Red algae (Phylum Rhodophyta) are a very large group of species that predominantly include marine, multicellular algae. Originating in the Mesoproterozoic, red algae are an ancient group of organisms. In fact, the oldest fossil of a eukaryotic, multicellular organism is red algae. What makes red algae multicellular versus colonial is the presence of differentiated tissues, notably the presence of sexual organs. Red algae represent the first evidence of sexual reproduction in the fossil record. Similar to glaucophyte algae, red algae photosynthesize via chlorophyll a and phycobiliproteins, which collectively absorb all colors except for red making them appear red.

 Coraline Red Algae. Photo: Peter Southwood 2012. Source: Wikimedia Commons. 

Coraline Red Algae. Photo: Peter Southwood 2012. Source: Wikimedia Commons. 

 Rhodophyta.  Ptilothamnion sp . Photo: Pallastrelli 2011. Source: Wikimedia Commons. 

Rhodophyta. Ptilothamnion sp. Photo: Pallastrelli 2011. Source: Wikimedia Commons. 

 Rhodophyta . Haliclystus antarcticus.  Photo: André C. Morandini 2010. Source: Wikimedia Commons. 

Rhodophyta. Haliclystus antarcticus. Photo: André C. Morandini 2010. Source: Wikimedia Commons.