BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VI. THE PLANT SYSTEM

LOWER SPORE PLANTS

GENERAL CHARACTERISTICS AND FEATURES

Division Brown Algae - Phaeophyta

Eukaryotic, multicellular, predominantly macroscopic (up to 60 m) marine Algae with filamentous, heterotrichous, pseudo- or parenchymatous thallus Organization, featuring leaf-like, stem-like, and rhizoid-like structures. They are usually attached to the substrate by rhizoids or a disc-shaped holdfast, currently numbering about 2,000 species. They appeared on Earth 400–570 million years ago (Silurian-Devonian). In terms of their Structure and general biological organization, they belong to the "yellow" pigment group and are closely related to yellow-green algae. They contain chlorophylls $\alpha$ and c₁, c₂, $\beta$- and $\varepsilon$-carotenes, and xanthophylls: 1) of the yellow pigment group — fucoxanthin, diatoxanthin, diadinoxanthin, and 2) of the lutein series — violaxanthin, zeaxanthin, antheraxanthin, neoxanthin.

Due to the massive accumulation of carotenoids — fucoxanthin and violaxanthin — chlorophyll is "masked," and the METABOLISM/14.html">Chloroplasts acquire a bright yellow or brown color. Chloroplasts are parietal, numerous, and small.

Each chloroplast is covered by four membranes, of which the two outer ones form the chloroplast Endoplasmic reticulum that merges with the nuclear envelope, while the two inner ones constitute the chloroplast envelope proper. A periplastid space is located between the outer and inner membranes. Plastids are secondarily symbiotic. Thylakoids are arranged mostly in threes, with a girdle lamella running along the periphery.

In many brown algae, the chloroplast contains a pyrenoid, often rudimentary, which is not penetrated by thylakoids, with laminarin deposited around it. The products of assimilation are CARBOHYDRATES soluble in The Cell sap: laminarin (a polysaccharide), Lipids, and mannitol.

Mannitol is a six-carbon carbohydrate that can account for up to 20–30% of the alga's dry mass and is deposited outside the chloroplasts. Low-molecular-weight compounds — mannitol, sucrose, and glycerol — prevent freezing in cold seas and stabilize the Osmotic Pressure of the cell, performing an osmoregulatory function.

The Cell wall consists of two layers: an outer layer containing pectic substances, protein compounds of alginic acid and its salts (insoluble calcium alginate), and an inner Cellulose layer with a fibrillar structure; however, this cellulose differs from that of flowering plants and is called algulose. Most representatives contain algin, a soluble sodium salt of alginic acid, as well as other salts of monovalent metals. Cells are interconnected by plasmodesmata. Pores are present. Small vacuoles are visible in the Cytoplasm, among which extremely tiny ones are physodes filled with fucosan (phlorotannin), a tannin-like substance. Phlorotannins are unique to brown algae and apparently play a protective role as radioprotectors, adsorbents for heavy metals, and deterrents against herbivory and epiphytic fouling. The Nucleus is single, though multinucleated in some species.

Brown algae reproduce both asexually and sexually. Asexual reproduction occurs via zoospores, aplanospores, tetraspores, as well as thallus fragmentation and gemmae. Zoosporangia can be unilocular (single-chambered) or plurilocular. The sexual process is isogamous, oogamous, or heterogamous. Female sex cells produce low-molecular-weight compounds called pheromones to attract male Gametes. Gametes are formed in plurilocular gametangia, one in each chamber. Zoospores and gametes are pear-shaped (pyriform), with a single chloroplast and two unequal flagella located on the ventral side of the cell. One flagellum is long and tinsel-type (with mastigonemes), directed forward; the other is shorter and smooth, directed backward—meaning they are heterokont and heteromorphic. The stigma, characteristic of flagellated stages, acts as a lens focusing light onto the flagellar Swelling, which Functions as a photoreceptor.

The life cycle of brown algae is haplo-diplophasic with Morphology/12.html">ALTERNATION OF GENERATIONS (isomorphic or heteromorphic), or diplophasic without alternation. Representatives with a haplo-diplophasic life cycle feature a diploid sporophyte, in the unilocular sporangia of which Meiosis occurs, producing haploid zoospores and tetraspores. These grow into haploid gametophytes that form sexual structures (gametangia) producing numerous small microgametes and a much smaller number of large macrogametes. Following copulation, the zygote develops directly into a new sporophyte without a resting period. When the alternation of generations is heteromorphic, the gametophyte is typically microscopic, while the sporophyte is macroscopic (Fig. 18).

Representatives with a diplophasic life cycle possess a diploid sporophyte but lack zoospores; meiosis occurs during gamete formation, and the sexual process is predominantly oogamous. Oogonia and antheridia are formed on specialized fertile branches (receptacles) within semi-enclosed cavities called scaphedia. The zygote, formed after Fertilization of the egg cell outside the oogonium, grows without a resting stage inside the scaphedium, giving rise to the next generation.

Brown algae are extremely widespread in the seas of polar and temperate regions, inhabiting littoral and sublittoral coastal zones where they form dense underwater forests. In tropical regions, they thrive during colder seasonal periods (e.g., species of the genus *Sargassum*, which form significant biomass in the Sargasso Sea) under lower Temperature regimes. Freshwater forms (species of the genera *Heribaudiella*, *Sphacelaria*, *Pseudobodanella*, *Lithoderma*, *Pleurocladia*, and *Porterinema*) are rare and comprise very few species (about 10). Endophytic species growing within the Tissues of other algae are also known.

The practical utilization of brown algae is driven by their developmental and growth characteristics (giant sizes and rapid biomass accumulation) as well as the specific Chemical composition of their thallus. The main avenues of use include the extraction of alginic acid and alginates (for the chemical, food, textile, and medical industries), the cultivation, harvesting, and Processing of edible species, and the application of biomass as organic fertilizers rich in potassium and nitrogen. In coastal countries, storm-cast marine algae are used as raw material rich in potassium and nitrogen for soil fertilization and livestock feed. The primary targets of practical use are species of the genera *Laminaria*, *Macrocystis*, *Fucus*, *Sargassum*, *Cystoseira*, etc. However, the leading aquaculture species belong to the genera *Laminaria* (*L. saccharina*, *L. japonica*, *L. digitata*) and *Macrocystis* (*M. pyrifera*), which are marketed collectively as "kelp" (sea kale).

The modern Classification of the division Phaeophyta is based on a synthesis of classical morphological and anatomical Research Methods (type of thallus organization and growth pattern, type of sexual reproduction, Features of the reproductive cycle, as well as the presence and structure of zoospores and pyrenoids) combined with modern molecular-biological analyses of representative taxa via nucleotide sequencing of various genes.

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Fig. 18. Diagram of the developmental cycles of selected Representatives of the division Phaeophyta:

A – Ectocarpus (isomorphic alternation of generations), B – Laminaria (anisomorphic alternation

of generations), C – Fucus (without alternation of generations).

Based on these analyses, a single class of brown algae is recognized (Phaeophyceae), which is subdivided into seven (sometimes up to 17) orders (Ectocarpales, Scytothamniales, Cutleriales, Dictyotales, Laminariales, Sphacelariales, Fucales, etc.). The best-known representatives belong to the orders Laminariales and Fucales.