Martin, R., Walther, P., & Tomaschko, K. H. 2013. Phagocytosis of algal chloroplasts by digestive gland cells in the photosynthesis-capable slug Elysia timida (Mollusca, Opisthobranchia, Sacoglossa). Zoomorphology 132: 253-259.
Members within genus Elysia are mysterious due to their incredible ability to select chloroplasts from ingested algae and incorporate these plastids into their own body - a process called kleptoplasty. These newly acquired chloroplasts (kleptoplasts) remain active within the host, allowing the organism to carry out photosynthesis temporarily. However, the actual mechanisms by which Elysia sequester chloroplasts are unclear.
Phagocytosis has been a popular suggestion, but previous studies have yet to illustrate clear evidence of a phagosome membrane surrounding the kleptoplasts. Most just appear to be directly exposed to host cytoplasm. In this study, the authors attempt to trace the membrane origins surrounding kleptoplasts present in the digestive tract of Elysia timida. They used high-pressure cryofixation, which allows for better preservation of membranes.
Prior to cryofixation, the specimens of E. timida were fed algae. Then pieces of their parapodia were frozen in liquid nitrogen under immense pressure. Fixatives were added to stabilize the ultrastructure of specimens and samples were stained for electron microscopy. Algal chloroplasts getting phagocytised into the digestive cells of E. timida were observed. Unexpectedly, only one membrane was observed surrounding the thylakoid stacks, presumed to be the phagosome.
The mechanism is now clear that Elysia sequester chloroplasts via phagocytosis; however the notion that chloroplasts are incorporated as "nude thylakoid stacks" is perplexing. There are protein-transport systems present in outer/inner membrane of chloroplasts that import proteins essential to photosynthesis from the cytoplasm. One now has to wonder how Elysia can sustain long-term photosynthesis without these mechanisms.
(250 words)
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(posted Jan. 4 2016)