Читать книгу Parasitology - Alan Gunn - Страница 119

Commonly known as the green algae, the Chlorophyta is a paraphyletic phylum – that is, the species within it derive from several different ancestors. Because most of them contain chloroplasts, they are often referred to as plants. Furthermore, these chloroplasts are similar in appearance to those in multicellular plants, such as wheat and roses, have a very similar physiology, and contain both chlorophyll a and chlorophyll b. However, the Chlorophyta are single‐celled organisms (although some are colonial) and are usually classed within the Protista rather than Plantae – although this remains a source of debate. Several species have close symbiotic relationships with invertebrates. For example, Chlorella spp. lives in association with the cnidarian Hydra viridis, and some sea slugs extract the chloroplasts from their algal food and utilise them as photosynthetic organelles within their own cells. Although various algae grow on the pelage of sloths and the skin of certain lizards, there are few reports of them becoming intracellular symbionts of vertebrates. An instance where it does occur is between the alga Oophila amblystomatis and the egg masses of certain amphibia. This alga enters the embryos of the spotted salamander Ambystoma maculatum and is maternally transmitted (Kerney et al. 2019). The alga utilises nitrogenous waste generated by the host cells and undertakes photosynthesis; the relationship therefore appears to be beneficial to both organisms. The alga Nannochloris eukaryotum will enter an endosymbiotc relationship with human cells under in vitro culture conditions. However, this is mainly of interest for the development of molecular machines (Black et al. 2014) and not something that happens naturally outside the laboratory.

There are isolated case reports of Chlorella spp. infecting wounds in humans and other mammals. There are also accounts of fatal disseminated infections in sheep that were presumably acquired via the digestive tract after consuming contaminated drinking water (Ramírez‐Romero et al. 2010). These, presumably, represent rare opportunistic infections. Some species of algae lost their chloroplasts during evolution. Amongst these are members of the genus Prototheca, which includes species that parasitize mammals and the genus Helicosporidium that are parasitic in insects.