Algae

Algae are plant organisms that carry out photosynthesis but which, unlike plants, have a poorly differentiated body, or thallus, and very simple reproductive organs. Due to the absence of roots and vessels,   absorption of nutrients and gas-exchange take place on the entire surface of the algae. Algae reproduce in two different ways, like all the other plant organisms, by  both sexual and asexual reproduction. In asexual reproduction, a production of cells that develop from one organism and give rise to a new organism, is observed.  In the case of sexual reproduction, instead, a fusion of female and male gametes takes place. From this union an organism that is similar to the parents, but which has intermediate genetic characteristics of the two parents is originated, and therefore it is not identical to one of them as in the case of asexual reproduction.  Algae are generally subdivided, according to the type of chlorophyll and pigments that are present, into Green, Brown and Red Algae.
Green algae
Green algae are characterized by a brilliant green colour and there are approximately 7000 species. Their structure is extremely varied, some consist of a single cell and some have a very differentiated thallus. Due to the presence of chlorophyll, they are usually present in the more illuminated surface areas , where they can receive red radiations that disappear in the deeper waters. However, the alga  which has been found in the deepest water is in fact a green alga, Halimeda copiosa,  the tropical equivalent of   “alga moneta” (Halimeda tuna) of the Mediterranean Sea. Green algae have a high calcium content and their “skeleton” forms thin fragments which are the most common sediment in most reefs.
Brown algae
There are approximately 1500 species of brown algae and they are almost exclusively marine plants. They are coloured brown, yellow or golden, due to the presence of  pigments that differ from chlorophyll, the carotenoids. At times the thallus is covered with a thin film of calcium carbonate that gives the algae a characteristic whitish colouring. Among the brown algae we wish to point out the Padina boergesenii of the Caribbean which forms tufts of fan-like leaf lamina. The striped colours vary from yellow to green and brown with brilliant iridescent blue-green colouring, and the upper margin is often lighter.
Red algae
Red algae are a group of algae with a particular cell pigment, phycoerythrin which gives them their red colouring. There are approximately 4000 species, and they are particularly abundant in warm and temperate water. Due to the presence of this pigment they can carry out photosynthesis even at great depths, however they do not disdain areas closer to the surface, where it is easy to see species like the Peyssonnelia squamaria.

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