Green algae

2.3.1 Green algae

Green algae, which we have seen are the progenitors of land plants, may contain a range of biologically active compounds, including diterpenes that are toxic to bacteria (Ahmad et al., 1994), and thus probably defend the plant against disease. Paul and Fenical (1986) investigated 40 species of multicellular green algae and found that they produced a range of terpenes of unusual and distinctive types that are toxic to micro-organisms, sea urchin larvae, molluscs, and fishes. These compounds were produced in greater concentrations when herbivory was more intense, and in particular in the softer and more vulnerable growing points of organs, although there was no difference in the concentration between different organs. They concluded that biologically active compounds are produced in order to defend tissues against herbivory. Paul et al. (1987) then confirmed that sesquiterpenes in Caulerpa ashmeadii deter feeding by herbivorous fishes. Similarly, the marine green alga Bryopsis maxima produces (Z)-8-heptadecene (which has a range of suppressive effects on her- bivorous marine invertebrates) only when wounded (Akakabe et al., 2007). In temperate waters, the green alga Ulvaria obscura can sometimes form “blooms” that choke subtidal marine communities, mainly because of its toxicity and thus lack of grazing by invertebrates. This alga produces dopamine, a common neurotransmitter in animals, that interferes with animal nervous systems and thus acts as a feeding deterrent (Van Alstyne et al., 2006). Leflaive and Ten-Hage (2007) review evidence that green algae also produce alkaloids that are used to deter herbivores and fatty acids and as-yet unidentified compounds to inhibit competing algae and aquatic flowering plants (note that the chemical suppression of competitors is known as “allelopathy” and is also known for flowering plants). Indeed, blooms of the green alga Botryococcus braunii follow the excretion of free fatty acids that inhibit growth of both zooplankton and phytoplankton (i.e., suppressing both herbivores and competitors; Chiang et al., 2004). Algae produce greater concentrations of allelopathic compounds in conditions of stress, induced by suboptimal nutrient availability, pH, or temperature, and also in response to the presence of waste compounds produced by competing species—this can alter the structure of the algal community (Leflaive and Ten-Hage, 2007, and references therein).

Thus, green algae, which essentially live in a microbial soup and are also surrounded by animal enemies and plant competitors, use secondary metabolites to defend themselves against disease and herbivory and to increase their chances of competitive success. Land plants have inherited these survival tools.

Soure: Giacinto Bagetta, Marco Cosentino, Marie Tiziana Corasaniti, Shinobu Sakurada (2012); Herbal Medicines: Development and Validation of Plant-derived Medicines for Human Health; CRC Press