The Rhodophyta are the lineage of so-called red algae or red seaweed, that is eukaryotic organisms living in the sea, and characterized by red pigments, including phycoerythrin, phycocyanin and allophycocyanins. These pigments serve for photosynthesis.
Though they may loosely be described as 'algae' and even 'plants', these terms do not reflect their genetic relationships. They are considered a branch of Eucarya (the eukaryotes) distinct from the green plants. This would make them a separate kingdom. Individual members are called rhodophytes.
Formerly the Rhodophyta (literally 'rose-coloured plants') were divided into two classes, the Bangiophyceae and Florideophyceae, but the former is no longer used, and is recognized as different classes Porphyridiales, Compsopogonales, and Bangiales. There is also a fifth class called Cyanidium.
Commercially important rhodophytes include the seaweed that produces carrageenan gum.
There are about 700 genera, mainly marine, containing about 4000 known species. RNA analysis shows that internal divisions in lineage are much greater than in plants. As a clade they are much more ancient than plants. They are the earliest multicellular photosynthesizers to appear in the fossil record. They make lasting fossils because many of them have a coating of calcium carbonate.
Internally, rhodophytes are notable for the absence of flagella and centrioles, and the free deposition of starch in the cytoplasm as their energy source. The chloroplasts of rhodophytes contain the chlorophyll a form of chlorophyll, and also the red colourings collectively known as phycobilins or phycobiliproteins.
The presence of phycobiliproteins links them with cyanobacteria, because they share those with Synechococcus, which also has chlorophyll a; and no other eukaryote has these pigments. It is therefore probable that their chloroplasts are descended from cyanobacteria. Some scientists believe chloroplasts in different eukaryotes arose from different prokaryotes, but others hold that cyanobacteria gave rise to them all, in which case rhodophytic chloroplasts are simply retaining primitive versions.
The chloroplasts of rhodophytes and plants have a double membrane, suggesting that they were acquired by engulfing free-swimming prokaryotes, whereas the comparable plastids of other kingdoms have three or four layers, suggesting they were acquired later from plants or rhodophytes.
Colin Tudge, The Variety of Life, Oxford, 2000