Plastid Structure, Diversification and Interconversions I. Algae

Katalin      Solymosi

Abstract

During evolution, different heterotrophic organisms have acquired photosynthetic prokaryotes to transform them into their „tiny green slaves”, the chloroplasts. During endosymbiogenesis plastids have partially lost their independence, and tight co-regulation between the host nucleus and the organelle has been developed. In this work, the general features and the special characteristics (such as plastid morphology, number of plastid bounding membranes, periplastidial space and nucleomorph, thylakoid arrangement, plastid genome, plastid-located storage materials, carboxysomes and pyrenoids, plastoglobuli and eyespots, and other plastid inclusions) of the chloroplasts of the most important algal groups are reviewed in details. Several unicellular algae possess only (one or a few) chloroplast(s). In more complex organisms, such as for instance brown algal thalli and land plants, plastid form and function have been diversified and plastids got gradually specialized in parallel with the evolution of cells with different specific functions. Cells that have lost their photosynthetic activity developed special plastid types with less or no chlorophyllous pigments and thylakoids, but with specific functions such as storage (leucoplasts) or carotenoid synthesis (chromoplasts). In other cells (e.g. in dividing regions) proplastids can be found with poorly developed thylakoid system. In algae, plastid diversification and the interconversion of the different plastid types into each other are discussed as a consequence of (i) changes in the trophic mode of the algae, (ii) increasing complexity of the vegetative body, (iii) increasing complexity of the life cycle, (iv) senescence processes and (v) changes induced in plastid structure by environmental stimuli.

Keywords: Algae, chloroplast, plastid differentiation, plastid interconversions, plastid types, thylakoids, eukaryotic algae, proteobacteria, protoeukaryotes, kleptoplasts