Photosynthesis: Exclusive to Plants?

Photosynthesis: Exclusive to plants? Over the past 4 years, there have been findings of processes similar to photosynthesis in non-plant organisms. In November of 2008, it was discovered that Elysia chlorotica (green sea slug) was able to live off of solar power.

This doesn't provide evidence for the existence of photosynthesis in non-plants; however, it is part of a trend I have started to follow dealing with photosynthesis-like processes that are occurring in organisms.

The abstract of the paper basically states that the slug aquires plastids through ingestion of Alegae. The organelles it acquires are taken into the digestive epithelium. They photosynthesize there for months even without a nucleocytoplasm which is where the confusion kicks in. These slugs, are able to steal plastids from Algae and live off its photosynthesizing for week even WITHOUT the algal cytoplasm.

--> Plastid Metabolism is dependent on the nuclear genome for the proteins. So this is where it becomes a puzzle and the paper provides 2 explanations for the ability of photosynthesis to take place in these slugs

1. The plastids are able to survive without the nucleocytoplasm and are retain their autonomy. 
2. The molluscs provide the plastids with the proteins it needs. 

The paper goes on to argue for the second point, that if it is true, then it means genes that are required for hte process of photosynthesis would have been aquired by animals with a horizontal gene transfer. It also means that the already encoded proteins are retargeted to the plastids.

I realized that this process sounded incredibly similar to Phagocytosis, a concept we learned about a few chapters back. It is basically a process where the cell devours particles with its membrane to form a phaosome. Phagocytes and protists use this process. This is a form of endocytosis which is what we studied in particular. The  main function for phagocytosis is for removing pathogens, but it seems as if these snails are using this process to acquire a photosynthesis fueled system. It marvels me how these processes can be used   interchangeably and can be connected just through a process that was used for other primary functions! The process of 'eating' is only available with single-celled organisms, but in multi-cellular animals, the process is only used for eliminating debris and pathogens. It is amazing how this snail is able to use it as fuel for cellular processes.

When they sequenced the whole plastid genome, they found that it lacked the required genes for photosynthesis but:

 In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3′ flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty (“green animals”) and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.

 This http://www.pnas.org/content/105/46/17867 http://www.newscientist.com/article/dn16124-solarpowered-sea-slug-harnesses-stolen-plant-genes-.