Coral reef bleaching & malaria

http://www.afreeman.org/2008/02/28/great-interview-week-scientific-serendipity-in-sydney-2/

Chromera velia, a unicellular photosynthetic symbiotic brown algae found associated with coral reefs, which leaves when the reef water becomes too warm resulting in bleached coral, has been found to be related to malarial Plasmodium and other apicomplexan parasites like toxiplasmosis.

Apparently, these blood-borne parasites derive from coral cell symbiont alga, and still contain vestigial chloroplasts (chlorophyll compounds, like in tree leaves that construct tissue from hydrocarbons using sunlight energy), but now obtain energy from their animal hosts.

http://news.bbc.co.uk/2/hi/science/nature/6468381.stm

Now, the question remains, how did a salt-water-loving coral-symbiont-algae evolve into a freshwater-nymphal-mosquito carried vertebrate-blood parasite?
or... did it?