Diversity and classification of marine benthic algae
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Chlorophyllian photosynthesis is responible of almost all primary production at global
level and represents the most fundamental process for the support and maintenance
of life on our planet. Organisms performing photosynthesis occur in every habitat in
which sunlight is available. Whereas vascular plants are largely responsible of
photosynthesis on land, in aquatic habitats this process is mainly perfomed by the
wide and heterogeneous complex of organisms called algae.
In the widest sense, algae are defined as oxygenic photosynthesizers other than embryophyte land plants (Cavalier-Smith, 2007). In recent years our knowledge of these organisms has greatly advanced, thanks to new types of data (mainly electron microscopy observations and DNA sequence data). Based on these data, we know now that algae represent an artificial and unnatural agglomeration of very different organisms, sharing the only common characteristic of living in aquatic habitats. Algae living in the sea are typically subdivided in plankton (the complex of microscopic algae not visible with unaided eye, which live floating in the water) and benthos (the collectivity of the algae that live attached to the sea bottom).
The algae that form the benthos are plant-like organisms, very diverse and very different in size, shape and colour; they are are typically designated with the term seaweeds. Despite of the undeserved negative connotation associated with such a name, seaweeds play a fundamental role marine ecosystems, where they have a multitude of beneficial effects. Although unrecognized, they also have an important part in our everyday life. Substances extracted from seaweeds occur in toothpastes, shampoos, cosmetics, drugs, soups, ice creams, soft drinks, beer, jellies, marmelades, salad dressings, chewing gums and many other products of large use. In China, Korea and Japan some species of seaweeds are among the most popular foods, and for this reason they have been farmed for many centuries.
Seaweeds occur on any shore where a hard bottom or any other type of stable surfaces are present. From a systematic point of view they are divided in three groups, whose empirical distinction is based on the colour: green algae, red algae and brown algae. The distinction of these three groups, however, is much more substantial than indicated by this simple designation. Besides the colour, they differ in their photosynthetic pigments, storage compounds, composition of cell walls, presence/absence of flagella, ultrastructure of mitosis, fine structure of the chloroplasts and several other characters. The separation of these three groups is so deep and substantial that in recent classification schemes they are classified into different kingdoms. While green and red algae are considered plants (kingdom Plantae), brown algae belong to a different kingdom called Chromista (Cavalier- Smith, 2007). This separation is due to the evolutionary origin of these groups. All eukaryotic algae originated through a process called endoymbiosis, in which a unicellular organism was captured and incorporated by another unicellular organism.
Instead than being killed and digested, the organism incorporated remained alive in its host and established a symbiotic relationship with it. Green algae and red algae were created by primary endosymbiosis. In this process, a unicellular photosynthetic cyanobacterium was incorporated by a larger non-photosythetic eukaryote. Performing photosynthesis, the cyanobaterium had a beneficial effect on its host and was maintained alive; gradually in time, the cyanobacterium established a strict metabolic relationship with the host and underwent deep modifications, becoming the chloroplast of the modern green and red algae. Conversely, brown algae originated through secondary endosymbiosis. In this process, a unicellular red alga was incorporated by a unicellular, non-photosyntetic eukaryote. Once again, the red alga was kept alive by his host and established a strict metabolic relationship with it; the symbiosis of the two organisms produced the ancestral brown alga.
Green algae
The green algae represent a very diverse group, which includes members distributed not only in the sea, but also in freshwater and terrestrial habitats. Their classification has been revised in recent years, based on DNA sequence data. In the current definition, green algae do not form a homogeneous and coherent entity; they are part of a larger group called Viridiplantae, in which the land plants are also included (Lewis & McCourt, 2004). However, all marine green algae are classified in a common class, called Ulvophyceae. The Ulvophyceae are a very diverse group and include about 920 species, which are distributed in all seas of the world.
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