genes in the cyanelle genome show that cyanelles are genetically plastids, they have retained cyanobacterial features which are never (peptidoglycan walls) or only very rarely (carboxysomes) found in other plastids. The hypothesis presented here relates the peptidoglycan wall and the function of carboxysomes to the involvement of active water efflux using contractile vacuoles or their functional equivalent in volume regulation of non- cyanelle compartments of the cell in at least some parts of the life cycle. The peptidoglycan wall could permit a relatively high intracyanelle osmolarity which is presumably needed for cyanelle growth. A high intracyanelle osmolarity permits the high inorganic carbon concentration in the cyanelle during photosynthesis which is required for a carboxysome-based inorganic carbon concentration mechanism. The occurrence of a contractile-vacuole-based mechanism of volume regulation in at least part of the life cycle argues on energetic grounds for a low osmolarity of the cytosol, with a consequent lack of potential for a high inorganic carbon concentration in the cell as a whole. These predictions are susceptible to experimental test.