Algae also are common in desert habitats. Here, in the Sonoran Desert of Arizona, diverse green algae and other photosynthetic microbes and lichens live on the surface of the soil forming biotic crusts (aka biological soil crusts or cryptogamic crusts). In this photo the microbial community appears darker than the surrounding soil.
Desert algae are able to survive tremendous temperature extremes and long periods, even years, of drought. They begin actively photosynthesizing once the rain falls and can stay active for several days.
Desert green algae evolved from freshwater algae and are found in several taxonomic groups, such as the classes Chlorophyceae, Trebouxiophyceae, Klebsormidiophyceae, Zygnematophyceae.
The following links provide more detail about the diversity and biology of desert green algae:
The photos below illustrate three different microscopic green algae, Myrmecia, a terrestrial species of Scenedesmus, and Pseudotetracystis (L to R).