There are several branches of life that include unicellular organisms capable of photosynthesis. All of these very diverse organisms are called microalgae because they share features such as a small size, the presence of photosynthetic pigments, the capacity to grow by capturing atmospheric CO2. However, there is not a single group that would bring together all microalgae: several groups appeared at different times in evolution.
The most ancestral group, known as the cyanobacteria, is composed of cells of very simple structures, containing photosynthetic membranes. Cyanobacteria formerly called « blue algae » are sometimes considered a group apart from microalgae, in strict meaning.
Through an initial process called a “primary endosymbiosis”, a cyanobacterium has been “engulfed” within a cell of another origin, leading to the emergence of a novel cell structure giving rise to several major lineages of algae including green algae (Chlorophyta) and red algae (Rhodophyta).
Later, at several times in evolution, other “engulfments” occurred (called secondary endosymbiosis events), leading to organisms of a higher level of complexity, including the majority of marine microalgae, such as diatoms.
In terrestrial environments, the major group of microalgae populating soils, the surface of rocks, the rivers, snow, etc., is made up of green algae. This is the group studied in the ALPALGA project.