Rhodoplantae (ro-do-PLAN-tee) is derived from two roots: a Greek word that means "rose" (rodon - ρόδον), and a Latin root that means "shoot" (planta).   The reference is to plants (photosynthetic organisms) that are rosy or red.  The common name for the group, Red Algae, is reflected in the formal name.



The Rhodoplantae (Clade 1) are the red algae in a broad sense plus the Glaucophytes.  They range in form from unicells to complex filaments and thalloid plants.  Though mainly organisms of warm marine, the reds can be found in almost all aquatic environments.  They are united by the presence of phycobillin, an accessory photosynthetic pigment also characteristic of the Cyanobacteria.



FIGURE 1. Major Clades of the Rhodoplantae.  The topology of this cladogram is a modification of Doweld (2001) and Cavalier-Smith (1981, 1998).



The Red Algae 

The Red Algae include two unequal clades, the thermophilic Cyanidiophyta (Clade 3) and the very diverse Rhodophyta (Clade 4).  Although they both possess phycobilliproteins, they have almost uniting synapomorphies.  Patterson (1999) struggles to identify a synapomorphy for the red algae and concedes only that pit connections might be.  In general, they are united by the absence of characters: no chlorophyll b or c, no motility.  Also, they store their photosynthate as floridean starch, which is very similar to cyanophycean starch of the Cyanobacteria.


The Glaucophytes

The Glaucophytes are a very small, likely a remnant of a once diverse group.  They are motile with some flagellar characters that are similar to those of the green plants (Viridiplantae).  They are somewhat similar to the Red Algae in that the Glaucophytes have phycobilliproteins and chlorophyll a only.  Their chloroplasts give very strong support to the endosymbiosis theory because they still retain a trace layer of murein in the chloroplast wall.

This system roughly corresponds to some of the systems proposed by Cavalier-Smith (1981, 1998) in which the phycobillin-containing taxa are grouped together into a taxon that he called Billiphyta.  Similarly, Doweld (2001) united the glaucophytes and rhodophytes along with the cryptomonads as subkingdoms within a kingdom that he called Rhodymeniobiota.  Although the cryptophytes are troublesome with regard to their phylogenetic relationships, they clearly are the products of a secondary endosymbiosis, and, therefore, do not belong in this kingdom or supergroup of primary photosynthetic symbionts.    The relationships within the kingdom are even more problematic (Freshwater et al., 1994; and Garbary and Gabrielson, 1990), but are well on the way to being resolved by the system of Saunders and Hommersand (2004).  The "Cyanidiales" consistently emerge as a sister group to the other red algae (e.g. Gross et al. 2001; Ciniglia et al. 2004; Yoon et al. 2002; Muller et al. 2001; Saunders and Hommersand 2004).  Thus, I have followed the system of Saunders and Hommersand (2004) and set the Cyanidiales apart as a distinct phylum.  The inclusion of the Glaucophyta into this kingdom is quite tentative and harkens back to the systems of Cavalier-Smith (1981, 1998) and Doweld (2001).  I include them here as a third phylum of primary photosynthetic endosymbionts with phycobillins as a phylum of incertae sedis.





RHODOPHYTA (Wettstein 1922)

CYANIDIOPHYTA (Saunders and Hommersand 2004)

Incertae Sedis: GLAUCOPHYTA (Skuja 1954)



A more complete taxonomy (to the ordinal level) of the Kingdom Rhodoplantae.





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Gross, W., I. Heilmann, D. Lenze, and C. Schnarrenberger. 2001. Biogeography of the Cyanidiaceae (Rhodophyta) based on 18S ribosomal RNA sequence data. European Journal of Phycology. 36: 275-275.

Müller K. M., M. C. Oliveira, R. G. Sheath, and D. Bhattacharya. 2001. Ribosomal DNA phylogeny of the Bangiophycidae (Rhodophyta) and the origin of secondary plastids. American Journal of Botany. 88: 1390-1400. 

Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124.

Saunders, G. W. and M. H. Hommersand. 2004. Assessing red algal supraordinal diversity and taxonomy in the context of contemporary systematic data. American Journal of Botany. 91(10): 1494-1507. 

Yoon, H. S., J. D. Hackett, G. Pinto, and D. Bhattacharya. 2002b. The single, ancient origin of chromist plastids. Proceedings of the National Academy of Sciences (USA)  . 99: 15507-15507.


By Jack R. Holt.  Last revised: 03/17/2013