Phaeophyta (Brown Algae) – Characteristics, Occurrence, Thallus Organization, Cell Structure and Reproduction

What is Phaeophyta (Brown Algae)?

• Phaeophyta, also known as brown algae, belong to the class Phaeophyceae and represent a large and diverse group of algae. With around 240 genera and over 1,500 species, they are predominantly marine organisms, though a few species thrive in freshwater environments. Examples of these freshwater species include Pleurocladia, Heribaudiella, Pseudobodanella, Lithoderma, and Sphacelaria, with Pleurocladia lacustris being capable of growing in both freshwater and marine habitats. In India, about 32 genera and 93 species have been reported.
• The characteristic brown coloration of Phaeophyta is due to the presence of the pigment fucoxanthin (C40H54O6) in their chromatophores, which gives them their distinct golden-brown hue. Fucoxanthin, along with other pigments such as chlorophyll a, chlorophyll c, and carotenoids, determines the exact color, which can range from olive green to deep brown depending on their concentration. The abundance of fucoxanthin typically masks the green of chlorophyll, giving the algae their brown appearance.
• These organisms are multicellular, and they can vary significantly in both size and form. While some brown algae are small and filamentous, others, like the giant kelps, can grow to immense sizes, forming underwater forests that play essential ecological roles. Notable examples of brown algae include Ectocarpus, Fucus, Sargassum, and giant kelps.
• Most brown algae thrive in marine environments, often found in colder coastal waters where they attach to rocks and other surfaces. Their ability to survive and flourish in these environments is due to their specialized structure, including a holdfast that anchors them to the substrate, a stipe that resembles a stem, and blade-like structures that allow them to capture light for photosynthesis.
• Besides serving critical roles in marine ecosystems, brown algae are also economically important. Species like Laminaria and Macrocystis are harvested for their alginates, which are used in food, cosmetics, and pharmaceuticals as stabilizers and thickening agents.

General Characteristics of Brown Algae (Phaeophyta)

Brown algae, or Phaeophyta, exhibit a variety of unique structural, cellular, and reproductive features that make them an important group within the marine ecosystem. The following points detail the essential characteristics of this group:

• Thallus Organization: Brown algae show a range of thallus (body) organization, from simple to highly complex. Their structures include:
  • Heterotrichous forms like Ectocarpus.
  • Uniaxial pseudoparenchymatous forms.
  • Multiaxial forms.
  • Parenchymatous forms.
• Thallus Differentiation: The thallus is typically differentiated into three main parts:
  • Holdfast: Anchors the algae to substrates like rocks.
  • Stipe: A stalk-like structure connecting the holdfast to the blades.
  • Blade: The flat, leaf-like portion that participates in photosynthesis.
• Attachment: Brown algae are firmly attached to rocky surfaces by either a discoid or branched holdfast, which secures them in their marine environment.
• Cell Structure: Brown algae are eukaryotic, meaning their cells have a distinct nucleus and other organelles enclosed within membranes.
• Cell Wall: The cell wall is composed of multiple layers:
  • The inner layer is primarily made of cellulose.
  • The outer mucilaginous layer is composed of pectin, alginic acid, and fucinic acid.
• Nucleus: Each cell contains a single large nucleus.
• Chromatophores: The chromatophores, responsible for photosynthesis, are generally parietal (located near the cell wall) and can vary in number from one to many per cell.
• Pigmentation: The characteristic brown color of these algae is due to the dominance of the carotenoid pigment fucoxanthin. Other pigments present include:
  • Chlorophyll a
  • Chlorophyll c
  • Xanthophylls
• Pyrenoid-Like Bodies: Cells may contain single, stalked, or projected pyrenoid-like bodies, which are involved in the storage of photosynthetic products.
• Fucosan Vesicles: The cytoplasm contains numerous small, colorless vacuoles called fucosan vesicles, which are metabolic by-products.
• Food Reserves:
  • The main long-term storage product is Laminarin.
  • Mannitol, a sugar alcohol, is another important storage product resulting from photosynthesis.
• Flagella: Flagellated structures, when present, have two unequal flagella:
  • The anterior flagellum is pantonematic (covered with fine hairs).
  • The posterior flagellum is acronematic (smooth).
• Eye Spot: Male gametes contain an eye spot, which is connected to the chromatophores and helps in detecting light.
• Reproduction: Reproduction in brown algae occurs through various methods:
  • Vegetative Reproduction: Occurs through fragmentation or the production of propagules. Detached fragments or propagules can grow into new thalli.
  • Asexual Reproduction: Involves zoospores, except in certain orders like Dictyotales and Fucales, where zoospores are absent.
  • Sexual Reproduction: Can be isogamous (similar-sized gametes), anisogamous (different-sized gametes), or oogamous (large, non-motile egg and small, motile sperm). In most species, fertilization occurs externally in water, with gametes fusing outside the gametangium.
• Fertilization: External fertilization is common, where gametes meet and fuse in the water outside the reproductive structures (gametangia).
• Zygote Germination: After fertilization, there is no meiosis during zygote germination, leading to the formation of a diploid thallus.
• Life Cycle: The life cycle of brown algae varies and can follow:
  • Isomorphic: Where the diploid and haploid stages appear similar in morphology.
  • Heteromorphic: Where the diploid and haploid stages differ in form.
  • Diplontic: Where the diploid stage dominates the life cycle.

