Abstract
Rhodophyta, or red algae, comprises a monophyletic lineage within Archaeplastida that includes glaucophyte algae and green algae plus land plants. Rhodophyta has a long fossil history with evidence of Bangia-like species in ca. 1.2 billion-year-old deposits. Red algal morphology varies from unicellular, filamentous, to multicellular thalloid forms, some of which are sources of economically important products such as agar and carrageenan. These species live primarily in marine environments from the intertidal zone to deep waters. Freshwater (e.g., Batrachospermum) and terrestrial lineages also occur. One of the major innovations in the Rhodophyta is a triphasic life cycle that includes one haploid and two diploid phases with the carposporophyte borne on female gametophytes. Red algae are also well known for their contribution to algal evolution with ecologically important chlorophyll-c containing lineages such as diatoms, dinoflagellates, haptophytes, and phaeophytes all containing a red algal-derived plastid of serial endosymbiotic origin. Analysis of red algal nuclear genomes shows that they have relatively small gene inventories of 6,000–10,000 genes when compared to other free-living eukaryotes. This is likely explained by a phase of massive genome reduction that occurred in the red algal ancestor living in a highly specialized environment. Key traits that have been lost in all red algae include flagella and basal body components, light-sensing phytochromes, and the glycosylphosphatidylinositol (GPI)-anchor biosynthesis and macroautophagy pathways. Research into the biology and evolution of red algae is accelerating and will provide exciting insights into the diversification of this unique group of photosynthetic eukaryotes.