Seeps, wet rocks, waterfalls, etc., can harbor diverse assemblies of algal species. Algae are often among the “pioneer species” that colonize bare rock surfaces. Many of these are not green algae: cyanobacteria tend to dominate, along with diatoms (especially in moss mats). However, green algae – filamentous as well as unicellular – are often present in these communities.
Algae in subaerial habitats must be able to survive a wide range of environmental conditions – the rock surface experiences fluctuations in water availability, temperature and sunlight. These differences can be extreme even within a single day and algae have a variety of mechanisms to deal with these conditions. Red pigments are a common “sunscreen” protecting the organisms against excessive sunlight.
Conjugating green algae of the class Zygnemophyceae are common, even if not always abundant, inhabitants of subaerial habitats (a representative of the genus Mesotaenium in the image below, with a gelatinous, red-tinted extracellular sheath).