Classification of Phaeophyta

Phaeophyta, or brown algae, are classified based on their life history, morphology, and reproductive structures. The classification below is largely based on Kylin’s (1933) division, which is widely recognized for its practical utility. This system categorizes Phaeophyta into different classes and orders, allowing for a clearer understanding of the relationships among these algae.

• Plants without an alternation of generations:
  • Class: Cyclosporeae
    • Order: Fucales
      • Fucales lack alternation of generations and are characterized by a complex diploid plant body with apical growth. Sexual reproduction is oogamous, and meiosis occurs during gamete formation.
• Plants with an alternation of generations:
  • Class: Isogeneratae (alternation of macroscopic, usually isomorphic generations)
    • Orders:
      1. Ectocarpales
         • Thallus can be pseudoparenchymatous, true parenchymatous, or composed of branched filaments. Growth is typically intercalary. Reproduction is usually isogamous, sometimes anisogamous, with an alternation of isomorphic generations.
      2. Sphacelariales
         • Short, densely branched tufted plants with parenchymatous structures and radial organization. Apical growth is prominent.
      3. Cutleriales
         • Alternating generations are macroscopic, though not always isomorphic, marking an evolutionary transition toward heteromorphic alternation.
      4. Tilopteridales
         • Resembling Ectocarpus in habit, these deep-water plants exhibit filamentous thalli and marked trichothallic growth. They reproduce through monosporangia, which release quadrinucleate monospores.
      5. Dictyotales
         • Characterized by a parenchymatous thallus with apical growth. Reproduction involves non-motile tetraspores and oogamous sexual reproduction. There is an isomorphic alternation of generations.
  • Class: Heterogeneratae (alternation of heteromorphic generations)
    • Subclass: Haplostichineae (thallus composed of pseudoparenchymatous tissue)
      • Orders:
        1. Chordariales
           • Plants form pseudoparenchymatous tissues. Sporophytes exhibit trichothallic growth and produce unilocular sporangia.
        2. Sporochnales
           • Inhabit deep waters with a distinctive meristematic tip structure and exhibit alternation of macroscopic sporophytes with filamentous gametophytes.
        3. Desmarestiales
           • Alternation between large, erect sporophytes and microscopic gametophytes. Growth is initiated by a single filament.
    • Subclass: Polystichineae (thallus composed of true parenchymatous tissue)
      • Orders:
        1. Dictyosiphonales
           • The thallus is parenchymatous with longitudinal and transverse intercalary cell divisions. The order typically shows alternation between a macroscopic sporophyte and a microscopic gametophyte.
        2. Laminariales
           • These are large, parenchymatous forms with significant morphological and anatomical differentiation, including holdfast, stipe, and lamina. Reproduction involves oogamous sexual reproduction with zoospores, and the alternation of heteromorphic generations is evident.

