Structure and Life Cycle of Marine Water Brown Algae Ectocarpus

Marine water brown algae”Ectocarpus” is abundantly found throughout the world in cold waters. A few species have been reported to occur in freshwaters. The plant grows attached to rocks and stones along coasts both in the littoral and sub-littoral zones. Some species are epiphytes on other algae especially the members of Fucales and Laminariales. Ectocarpus stands for (ecto=external + carpus=fruit).

Structure or Plant Body of Ectocarpus

The thallus consists of sparsely or profusely branched uniseriate filaments and heterotriychous habit. There are two types of filaments, the prostrate and erect filament.

Prostate Filament

The prostrate system comprises of creeping, septate, irregularly branched filaments are attached to the substratum with the help of rhizoids. In epiphytic species, this system occasionally penetrates the host. In free floating species, the prostrate system is poorly developed.

Erect Filament

The projecting system arises from prostrate system and consists of well branched filaments. The branching is lateral and each branch arises beneath the septa. The main axis and the branches of the projecting system are uniseriate (cells joined end to end in a single series). The branches terminate into an acute point forming a hair. In some species, the older portions of main axis are ensheathed (corticated) by a layer of descending rhizoidal branches.

Growth of Thallus

The growth of the thallus in primitive brown algae is by an intercalary meristem at the base of a hair or filament. This type of growth is called trichothallic growth, e. g., in Ectocarpus.

Cell Structure

The cells are small, cylindrical or rectangular and uninucleate. The cell is surrounded by distinct cell wall composed of an inner cellulose layer and an outer pectin layer. The outer layer becomes slimy or gummy due to presence of pectic compounds. The most important constituent of the cell wall is alginic acid found in the outer layer of the cell wall. It forms a colloidal substance algin which is important economically. The reserve food is mannitol and laminarin starch. The reserve food is golden brown in color due to presence of dominant fucoxanthin. The other photosynthetic pigments are chlorophyll-a, -c, beta-carotene and other xanthophyll’s. Pyrenoid-like bodies are associated with the chromatophores.

Classification of Ectocarpus

Division: Phaeophyta

Class: Isogeneratae

Order: Ectocarpales

Family: Edocarpaceae

Genus: Ectocarpus

Reproduction

Ectocarpus reproduces asexually by zoospores and sexually by isogamy or anisogamy. The oogamous type of sexual reproduction is absent.

Asexual Reproduction

The asexual reproduction is either by means of biflagellate haploid zoospores produced in one-celled unilocular sporangia, or diploid zoospores formed in many-celled plurilocular sporangia. Both kinds of sporangia are present on the same diploid plant which is sporophyte. The sporangia are usually borne terminally and singly on lateral branches, but in rare cases these are borne seriately.

Unilocular Sporangia

A unilocular sporangium develops from a terminal cell of a short lateral branch. The sporangial initial enlarges in size and becomes globose or ellipsoidal. The number of Chromatophores also increase. The nucleus of the sporangium divides meiotically to Produce four haploid nuclei which undergo repeated mitotic divisions to produce 32-64 daughter nuclei. The cytoplasm of the sporangium cleavage and each bit surrounds a nucleus and a chromatophore to produce daughter protoplasts. Each daughter protoplast metamorphoses into a pyriform, biflagellate meiozoospore (zoospores produced as a result of meiosis). The flagella are laterally inserted and are of unequal size, the larger one directed forward and the smaller one is directed backward.

The meiozoospores are liberated from the sporangium through an apical pore in the sporangial in a gelatinous mass. After few moments, these are separated from each other and swim freely in all directions. After the liberation of zoospores, a new sporangium may be produced within the old sporangial wall.

Plurilocular Sporangia

The plurilocular sporangia are stalked or sessile, elongated, cone-like multicellular structures. Like unilocular sporangia these also develop from a terminal cell of a short lateral branch. The sporangial initial enlarges in size and undergoes repeated transverse mitotic divisions to produce a vertical row of 6-12 cells. These cells then divide by vertical and transverse divisions repeatedly to form a cone-like structure consisting of hundreds of small cubical cells arranged in 20-40 transverse tiers. Each cell represents a sporangium. The protoplast of each cell metamorphoses into a single, pear-shaped diploid, biflagellate mitozoospore (the zoospores produced as a result of mitosis). The flagella are of unequal size and are laterally inserted.

The mitozoospores are liberated either through a terminal or a lateral pore formed in the wall of the sporangium. The zoospores just beneath the pore escape as irregular mass, whereas those away from it are released singly and in an orderly manner.

Germination of Zoospores

The zoospores formed in unilocular sporangia (meioszoospores) swarm for some time and then come to rest on some solid object, withdraw their flagella and secrete a membrane around them. They germinate by the formation of a small germ tube.

Sexual Reproduction

Majority of the species are isogamous and homothallic. The gametes are produced in many-celled, elongated, sessile or shortly stalked, conical structures called plurilocular gametangia produced on haploid plants developing from the meiozoospores.

The development of gametangia is similar to that of plurilocular sporangia. These develop from terminal cell of a lateral branch. The gametangial initial gets inflated and divides mitotically by repeated transverse divisions to produce a vertical row of flat cells. These cells undergo repeated vertical and transverse divisions to form many hundred small cubical cells arranged in 24-40 transverse rows. The protoplast of each cell metamorphoses into a single, pyriform, biflagellate, haploid zoogamete. The flagella are laterally attached. The only difference between the zoospores and the gametes is that the gametes are relatively smaller in size. The gametes are liberated from the gametangium in through an apical pore formed in the Cell Wall of the sporangium,

Fertilization and Germination of Zygote

Fertilization results in a diploid zygote. The zygote germinates into a diploid sporophyte without undergoing any resting period.

Alternation of Generations

Ectocarpus shows isomorphic alternation of generations. The sporophyte is diploid and bears two types of sporangia, the plurilocular sporangia in which zoosporesare formed as a result of mitosis (mitozoospores), and the unilocular sporangia in which the zoospores are produced meiotically (meiozoospores). The mitozoospores germinate into a diploid sporophyte whereas the meiozoospores germinate to give rise a haploid gametophyte plant that bears plurilocular gametangia. These gametophytes are similar to the sporophyte in morphology. Haploid gametes are produced in the gametangia which on fusion result in a diploid zygote that germinates into a diploid sporophytic plant. The diploid mitozoospores serve the purpose of reduplication of sporophyte generation. In some species, the gametophyte generation is also reduplicated by the parthenogenetic germination of the gametes.