Not all of us, me included, are fully aware that the algae include pathogens of many vertebrates, such as humans and domesticated animals. The most important ones are Prototheca bovis (once P. zopfii ) and P. wickerhamii. A fine overview of this subject by Shave, Millyard and May from the University of Birmingham, United Kingdom, has been recently published. It carries the most appropriate title: "And Now for Something Completely Different: Prototheca, Pathogenic Algae."
The protothecae are non-photosynthesizers, although some photosynthesizing algae (Chlorella and Desmodesmus) have also been found to be pathogenic. They commonly produce in cattle a mastitis that reduces milk yield and is not receptive to treatment. Human infections, though relatively rare, are most likely to be acquired from wounds. Rare or not, they can be fatal and be difficult to diagnose.
Right now, we don't know much about the mechanism of pathogenesis of these algae. Among other things, they seem to be quite resistant to phagocytosis. The authors consider various possibilities for the pathogenic mechanisms involved. In a section of their review entitled "What do we still not know?" they conclude: "Quite a lot." Given their importance in human and domestic animal medicine, we can only hope that our knowledge of these unexpected agent will improve.