Types of Common Brown Algae

Brown algae, or Phaeophyta, are diverse and include several notable species that play key roles in marine ecosystems. Below are descriptions of common types of brown algae, highlighting their structure, reproductive methods, and practical uses.

• Laminaria:
  • Laminaria is a widespread, large-sized brown alga, often referred to as “devil’s apron” due to its appearance. The size of this kelp ranges between 1-3 meters.
  • The plant body is a sporophyte, differentiated into three main parts: a basal holdfast that anchors the alga, a near-cylindrical stipe (stalk), and a flattened blade or lamina.
  • The life cycle involves heteromorphic alternation of generations.
  • Laminaria serves as a source of food, manure, and commercially valuable products such as algin and iodine.
• Dictyota:
  • Dictyota is a ribbon-shaped brown alga that grows in shallow marine waters, characterized by its dichotomously branched fronds.
  • The surface of the frond bears hair and unilocular sporangia, which produce haploid tetraspores.
  • Each tetraspore gives rise to a haploid gametophytic thallus that is morphologically similar to the sporophytic thallus.
  • Sex organs develop in clusters, with male antheridia producing uniflagellate sperms. Fertilization results in a diploid zygote that grows into a diploid plant body.
• Fucus:
  • Fucus is a small, leathery, flat-branched perennial brown alga.
  • The frond is branched both dichotomously and monopodially. At the points of branching, pairs of air bladders (pneumocysts) are present, which help the alga maintain buoyancy.
  • Fucus contains conceptacles—flask-shaped cavities that house reproductive organs.
  • Meiosis occurs during gamete formation, contributing to the reproductive cycle of the plant.
  • This alga has been traditionally used as fodder, manure, and a source of algin.
• Sargassum:
  • Sargassum is commonly known as “gulf weed” and serves as both fodder and manure. An antibacterial and antifungal extract called sarganine is derived from it.
  • Sargassum exists in two forms: free-floating and attached. Free-floating forms are abundant in the North Atlantic’s Sargasso Sea, where they can hinder shipping due to their density.
  • In attached forms, the plant is divided into a holdfast, a main axis, and laterals. Short laterals, or leaves, grow on the long laterals of the main axis.
  • Air bladders, or pneumocysts, are located in the axils of leaves. In free-floating forms, these bladders provide buoyancy, while in attached forms, they help keep the plant upright.
  • Reproductive organs are housed in two distinct types of conceptacles.
• Ectocarpus:
  • Ectocarpus is a filamentous brown alga with both upright and prostrate regions, known as heterotrichous growth.
  • Upright branches exhibit evection, where the parent branch is pushed forward, creating an appearance of dichotomy.
  • Attachment to solid substrates occurs through rhizoids present in the prostrate portion of the plant.
  • Reproduction occurs through fragmentation, and the alga can also multiply asexually by forming diploid biflagellate zoospores in plurilocular sporangia.
  • The sporophytic body bears unilocular sporangia where sporic meiosis takes place, forming haploid biflagellate meiozoospores.
  • These meiozoospores germinate into gametophytic thalli, which release biflagellate gametes from their plurilocular gametangia. The fusion of gametes results in a diploid zygote that develops into a diploid plant.

Reproduction in Phaeophyceae

The brown algae, classified under Phaeophyceae, exhibit a remarkable diversity in reproductive strategies. Their ability to reproduce through vegetative, asexual, and sexual methods enables them to adapt to various environmental conditions and enhances their survival. Each mode of reproduction serves a specific function and occurs in different species, highlighting the complexity and adaptability of these organisms.

