Rhodophyta (ro-DA-fa-ta) is made of two Greek terms that mean rose (rodon - ρόδον); and plant (phyto -φυτό).  The reference is to the red (rosy) color that dominates the pigments of many taxa.



The red algae are common seaweeds of warmer marine waters, and a few taxa occur in freshwater.  Some are small and little more than individual cells, simple filaments, or very thin thalli.  Most are decidedly multicellular and made of large thalli (pseudoparenchymatous) or complex filaments. They are large and multicellular (most species) with some of the most complex life cycles of any of the eukaryotes.  Some go through a typical biphasic alternation of generation with may be isomorphic or heteromorphic.   The complex red algae, most of which are in the Class Florideophyceae, however, go through a triphasic life cycle. 

The rhodophytes are made up of three major groups, which, in the system of Saunders and Hommersand (2004), are defined as three subphyla.  Figure 1 shows the groups nested with Rhodellophytina (clade 2) as the sister to the rest of the phylum.  Metarhodophytina (clade 4), a group with filaments and simple pseudoparenchymatous forms is sister to the complex reds (Eurhodophytina, clade 5).

FIGURE 1. A cladogram showing the relationships between the major groups of the Rhodophyta after Saunders and Hommersand (2004).  F/T is the clade of filamentous and thalloid taxa.  Eu is the clade of the subphylum Eurhodo- phytina.  



Subphylum Rhodellophytina

The simpler taxa occupy two subphyla: Rhodellophytina and Metarhodophytina, each with a single class.  Members of the Rhodellophytina are simple unicells (Figure 2) or pseudofilaments, cells held in a linear array by the common gelatinous covering (Figure 3).  They have no sexual reproduction.  However, some taxa, like Porphryidium (Figure 2) can produce large amoeboid forms whose particular function is unknown (Bold and Wynne 1978).  

Subphylum Metarhodophytina

Members of the Metarhodophytina tend to be filamentous or pseudoparenchymatous.  For example Composogon (Figure 4) develops a pseudoparenchymatous thallus at the base from which branched uniseriate filaments emerge.   They tend to have a biphasic life cycle (that is, alternation of sporophyte and gametophyte stages) in which the spermatia are simple and derived from vegetative cells.

Subphylum Eurhodophytina

The subphylum Eurhodophytina is the most speciose of the three and contains two classes: Bangiophyceae and the Floridiophyceae.  The defining synapomorphy is the occurrence of pit plugs in at least one of the phases of the life history.  The pit plug, sometimes called a pit connection, is a lens-shaped mucilaginous structure in the walls of adjoining cells.  The plug fills an aperture, which otherwise would allow the flow of cytoplasm from cell to cell.

Class Bangiophyceae

Members of the Bangiophyceae have a simple alternation of heteromorphic generations in which the sporophyte is a small, prostrate filament called a conchocelis that releases meispores called conchospores.  The sporophyte is the stage that has pit connections.  The gametophyte can be variable in this group and range from filamentous (Figure 5) to foliose (Figure 6).  Porphyra is the source of  Nori, the black seaweed that wraps sushi; so, the discovery of the its history opened the door for its culture and the global availability of Nori.  Specialized cells in the foliose gametophyte of Porphyra form the spermatia, and other large cells function as eggs.  Following syngamy, the zygote settles down on a mollusk shell and develops into the sporophyte (see Figure 7). 

FIGURE 7.  A life cycle of Porphyra.

A. the thalloid gametophyte. 

B. production of spermatia.

C. fusion of gametes

D-E. release and germination of 2n carpospores

F. the 2n conchocelis stage

H-K. meiosis forming haploid conchospores, which germinate and form the haploid thallus.

From Scagel et al. (1982)

Class Floridiophyceae

The Floridiophyceae contains most of the taxa in the phylum.  These plants tend to be complex, either filamentous or pseudoparenchymatous and tend to be seaweeds of warmer waters.  The polysaccharides common in the cell walls of many in this group are the sources of agar, agarose, and carrageenin, common food additives.  Chondrus crispus (Figure 8) is the red most commonly harvested on the coast of the eastern US as a source of agar.  Corallina (Figure 9) is a taxon that impregnates its cell walls with calcium carbonate forming filaments that appear armored and segmented.

