Ectocarpus (A Genus of Brown Alga)

Ectocarpus (Ecto = External, Carpus = Fruit) is a genus of brown algae belonging to the class Phaeophyceae. Brown algae are multicellular, photosynthetic organisms found primarily in marine environments. 

Ectocarpus is known for its filamentous and branched structure. This marine algae has a wide distribution range, ranging from tropical to temperate oceans. It is found growing as epiphyte or epilithic tufts in littoral and sublittoral zones.

1.0Classification of Ectocarpus 

The classification of Ectocarpus, provides a hierarchical framework that illustrates its evolutionary relationships and taxonomic position in the broader context of living organisms.

This systematic arrangement helps scientists and researchers understand its place in the diversity of life and aids in the study of its biological characteristics, ecology, and evolutionary history.

Indian species are-  E. filife, E. enhali, E. coniger, E. rhodochortonoides

The name Chromista means "colored", and although some chromists, like mildews, are colorless, most are photosynthetic. Unlike plants, the Chromista have chlorophyll c, and do not store their energy in the form of starch.

2.0Habitat of Ectocarpus

The habitat of ectocarpus includes intertidal zones, rocky shores, and coastal areas. Ectocarpus attaches to hard substrates such as rocks, shells, or other submerged surfaces by the help of rhizoids. Many species of Ectocarpus are free floating.

3.0Thallus structure of Ectocarpus

Ectocarpus algae is characterized by a branched, uniseriate filamentous thallus with heterotrichous features, differentiating into prostrate and erect filaments. The erect system displays a higher degree of branching compared to the prostrate system. Branching typically occurs just below the septum, with branches often terminating into elongated tapering hyaline, vacuolated cells forming colorless hair-like structures. 

The cells are the main filament and the branches that arise are usually arranged one above in a single strand to form uniseriate, branched filaments. The thallus attaches to the substratum through branched rhizoids originating from the lower cells of the lower branches. 

4.0Cell structure of Ectocarpus

The uninucleated cells are small, cylindrical and barrel-shaped. The cell wall, mainly cellulose with additional alginates, provides strength and protection. Chloroplasts house essential pigments for photosynthesis, exhibiting ribbon-like or plate-like structures, some containing pyrenoids. 

A brown carotenoid pigment fucoxanthin is located in the plastids in addition to chlorophyll a, chlorophyll c, carotenes and xanthophyll. Ectocarpus cells are uninucleate, with a single nucleus regulating cellular activities. Mature cells feature large vacuoles, crucial for turgor pressure, nutrient storage, and cellular processes. Branched rhizoids anchor Ectocarpus to substrata, and septa partition cellular compartments. 

Terminal branches are distinctive, clear in appearance due to the absence of chloroplasts. This highlights Ectocarpus' intricate cellular structure, adapted for marine survival.

5.0Reproduction in Ectocarpus

Both asexual and sexual reproduction are found in Ectocarpus. 

1. Asexual reproduction in Ectocarpus : 

It takes place by biflagellated zoospores which are produced in two different types of sporangia a multicellular plurilocular sporangia and single- celled unilocular sporangia. 

Plurilocular sporangia : These structures are elongated cone-like formations, either sessile or stalked, and are situated at the end of the lateral branchlets on diploid plants. The terminal cells of the branchlet act as spore mother cells, initiating a series of repeated transverse divisions that give rise to a vertical row of 6-12 cells. Each of these cells undergoes subsequent vertical division. 

The nuclear divisions during this process are mitotic. The outcome is an elongated, multicellular cone-like structure composed of several hundred small cubical cells arranged in 20-40 transverse tiers, known as plurilocular sporangia. 

Each cell within these structures transforms into a single, biflagellate zoospore, characterized by diploid genetics and classified as mitospores.

Upon germination, the zoospore reabsorbs its flagella, assumes a rounded shape, and secretes a protective membrane. Subsequently, it germinates and develops into a prostrate system. Notably, the diploid zoospores generated in plurilocular sporangia function to replicate the sporophytic generation and do not participate in the alternation of generations phenomenon.

Unilocular sporangium : In addition to plurilocular sporangia, Ectocarpus also produces another type of sporangium known as unilocular sporangia. In the case of unilocular sporangia, the terminal cell of the branchlet undergoes significant enlargement, assuming a globose or ellipsoid shape. 

Within these structures, the diploid nucleus undergoes meiosis, resulting in the formation of four haploid nuclei. Subsequent mitotic divisions produce 32 to 64 daughter nuclei. Cleavage of the cytoplasm follows, giving rise to numerous uninucleate daughter protoplasts.

Each daughter protoplast transforms into a biflagellate haploid swarmer known as a meiozoospore. The meiozoospore shares similarities with the diploid zoospore but is haploid. Upon germination, the meiozoospore gives rise to the alternate haploid plant, or gametophyte, which is specifically involved in sexual reproduction. 

Some algal researchers consider meiozoospore formation as a stage within sexual reproduction, emphasizing its role in amplifying the beneficial effects of a single fertilization event.

2. Sexual reproduction in Ectocarpus : 

The majority of Ectocarpus species display isogamous and homothallic characteristics, while some exhibit heterothallism with physiological anisogamy. Oogamy is generally absent in this order. Gametes are produced within large, elongated, conical sex organs known as plurilocular gametangia. These structures closely resemble plurilocular sporangia but are borne on haploid or gametophyte plants, often physiologically distinct but morphologically similar.

In the gametangia development, the terminal cell of the lateral branchlet undergoes inflation followed by repeated transverse divisions, resulting in a vertical row of flat cells. Subsequent longitudinal and transverse divisions give rise to several hundred small cubical cells arranged in 24 to 40 transverse tiers. Each chamber's protoplast produces one or sometimes two biflagellate pyriform gametes, structurally similar to zoospores. These gametes are liberated from plurilocular sporangia.

Sexual fusion between gametes varies among species, with isogamy, physiological anisogamy, or morphological anisogamy observed. In dioecious species, fusion occurs between gametes from different plants. 

Morphologically identical but behaviorally distinct, one gamete is less active and often referred to as the female gamete. 

It becomes passive after a brief period, while more active male gametes cluster around and attach themselves to the female gamete through forward-directed flagella. Eventually, the two gametes fuse, combining their nuclei and cytoplasm to form the zygote. The remaining active gametes disperse. 

Despite morphological similarities, physiological anisogamy exists, designating the less active gametes as the female. The clustering of active gametes around the passive female gamete is known as clump formation.

6.0Life cycle of Ectocarpus

Ectocarpus has a haploid–diploid life cycle that involves alternation between two multicellular generations, the sporophyte and the gametophyte.

Ectocarpus exhibits an alternating heteromorphic type of sexual life cycle with isomorphic alternation of generation . Morphologically similar multicellular gametophyte and sporophyte generations are easy to identify in cultures under laboratory conditions, where the gametophytes are free floating and sporophytes form compact thalli that are attached to the substratum.

Sporophytic generation (a heterotrichous thallus) develops from diploid zygote that undergoes a bipolar germination to produce two germ tubes. Both plurilocular and unilocular sporangia are produced on the upright filaments.