• Vegetative Reproduction:
  • Some species of Phaeophyceae engage in vegetative reproduction through a process known as fragmentation. During this process, a part of the thallus detaches from the parent organism and can float away, eventually developing into a new plant body.
  • In certain species, specialized branches called propagula develop. These propagula separate from the main plant body, facilitating the growth of new individual algae. This method allows for rapid population increase and colonization of new areas.
• Asexual Reproduction:
  • Asexual reproduction in Phaeophyceae predominantly occurs through the formation of zoospores, which are motile cells that can develop into new organisms.
  • Zoospores are produced within the diploid sporophyte and are housed in two types of sporangia:
    • Unilocular Sporangia: These produce haploid zoospores. Once released, these haploid zoospores develop into a haploid thallus known as the gametophyte.
    • Plurilocular Sporangia: In contrast, these sporangia yield diploid zoospores, which subsequently develop into a diploid thallus. This differentiation allows brown algae to maintain a balance between haploid and diploid life stages.
• Sexual Reproduction:
  • Sexual reproduction in Phaeophyceae encompasses three distinct forms: isogamy, oogamy, and anisogamy. Each method involves the fusion of gametes, contributing to genetic diversity and adaptation.
    • Isogamy: In this mode, fertilization occurs between two identical gametes, meaning both gametes are morphologically similar. This method is relatively simple and does not involve specialized reproductive structures.
    • Oogamy: This method involves fertilization between two different types of gametes—specifically, one non-motile, non-flagellated gamete (the egg) and a motile, flagellated gamete (the antherozoid). The male reproductive structure is called the antheridium, while the female reproductive structure is referred to as the oogonium. This strategy promotes greater genetic variation and adaptability in fluctuating environments.
    • Anisogamy: In anisogamy, fertilization occurs between gametes of unequal size, leading to a greater differentiation in reproductive roles. This fertilization results in the formation of a diploid zygote, which develops into a diploid thallus, known as the sporophyte.

Economic Importance of Brown Algae

Brown algae, a prominent group of marine algae, play a significant role in various economic sectors due to their diverse applications and natural resources. From food and medicine to industrial use, brown algae are invaluable in multiple fields. Their broad utility stems from the presence of unique compounds and their abundant growth in marine ecosystems.

• Food Source:
  • Brown algae, including species like Laminaria, Alaria, Macrocystis, and Sargassum, are widely consumed as food in many countries. They are highly nutritious and form a key part of the diet in several coastal regions. Besides being used as human food, these edible brown algae are also used as fodder for livestock, enriching animal feed.
• Fouling of Ships:
  • Certain brown algae, such as Ectocarpus, are notorious for attaching themselves to the hulls of ships, causing problems for maritime transport. Other species, like Sargassum, can float in large masses, creating obstacles for ships. This fouling presents a nuisance in navigation and increases the maintenance cost of vessels due to cleaning and repairs.
• Iodine Source:
  • Species such as Fucus and Laminaria are rich sources of iodine, a crucial element for human health, particularly in preventing conditions like goiter caused by iodine deficiency. Historically, brown algae have been harvested specifically for their iodine content, which remains relevant in medical treatments and supplements. Additionally, potash is extracted from species like Macrocystis and Nereocystis, further contributing to their industrial value.
• Medicinal Uses:
  • Brown algae provide important medicinal compounds. For instance, sodium laminarin sulfate, derived from brown algae, is used as an anticoagulant, helping prevent blood clotting. Additionally, species such as Laminaria and Ascophyllum possess antibiotic properties, making them useful in treating infections. Durvillea is known for its vermifuge properties, aiding in the expulsion of intestinal worms, which highlights the medicinal versatility of brown algae.
• Alginic Acid:
  • One of the most economically valuable compounds found in brown algae is alginic acid, a phycocolloid extracted commercially from species like Laminaria, Macrocystis, Nereocystis, Fucus, and Sargassum. Alginic acid and its salts are extensively used in various industries for their emulsifying and thickening properties. It finds applications in products such as ice creams, ointments, toothpastes, cosmetics, creams, and shampoos. Besides food and personal care products, alginic acid is crucial in textile sizing, flame-proof plastics, security glass, and even the formation of pills and surgical threads.
• Industrial Uses:
  • Brown algae also have a notable industrial presence beyond alginic acid. Kelp, for example, has historically been used to produce soda ash, which is a fundamental component in the production of soap and glass. The utilization of these algae in large-scale production processes underscores their economic importance to industries reliant on raw materials derived from marine resources.

