A Tale of Two Flagella is written by Olivia Judson, one of the best science writers there is.

Dinoflagellates are single-celled organisms that make coral reefs possible; they have a symbiotic–mutually beneficial–relationship with corals that make corals grow faster. Corals with certain kinds of dinoflagellates can cope with warmer water better than corals without them.

But other dinoflagellates are bad news. Ciguatera, a nasty form of poison found in some fish, and parlytic shellfish poisoning, a nasty form of poison found in some shellfish, both are caused by dinoflagellates. Red tides (in which massive numbers of dinoflagellates accumulate rapidly in one area) kill many animals, including dolphins, turtles, and other endangered species.

Dinoflagellates are weird; they can photosynthesize, despite being mobile cells; they have taken over the chloroplasts (the sun-utilizing bodies within the cell) of other organisms. And their DNA is unusual too.

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