Large, diverse, mostly multicellular clade consisting predominantly of photosynthetic protists, many members of which are adapted to living at substantial depths.

Red algae, like green algae, are products of in terms of chloroplast acquisition (that is, versus secondary endosymbiosis). They possess the , , which is responsible for both their ability to gather photons at great depths (by changing the used for photosynthesis) and their generally red color.

Red algae often display alternation of generations but lack a stage even among gametes. They are members of phylum

Note that the "redness" of red algae varies with the depth at which the algae photosynthesize, ranging from at best reddish (shallow waters) to red to the point of blackness (very deep waters, e.g., up to hundreds of meters).

Starting with green algae as a kind of baseline algal ecology, we can view red algae as exploiting what presumably was a photosynthetic adaptive zone of growth at greater depths. Going essentially in the other direction, we can view land plants as exploiting again what presumably was a photosynthetic adaptive zone of "lesser" depths, that is, out of the water entirely.