Ectocarpus

Occurrence of Ectocarpus

Ectocarpus is a genus of brown algae that exhibits a global distribution, primarily thriving in the colder marine environments of temperate and polar regions. Its adaptability to various aquatic ecosystems makes it a significant organism within marine biology and ecology. The following points summarize the occurrence of Ectocarpus, particularly in relation to its habitat and presence in India:

• Global Distribution:
  • Ectocarpus is found worldwide, with a pronounced presence in colder waters. It is particularly abundant in the temperate and polar regions, where it contributes to the biodiversity of marine ecosystems.
  • The genus is notable for its ability to colonize diverse environments, showcasing resilience to varying water temperatures and salinities.
• Habitat in India:
  • In India, Ectocarpus is commonly located along the western coastal regions. This area provides a conducive environment for its growth due to the availability of appropriate substrates and nutrient-rich waters.
  • Ectocarpus typically grows epiphytically on various sea plants or may be found attached directly to rocks. This epiphytic growth allows it to benefit from the nutrient flow associated with the host organisms, enhancing its survival prospects.
• Species Diversity:
  • Approximately 13 species of Ectocarpus have been reported from India, reflecting a notable diversity within this genus. This diversity indicates the potential for various ecological roles and adaptations.
  • Common Indian species include:
    • Ectocarpus indicus
    • Ectocarpus coniferus
    • Ectocarpus geminifructus
    • Ectocarpus dermonematus
  • Each of these species may exhibit specific ecological preferences and adaptations that enable them to thrive in their respective habitats.

Morphology of Thallus in Ectocarpus

The thallus of Ectocarpus exhibits a complex and organized structure that supports its ecological functions and adaptations. This heterotrichous thallus is typically differentiated into two primary systems: a creeping or prostrate rhizoidal system and an erect branched system. The following points detail the structural components and characteristics of the thallus in Ectocarpus:

• Heterotrichous Structure:
  • The thallus is characterized as heterotrichous, meaning it possesses two distinct growth forms. This allows the organism to efficiently exploit its environment.
  • The creeping or prostrate rhizoidal system anchors the alga to substrates, facilitating attachment to rocks or other sea plants, while the erect branched system elevates the alga for better access to light and nutrients.
• Variability in Growth Forms:
  • In some species, one of the two systems may be reduced. For instance, in epiphytic forms, the prostrate system is often more developed compared to the erect system.
  • The thallus can exhibit varying degrees of branching, ranging from sparingly to profusely branched, contributing to the overall surface area available for photosynthesis.
• Cellular Arrangement:
  • The cells of Ectocarpus are uniseriate, meaning they are arranged in a single row and joined end to end, creating long filaments.
  • In certain species, the older portions of the main branch may become corticated, forming a layer of descending rhizoidal branches that provide additional structural support.
• Terminal Features:
  • The terminal portions of branches may end in colorless hairs, which are supported by a basal meristem. These structures can play a role in nutrient absorption and anchorage.
• Erect Thallus Characteristics:
  • The erect portion of the thallus is usually irregularly branched, contributing to the overall complexity and adaptability of the organism.
  • The filaments that compose the thallus are formed from uninucleate rectangular cells, which provide structural integrity.
• Cell Wall Composition:
  • Each cell features a cell wall composed of two distinct layers. The outer layer is gelatinous, made up of algin and fucoidan, while the inner layer is composed of cellulose. This dual-layer structure helps in maintaining cell shape and provides resistance to environmental stresses.
• Pigments and Chromatophores:
  • The thallus contains various pigments crucial for photosynthesis, including chlorophyll-a, chlorophyll-c, carotene (fucoxanthin), and xanthophylls. These pigments contribute to the alga’s coloration and its ability to capture light energy.
  • The chromatophores within the cells contain a significant amount of xanthophylls, which aid in the photosynthetic process.
• Storage Compounds:
  • Ectocarpus stores reserve food material primarily in the forms of laminarin and mannitol, which serve as energy sources for growth and reproduction.
• Growth Patterns:
  • Growth occurs through apical growth in the prostrate part, allowing for horizontal expansion, while intercalary growth in the erect part facilitates vertical elongation and branching.

