14.6D: Algae

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Algae can act as pathogens like any other microbe.

Learning Objectives

Discuss the various types of pathogenic algae

Key Points

While algal blooms can lead to negative consequences, the effect of an algal bloom are often indirect, the alga is not directly infecting a host.
Cephaleuros are a genus of parasitic alga which infect plants, causing red rust, which affects many commercial crops that humans consume.
Prototheca are a type of green alga that lack chlorophyll, that can infect mammals including humans causing the disease protothecosis.
To some degree the distribution of algae is subject to floristic discontinuities caused by geographical features, such as Antarctica, long distances of ocean, or general land masses.

Key Terms

thalloid: Of a plant, alga, or fungus lacking complex organization, especially lacking distinct stems, roots, or leaves.
alga: any of many aquatic photosynthetic organisms, whose size ranges from a single cell to giant kelps and whose form is very diverse
basionym: An earlier valid scientific name of a species that has since been renamed and from which the new name is partially derived.

Figure: **Cephaleuros virescens**: Infestation of the algal leaf spot (Cephaleuros virescens) on ther southern magnolia (Magnolia grandiflora); Green-orange algal spots or “green scruf” on leaf surface. The grayish-white and darker “crusts” are lichens of the genus Strigula resulting from fungal colonization of the alga.

Algae, are not normally considered common pathogens. Algal blooms are often associated with negative impacts on humans and the surrounding environment in which they occur. A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. HABs are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisonings. However, the damage to other organisms is not due to the algae infecting a host but rather indirectly excreting a toxin, or in some cases blocking out light or competing for resources.

However notable examples of algae acting as pathogens are known. For example Cephaleuros which is a genus of parasitic thalloid alga comprising approximately 14 species. Its common name is red rust. Chrooderma is its basionym. Specimens can reach around 10 mm in size. Dichotomous branches are formed. The alga is parasitic on some important economic plants of the tropics and subtropics such as tea, coffee, mango and guava causing damage limited to the area of algal growth on leaves (algal leaf spot), or killing new shoots, or disfiguring fruit. Members of the genera may also grow with a fungus to form a lichen that does not damage the plants.

Examples of algae acting as a mammalian pathogen are known as well, notably the disease Protothecosis. Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as Prototheca that lacks chlorophyll. It and its close relative Helicosporidium are unusual in that they are actually green algae that have become parasites.The two most common species are Prototheca wickerhamii and Prototheca zopfii. Both are known to cause disease in dogs, while most human cases are caused by P. wickerhami. Prototheca is found worldwide in sewage and soil. Infection is rare despite high exposure, and can be related to a defective immune system. In dogs, females and Collies are most commonly affected. The first human case was identified in 1964 in Sierra Leone.

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