Sexual reproduction is generally triphasic, such that isomorphic gametophyte and sporophyte generations are separated by the carposporophyte, a very different sporophyte that emerges from the development of the zygote.  Following syngamy and karyogamy, the zygote nucleus typically moves to another cell, the auxiliary cell, from which the carposporophyte begins to develop.  In general, the carposporophyte is a set of small filaments that terminate in diploid spores, carpospores.  These disperse and germinate to form the sporophyte.  This is generally pseudoparenchymatous and identical to the gametophyte.  Certain cells develop as sporangia in which meiosis occurs.  In the case of Polysiphonia (Figures 10 and 11), the axial cells of the corticated filaments function as sporangia.  Before this was recognized as a sexual cycle, the four meiospores that were produced in each sporangium were just referred to as tetraspores (4 spores), and this second sporophyte was called the tetrasporophyte.  A gametophyte that is identical to the tetrasporophyte emerges from the tetraspore following its germination.  The gametophytes have separate sexes, one produces spermatia in specialized spermatangia.  The other produces eggs (called carpogonia), each with an elongate hair like extension called a trichogyne.  When a spermatium encounters a trichogyne, it transfers the nucleus, which fuses with the egg nucleus and it travels to a auxillary cell to begin the cycle again.  As the carposporophyte develops in Polysiphonia,  a cup-like envelope, the cystocarp, develops around the carposporophyte (see Figure 11).  The life histories of the other red seaweeds are variations on the same theme.  In some cases, they vary primarily in the location of the auxillary cell and the ploidy of the carposporophyte.

The ability of nuclei and other organelles to move through the thallus of the red algae has given rise to a number of parasitic taxa.  These have specialized spores that fuse with cells of a target host plant.  Then, they inject their nuclei, which direct the mitosis and proliferation of more parasitic nuclei.  Then, they direct the development of spores and the formation of a sporangium.

FIGURE 11. Life cycle of Polysiphonia.  The tetrasporophyte is a diploid, corticated filament within which meiosis occurs and produces haploid tetraspores.  The tetraspore germinates into a haploid filament that is isomorphic with the tetrasporophyte.  The gametophytes are dioecious with spermatangial (male) and carpogonial (female) plants.  A 2n carposporophyte develops on the carpogonial sporophyte after fertilization occurs.  The carposporophyte produces 2n carpospores which germinate into the tetrasporophyte.

From Lee (1980)

The Rhodophyta seems to have a very long fossil history that might date back as far as 2,000 million years old (Gabrielson et al. 1990; Tappan 1976; Saunders and Hommersand 2004).  Gabrielson et al. (1990) report that fossils from the Gunflint Chert (1,900 million years old) have been interpreted as a Porphyridium-like rhodophyte. They also report the occurrence of fossil multicellular eukaryotes that are interpreted as "bangiophyte" algae from the Paradise Creek Formation (1,600 million years old).










In the past, the red algae suffered from the assumptions of what was primitive.  Taylor (1976) placed the reds at the base of his phylogenetic tree of motile protists.  They seemed to be primitive because they were nonmotile and had chloroplast characters that seemed very similar to those of the cyanobacteria (no stacked thyllakoids, chlorophyll a only, phycobillins in phycobillisomes).  However, their complex structures and complex life histories indicate high levels of specialization rather than a primitive state.  

Traditionally the red algae has been divided into two large groups: Bangiophyceae and Floridiophyceae (Sleigh et al. 1984; Dixon 1973; Bold and Wynne 1985; Van den Hoek et al. 1995; Graham and Wilcox 2000).  This is the way that Margulis and Schwartz (1988, Pr-13; and 1998, Pr-25) treat the red algae.  Garbary and Gabrielson (1990) question the need of dividing the red algae into two groups and prefer to lump all of the orders into a single class. In particular, Garbary and Gabrielson (1990) and Gabrielson et al. (1990) do not consider the "Bangiophyceae" to be a monophyletic group. Indeed, they suggest that since taxa within the Bangiales share features like pit plugs, cellulosic cell walls, peripheral plastid lamellae, band-shaped plastids, a cell vacuole and apical growth with the "Florideophyceae," the Order Bangiales belongs to the "Florideophyceae." Since this would place the genus Bangia in the "Florideophyceae," a nomenclatural conundrum would ensue; so, Garbary and Gabrielson (1990) find that the taxonomic problem is most easily solved by having a single class.  Freshwater et al. (1994) who compare plastid DNA within the phylum show that the orders of the Florideophyceae form natural groupings.  However, the other taxa appear to be paraphyletic.  

We elect to follow the taxonomic system of Saunders and Hommersand (2004) who have attempted to rectify the past problems with rhodophyte classification systems by application of molecular phylogenetics.  Their system separates the taxa of the "Cyanidiales" into a sister phylum (a concept promoted by Doweld 2001).  The remaining phylum, that they call Rhodophyta, has three subphyla and four classes.





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