Mode of Reproduction/Life cycle of Ectocarpus

Ectocarpus exhibits two primary modes of reproduction: asexual and sexual. These reproductive strategies enable the organism to adapt to various environmental conditions and contribute to its distribution and survival in marine ecosystems.

• Asexual Reproduction:
  • Asexual reproduction in Ectocarpus occurs through the formation of zoospores. These zoospores are biflagellate, meaning they have two flagella that aid in motility.
  • The sporophytic (diploid) plant produces two types of sporangia:
    • Unilocular Zoosporangia:
      • These sporangia develop singly at the tips of small branchlets.
      • The terminal cell of the branchlet enlarges to serve as the sporangial initial, undergoing meiotic divisions followed by mitotic divisions to produce several hundred small cubical cells.
      • Each of these cubical cells is haploid and forms a biflagellate zoospore, which is liberated through a terminal or lateral pore.
      • After liberation, the zoospores settle on solid substrates with their anterior end, where they grow into a haploid or gametophytic thallus.
    • Plurilocular Zoosporangia:
      • These sporangia contain multiple locules, with the protoplast of each locule developing into a diploid zoospore.
      • Upon liberation, these diploid zoospores settle down and develop into a diploid or sporophytic thallus, which subsequently bears both unilocular and plurilocular sporangia.
• Sexual Reproduction:
  • Sexual reproduction in Ectocarpus can occur through isogamy, anisogamy, or oogamy, though most species primarily exhibit anisogamy.
  • Anisogamy can be classified as physiological or morphological. In this context, the gametes develop in the gametophytic thallus, which forms plurilocular gametangia similar to plurilocular sporangia.
  • Gamete Formation:
    • Zoogametes are produced singly from each locule and are equal in size, morphologically identical to the zoospores.
    • In species displaying morphological anisogamy, two types of gametangia are produced:
      • Megagametangium: Characterized by larger locules that yield larger gametes.
      • Microgametangium: Features smaller locules that produce smaller gametes.
  • Gamete Interaction:
    • Typically, the male gametes are active and motile, while female gametes are passive and sluggish.
    • A clustering of male gametes around a female gamete results in clump formation, facilitating fertilization.
    • Fusion occurs when one male gamete unites with the female gamete, resulting in the formation of a zygospore or zygote.
  • Zygote Development:
    • The zygote germinates directly into a new diploid thallus, and during this process, no meiosis occurs.
    • The newly developed diploid thallus subsequently bears both unilocular and plurilocular sporangia, continuing the cycle of reproduction.

Sargassum

Occurrence of Sargassum

Sargassum is a diverse genus of brown algae, consisting of approximately 150 species. It is predominantly found in warmer regions of tropical and subtropical seas, particularly in the southern hemisphere.

• Global Distribution:
  • The Sargasso Sea, located in the Atlantic Ocean near the African continent, is renowned for its dense populations of Sargassum. This area is characterized by its floating mats of the algae, which contribute significantly to the marine ecosystem.
  • The abundance of Sargassum in this region has led to the name “Sargasso Sea,” highlighting its ecological importance and the unique habitat it provides for various marine organisms.
• Presence in India:
  • In India, Sargassum is represented by 16 distinct species, which are primarily located along the western and southern coastal areas.
  • Common Indian species include:
    • Sargassum tenerrium
    • Sargassum carpophyllum
    • Sargassum duplicatum
    • Sargassum plagiophyllum
    • Sargassum wightii
• Ecological Importance:
  • The presence of Sargassum along the coasts supports a variety of marine life, providing habitat and food for numerous fish and invertebrates.
  • Its floating mats can serve as nurseries for juvenile fish and as feeding grounds for various marine species.

