Marine Algae and Plants

Abstract

The broadly accepted definition of algae is ‘oxygenic photosynthesisers other than embryophyte land plants’, and multicellular algae are generally termed macroalgae. The numbers of green, brown, and red algal species are estimated to be approximately 1100, 1500, and 4000–6000, respectively, worldwide. Embryophytes living in marine environments are mainly seagrasses, which are angiosperms adapted to nearshore areas with soft sediment. Seagrasses are less diverse than macroalgae; there are less than 60 species worldwide. The three groups of macroalgae have different subcellular structures and evolutionary histories. Green and red algae evolved from a common ancestral photosynthetic eukaryote, which acquired their plastids through primary endosymbiosis, in which a cyanobacterium was engulfed by an ancestral eukaryotic host cell. Brown algae evolved by secondary endosymbiosis, in which a red alga was engulfed by a eukaryotic host protist. Because of this, green and red algae have plastids with two envelope membranes, while brown algae have plastids with four envelope membranes. The green and red algae differ in their photosynthetic pigment composition, thylakoid structure and storage products. The three macroalgal groups became multicellular independently of each other from different unicellular ancestors. Both macroalgal and seagrass beds are very productive ecosystems as their photosynthesis per unit area can be the highest in the world. These macrophytes support biodiverse coastal ecosystems by providing energy and structural complexity.