Then there's the genus Prototheca. They're spherical single-celled organisms lumped in with algae based on their genetic code, but are colourless since they don't do photosynthesis. They can also infect animals, which algae tend not to do. I'm reminded of Monotropa uniflora and Rhizanthella gardneri, two parasitic green-lacking plants that similarly can't use the sun's energy but nevertheless belong to a group that is typically defined based on this ability.
Under a microscope, some Prototheca cells will have a characteristic daisy-like appearance that is due to the presence of multiple structures in which spores are formed. When they aren't making spores, cells closely resemble yeasts (e.g. Candida).
|Prototheca algae (colourless, but have been dyed blue) infecting a person (Source)|
Species of Prototheca are widespread in nature, but tend to be associated with human activity. Among their weirder habitats are slime flux (a microbe-colonized solution that leaks from cracks or wounds in trees), sewage, and sediments contaminated with crude oil (all of which provide organic matter for them to eat). Since Prototheca are able to eat many of the hydrocarbons found in crude oil, they have been investigated for use in oil spill remediation. One approach involves embedding the algae in polyurethane foam, which apparently permits greater rates of oil degradation as well as the removal of particularly toxic polycyclic aromatic hydrocarbons (which are sopped up by the algae as it grows).
|Ooey gooey slime flux, likely Prototheca hangout (Source)|
Unlike bacteria, Prototheca can make vitamin C from glucose, via a pathway that resembles one found in plants. Current methods by which this vitamin is produced involve more than one step, so having a microorganism that can do the whole thing at once is potentially very useful. The algae has also been investigated as a possible means of producing biofuel.
One of the stranger characteristics of Prototheca is its ability to occasionally cause disease in humans and other animals (cows, dogs, cats). Human infection usually involves a traumatic inoculation, so it tends to get inside us via our extremities (e.g. scraping your hand on something the algae is hanging out on). The result is a skin infection (e.g. dermatitis that can resemble common fungal infections) and/or olecranon bursitis (elbow inflammation). If a person has a compromised immune system (e.g. due to cancer or organ transplant) the algae can spread throughout the body resulting in a general illness that can be fatal. A Prototheca infection can be difficult to treat, since we haven't exactly prioritized the development of anti-algae drugs. Some species are easily killed using antifungals (e.g. amphotericin B seems to work well), but others are resistant. Not surprisingly, resistance is associated with a poor prognosis. A possible line of defence is the use of antimicrobial peptides, which are currently being investigated for their drug potential.
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