In this article we will discuss about the general features and phylogenetic relationships of phaeophyta.
General Features of Phaeophyta:
The Phaeophyta or brown algae, in general, are restricted to the sea—the seaweeds, with a few exceptional fresh-water forms (species of Heribaudiella, Sphacelaria, Lithoderma, Pseudobodanella and Pleurocladia lacustris, A. Br.). Usually the marine members are abundant in cold-waters. But warm-water brown algae are also fairly well represented.
They are predominant in part of the arctic and antarctic seas. Their colour varies from dark-brown to olive-green.
Most marine forms grow attached to rocks or similar other submerged objects of the substratum by the development of certain mechanism for anchorage. Some members of this group grow either as epiphytes or as endophytes remaining in association with other algae.
There is marked variation in the vegetative structure of brown algae, which ranges from minute unicellular to abundant and luxurious growth comprising 100 metres or more in length having morphological and anatomical differentiation.
There are also various ranges of heterotrichous forms starting from simple heterotrichous individual with the gradual disappearance of heterotrichous condition being replaced by very elaborate development of the aerial portion with the prostrate portion being reduced to rudimentary structure serving the function of anchorage. Growth of the thallus may be apical or by intercalary meristem exhibiting trichothallic growth.
Wall of each individual cell is composed of two layers; the inner cellulose layer, and the outer layer is composed of a mixture of pectic material and algin—a colloidal substance which has commercial importance for being used in the preparation of adhesives and artificial silk. In general, the uninucleate vacuolate protoplast bears one to several chromatophores whose shape and position in a cell is extremely variable.
The chromatophores contain Chlorophyll a, Chlorophyll c, Carotene, and Xanthophyll which are masked by a special brown pigment—fucoxanthin. Due to the presence of this special pigment which is nothing but a mixture of two carotinoid pigments: Fucoxanthin a and Fucoxanthin b, the chromatophores are coloured brown. In some members, the pigment xanthophyll is also absent.
Reserve food of most frequent occurrence is laminarin—a group of dextrin-like polysaccharides. Besides laminarin there may be present a hexahydric alcohol—mannitol and some amount of fats or fat-like substances as reserve food. Certain brown algae are the sources of commercial iodine. Some are used as fertilizers.
Besides vegetative reproduction by fragmentation of the thallus, most brown algae, reproduce both asexually and sexually. Asexual reproduction is either by biflagellate zoospores or by non-flagellate tetraspores developed in sporangia. Each zoospore bears two unequal flagella which are inserted laterally and sometimes near the posterior end. Sexual reproduction ranges from isogamy to oogamy.
In oogamy, fertilization may take place either after the female gamete or gametes are being liberated out from the oogonium or when the ovum is retained in the oogonium. The number of female gametes in an oogonium may be one or eight. The antherozoids are uni- or biflagellate.
Except few, all the members of the group are characterized for exhibiting distinct alternation of generations which in most cases is isomorphic, and in others, hetero- morphic. The zygote resulting from the fertilization without having the zygotic nucleus dividing reductionally, germinates producing a diploid individual which, depending on species, may or may not resemble the haploid individual morphologically.
In such cases meiosis takes place at the sporogenesis in the sporangia. Moreover, there are other brown algae in which the plant body is diploid and the haploid condition is represented by the gametes only. In such case the reduction division takes place during the gemetogenesis in the gametangia.
Phylogenetic Relationships of Phaeophyta:
The Phaeophyta form a well-marked taxon not very closely related to the other algae. The nature of swarmer’s suggests that the Phaeophyta possibly arose from flagellate unicellular organisms.
The Phaeophyta are often considered parallel to the Chlorophyta (Fig. 118) in the nature of plant body and in the method of reproduction, although the Phaeophyta have evolved multicellular reproductive organs and a higher type of vegetative body.
The apically growing thallus of the Phaeophyta developing into an elaborate structure is often differentiated into epidermis, cortex, and medulla. Besides this, the development of sieve-tube cells and in some cases development of a cambium like region are the features which can very well justify a very high position for the Phaeophyta in the evolutionary scale.
Since they lack xylem for conduction and support, a cuticle for protection against evaporation, jacket layers around their multicellular reproductive organs for protection against desiccation, and the mode of sexual reproduction is not well advanced, the Phaeophyta did not receive any higher position although they possess high degree of specialization in their plant body.
The pigmentation of the Phaeophyta suggests a relationship to both the Ghrysophyta and the Pyrrophyta. All these groups have an excess of carotinoid pigments over chlorophylls, as compared to the Chlorophyta. Chlorophyll c is known only in the Phaeophyta, Chrysophyta and Pyrrophyta. Fucoxanthin, the principal xanthophyll of the Phaeophyta occurs also in the Chryosophyta but is unknown among the Pyrrophyta.
The sum of the evidence indicates that the Phaeophyta originated from a prechrysophytan stock after the pyrrophytan line had already diverged from the ancestors of this same stock.
A common ancestry of the Phaeophyta with the Chrysophyta has also been suggested considering the similarity of the flagellate cells in both these taxa.
The fossil record is hardly useful in determining relationships of the Phaeophyta. Fossils more or less resembling Fucus and other members of the Phaeophyta occur in rocks of the early Paleozoic. But they are not well preserved to be placed in any algal taxa, and hence they provide no indication of the ancestry of the Phaeophyta.