What are algae?

‘Algae’ was once a taxonomic designation uniting the ‘lower’, aquatic, photosynthetic organisms, but recent ultrastructural and molecular data have uncovered a bewildering diversity of species.  Algae are now recognized as 10 divergent lineages on the tree of life that ally with organisms as distinct as bacteria and eukaryotic protozoans, ciliates, fungi and embryophytes. Phycologists (those who study algae) have, therefore, struggled for a contemporary definition for algae, but have generally failed with some workers suggesting that the term be abandoned.   Despite these difficulties, ‘algae’ is valid in a biochemical context defining species characterized by chlorophyll-a photosynthesis (except land plants) and their heterotrophic descendents (secondary chloroplast loss).  In fact this biochemical link has a phylogenetic base as all eukaryotic chloroplasts are derived, via primary or secondary endosymbioses, from a common ancestry within the photosynthetic cyanobacteria (these prokaryotes generally included under the term ‘algae’). ‘Algae’ is thus a valid phylogenetic term on an organellar basis, that organelle, the chloroplast, the defining feature of what it means to be an alga.

Why are algae important?

Algae are generally aquatic in habitat living in fresh, brackish and saltwater environments worldwide, although some species live in habitats as diverse as snow, soil and hot springs.  Globally algae are considered to fix 50 % of CO₂, and they are the primary “primary producers” in aquatic habitats supporting rich food chains, and they oxygenate aquatic systems.  From a more anthropocentric perspective algae are used in biomedical research, cosmetics, and are consumed as food in many parts of the world.  Algae are also important for the damage that they can do to the aquatic habitat in the form of non-toxic and toxic algal blooms including the infamous red tides.