Light on Chromera

Electron microscopy of the Chromera photosynthetic organ, or plastid, revealed it is enclosed by four membranes - the same arrangement that Geoff McFadden found in the Plasmodium apicoplast.

The chromera plastid is pigmented solely by chlorophyll a. Carter suggests the absence of chlorophyll b, found in all higher plants and most marine algae, indicates Chromera is vulnerable to light stress, and would be killed by bright sunlight near the surface - consistent with it living in a protected environment where it is exposed only to diffuse sunlight.

The Chromera DNA sequence also has a very unusual feature. Nearly all higher plants and marine algae use the codon UGG to specify a tryptophan residue. Chromera uses UGA - a feature unique to Apicomplexa, including Plasmodium.

The combination of these two characters suggests Chromera is a photosynthetic cousin of apicomplexan parasites, which have dispensed with chlorophyll altogether, but retain a degenerate plastid, the apicoplast.

Chromera and Plasmodium are both members of a recently recognised super-group of protists, the Alveolata, which incorporates foraminerifa, ciliates, dinoflagellates and Apicomplexa.

With the discovery of Chromera, the mists around the origins of Plasmodium have begun to lift.

On this evidence, Carter and her colleagues place Chromera somewhere between Apicomplexa and dinoflagellates. More than a decade ago, Geoff McFadden guessed that Plasmodium had descended from a red alga, probably a dinoflagellate.

Not all dinoflagellates are comfortably settled in symbiotic relationships. Free-living species flourish in the shallows of the world's oceans and seas, and are notorious as the toxic agents of so-called "red tides" that episodically poison fish and oysters.

People who eat have dined on predatory fish from tropical reefs in the wake of natural disturbances like cyclones have sometimes developed a bizarre food-borne illness called ciguatera, that reverses the sensations of hot and cold.

Predatory fish like barracuda, Spanish mackerel and grouper sit at the apices of marine food webs that concentrate ciguatoxin from the toxic dinoflagellate Gambierdiscus.

The uniquely toxic dinoflagellate Pfiesteria thrives in polluted seawater, and has caused massive fish kills in polluted rivers and estuaries in the north-eastern US.

Pfiesteria emits toxic molecules into water and air that have caused skin rashes and chronic mental confusion and memory loss in fishermen, and in scientists who unwisely cultured the so-called "Cell from Hell" in aquaria in poorly ventilated laboratories.