It is known that dinoflagellates acquired a plastid from a red alga via secondary endosymbiosis. However, several dinoflagellates should have abandoned their original plastid and acquired again plastids from different sources. Engulfing free-living algae, some dinoflagellates use them as a temporary plastid. This phenomenon is considered to be an on-going process of plastid acquisition, and this temporary plastid is called “kleptochloroplast”. In a probably monophyletic groups of unarmoured dinoflagellates, various stages of endosymbiosis, namely, reduction of endosymbiont's organelles except plastid, are present, which would be good research targets for understanding evolutionary process of plastid acquisition. Besides dinoflagellates, kleptochloroplasts are distributed in a wide range of protists. In this mini review, we introduce diversity of kleptochloroplasts in protists in particular reference to the dinoflagellates.

Each symbiotic Chlorella sp. of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole derived from the host digestive vacuole, thereby the alga is protected from digestion by the host lysosomal fusion. Algae-free paramecia and isolated symbiotic algae from algae-bearing cells still have an ability to live each other. Algae-free cells can be easily reinfected with the algae isolated from algae-bearing cells by ingestion into digestive vacuoles. The symbiotic associations of these eukaryotic cells are excellent models for studying the evolution of eukaryotic cells through the secondary endosymbiosis between protozoa and algae. However, the detailed algal infection process had been unclear. Using pulse-label of the alga-free paramecia with the isolated symbiotic algae and chase method, we clarified the timing of the algal infection process to the algae-free paramecia. Furthermore, four important cytological events needed to establish endosymbiosis were found: (1) some algae show resistance to the host lysosomal enzymes in the host digestive vacuole, (2) algal escape from the host digestive vacuole by budding of the digestive vacuole membrane, (3) differentiation of the host digestive vacuoles wrapping the green algae into the perialgal vacuole membrane to protect from lysosomal fusion, and (4) an attachment of the green algae wrapped by the perialgal membrane to beneath the host cell membrane. This algal infection process is different from those so far known infection processes in other symbiotic and parasitic organisms.

Almost all of oxygenic photosynthetic organisms contain sulfolipid, sulfoquinovosyl diacylglycerol (SQDG) in the photosynthetic membranes. The physiological functions of SQDG in the photosynthetic membranes such as the maintaining of structural and functional integrity of photosystem II are well investigated from cyanobacteria to seed plants. On the other hand, the molecular mechanism of SQDG metabolism is just now emerging. In this paper, the systems of SQDG-synthesis and -degradation were reviewed and the regulation of the system was referred. The system of SQDG degradation required de novo synthesis of mRNA(s) and protein(s) and the regulatory proteins for sulfur acclimation under sulfur-starved conditions. Additionally, biodiversity of the capacity of SQDG degradation induced by the sulfur starvation was surveyed among some organisms. From these results, it was implied that the system of SQDG degradation was derived not from the ancestral cyanobacteria, the origin of chloroplasts, but from the host cell of primary symbiosis and was conserved through the evolution of eukaryotic photosynthetic organisms.

It is widely believed that all known plastids originated from a single primary endosymbiosis in which a cyanobacterium was engulfed and retained by a heterotrophic protist. However, there is an interesting organism called Paulinella chromatophora that may change this widely accepted view.

P. chromatophora, a cercozoan protist, is a fresh water testate amoeba that contains two cyanobacterium-like structures called “cyanelles” in the cell. Past researches have failed to cultivate the cyanelles separately from the host cells and demonstrated that the cyanelles divided within the host cells and were handed over to daughter cells. In recent studies, it has been revealed that the cyanelle of P. chromatophora does not share a common ancestor with known plastids but originated from a cyanobacterium that belongs to the Synechococcus/Prochlorococcus lineage.

These situation led the idea that P. chromatophora represent the second example of the primary endosymbiosis that is in progress. Further study on the symbiotic relationship between the cyanelles and the host seen in this organism would provide important insight for the mechanism of primary plastid acquisition.