In this article we will discuss about the general features and phylogenetic relationships of rhodophyta.
General Features of Rhodophyta:
The Rhodophyta or red algae are almost entirely confined to the sea. Both fresh- and salt-water members of this group are comparatively few in number. The freshwater forms prefer well-aerated water. The marine forms are of world-wide distribution being predominantly represented in the warm waters.
They generally grow attached mostly to rocks. Some grow on or in other plants either as epiphytes or as endophytes, whereas others thrive as parasites.
The red algae exhibit various colourations from bright red and purple to dark brownish red, brownish green, blue green, and black. The vegetative body, in the majority of cases, is multicellular which ranges from simple filamentous to branched, heterotrichous, to often expanded macroscopic form of apparently appearing parenchymatous structure having more than a metre or so in length.
Some members have very simple unicellular thalli.
The thallus of red algae is characterized by the presence of gelatinous material which, in some species, is utilized for the preparation of agar, a gelatinous material of great use as a medium of culturing fungi and bacteria.
In some of the Oriental countries red algae are extensively used as food. The thalli of red algae are composed of highly organized cells whose wall is again composed of a mixture of cellulose and pectic material. Depending on species and maturity, cells may be uni- or multinucleate. Each cell bears one to many chromatophores of varied shape.
One of the striking features of most red algae is the presences of a pore-like opening in the cell wall through which conspicuous, broad cytoplasmic connection is maintained from cell to cell, known as pit-connection. The chromatophores contain chlorophylls a and d, a- and β carotenes, lutein, taraxanthin, allophycocyanin, a special red water- soluble pigment r-phycoerythrin, and a blue pigment r-phycocyanin.
The colour of the vegetative body depends upon the quantities and kinds of pigments present in the chromatophores. The food reserves may be alcohols, oils, but the most important one is—floridean starch, an insoluble carbohydrate, a compound intermediate between true starch and dextrin. One of the outstanding characteristics of red algae is complete absence of flagellate reproductive structures.
In general, red algae reproduce sexually. Vegetative reproduction by the fragmentation of the vegetative body is not very common. Besides sexual and vegetative reproduction, most red algae reproduce asexually by the formation of monospore, bispore, polyspore, or paraspore. All these are non-motile single-celled spores and are formed inside respective sporangia.
Sexual reproduction is oogamous with or without elaborate post-fertilization stages which, of course, depend whether meiosis of the diploid zygotic nucleus is delayed or takes place immediately after its formation. The male reproductive organ—spermatangium is a very simple structure which bears a single uninucleate single-celled male gamete—spermatium.
Whereas, the female one—carpogonium has a long neck-like structure—trichogyne and a bulbous base—the carpogonial base where remains the female nucleus.
The red algae which have very elaborate post-fertilization stage develop an elaborate diploid phase, the earlier part of which is entirely dependent on the gametophytic individual and the latter part is represented by an independent sporophyte which may or may not be different from the gametophyte morphologically.
The sporophyte bears tetra-sporangia and tetraspores. Reduction division takes place during the sporogenesis in the sporangium of the independent sporophyte. Such forms exhibit isomorphic to heteromorphic alternation of generations.
Phylogenetic Relationships of the Rhodophyta:
The Rhodophyta are unique among the algae for having specialized pigments (phycoerythrin), reserve food (floridean starch), characteristic female reproductive organ (carpogonium), absence of any flagellate structure, and the sexual process. These algae with fossil record extending back to the Ordovician, possibly arose independently from non-flagellate unicellular organisms.
The Rhodophyta resemble no other algae, except the Cyanophyta which also lack flagellate structures and possess phycobilin pigments. Furthermore, floridean starch, the characteristic carbohydrate reserve of the red algae, is chemically allied to cyano- phycean starch, the characteristic carbohydrate reserve of the blue-green algae.
As such many of the phycologists including Kylin (1943) suggested affinity of the Rhodophyta with the Cyanophyta. In spite of these points of similarity there are very many fundamental points of difference between the Cyanophyta and the Rhodophyta.
They are: the presence of a nucleus, in having definite plastids, in having chlorophyll d in addition to chlorophyll a, in the usually more complex structure of the thallus, and sexual reproduction with often very complex life cycle. All these features of the Rhodophyta may be regarded as evolutionary advances over the condition in the Cyanophyta (Fig. 133).
Again within the Rhodophyta, the Bangioideae are less highly modified than the Florideae, and some of the genera of Bangioideae which have cyanophycean cell structure and lack sexual processes are very suggestive for being closely allied to the Cyanophyta.
It is possibly justifiable to consider both Cyanophyta and Rhodophyta as living representatives of evolutionary lines of descent which have diverged considerably from the same common ancestor in the extremely distant past (Fig. 118).
Some consider Rhodophyta to have derived from a member of the Chlorophyta (Fig. 132). But the structure of the plant body and pigment composition in both these taxa are so different from each other that it is difficult to justify such an evolutionary line. But in Prasiola, the structure of chloroplast and the similarity in morphological structure and reproduction make it possible to suggest as an ancestor of the Rhodophyta.
Two alternatives have been suggested by Klein and Cronquist (1967) about the ancestry of the Rhodophyta:
(i) They originated from some simple procaryotic blue- green algae, and
(ii) They were derived from some archaic eucaryotic algae which themselves originated from blue-green algae.
But the fundamental differences between the eucaryotic and procaryotic cellular organizations rule out the existence of any direct link between the red algae and the blue-green algae. Further, the biliproteins of Rhodophyta differ in spectral properties from those of Cyanophyta. It may be speculated that the red algae probably originated from some primitive non-flagellate eucaryotic ancestor possessing biliproteins.