Structure of Thallus in Sargassum

The thallus of Sargassum exhibits a highly advanced parenchymatous structure characterized by either bilateral or radial organization. This complex organization allows Sargassum to thrive in diverse marine environments.

• Thallus Composition:
  • The thallus is sporophytic (diploid) and is primarily differentiated into two main components: the holdfast and the main axis.
  • The holdfast serves a crucial role in anchoring the thallus to the substratum, ensuring stability. In some species, the holdfast has a stolon-like appearance, while in free-floating species, it may be entirely absent.
• Main Axis:
  • The main axis, also referred to as the stipe or stem, is erect, elongated, or flat, typically reaching lengths of up to 30 cm, although this varies among species.
  • This axis features numerous primary laterals that exhibit unlimited growth potential.
• Leaf-like Structures:
  • Both the main axis and the primary laterals are adorned with flat, leaf-like branches known as secondary laterals or leaves.
  • These leaf-like structures possess distinct characteristics, including a blade, veins, and petiole-like components. A midrib is present in most leaves, except for in Sargassum enerve.
• Air Bladders:
  • Air bladders are integral to the structure, facilitating the floating ability of the plant by enhancing buoyancy.
  • In certain species, these air bladders may culminate in leaf-like structures.
• Reproductive Structures:
  • Receptacles, arising from branches, bear reproductive organs housed in specialized flask-shaped structures known as conceptacles.
• Anatomical Structure:
  • The main axis can be divided into three distinct anatomical regions:
    • Meristoderm: This outermost layer acts as the epidermis and is meristematic in nature, contributing to growth.
    • Cortex: Comprising narrow, elongated parenchymatous tissue, the cortex serves as a storage region for reserve food materials.
    • Medulla: The central medulla is crucial for transporting water and metabolites throughout the thallus.
• Leaf Anatomy:
  • The leaves also exhibit a three-part differentiation:
    • Meristoderm: Forms the outer epidermis.
    • Cortex: Located between the meristoderm and medulla.
    • Medulla: Responsible for conduction within the leaf structure.
• Paraphyses:
  • Within the conceptacles, unbranched filaments called paraphyses arise from the wall, contributing to reproductive processes.
• Growth Mechanism:
  • The thallus of Sargassum grows through a quadrangular apical cell, allowing for continued development and adaptability to its environment.

Mode of Reproduction in Sargassum

Sargassum exhibits two modes of reproduction: vegetative and sexual reproduction. It is important to note that asexual reproduction is entirely absent in this genus.

• Vegetative Reproduction:
  • Sargassum reproduces vegetatively through the fragmentation of its thallus.
  • When injury, death, or decay occurs in the older parts of the thallus, the structure breaks into smaller segments.
  • These segments are capable of growing into new thalli, thus facilitating the continuation of the species.
• Sexual Reproduction:
  • Sexual reproduction in Sargassum is characterized as oogamous, involving distinct male and female reproductive organs that develop in specialized flask-shaped cavities known as conceptacles.
  • The male reproductive organ is termed the antheridium, while the female organ is referred to as the oogonium.
  • These reproductive organs are formed in separate conceptacles, ensuring the differentiation of sex in the reproductive process.
  • Developmental Stages of Sexual Reproduction:
    1. Conceptacle Formation: The conceptacle begins to develop from a single initial cell. This initial cell undergoes transverse division to produce two cells: a lower basal cell and an upper tongue cell. The tongue cell elongates and subsequently divides transversely, leading to the formation of a small filament that soon disappears.
    2. Fertile Layer Development: The fertile layer of the conceptacle originates from the basal cell, where the sex organs will develop.
    3. Antheridium Formation: The antheridium arises from the fertile layer of the conceptacle. The antheridial initial cell divides to create a lower stalk cell and an upper antheridial cell. The upper cell ultimately develops into the mature antheridium.
    4. Mature Antheridium Structure: The mature antheridium is oval and encased by a two-layered wall: the outer layer known as the exochite and the inner layer referred to as the endochite. The antheridium is anchored to the conceptacle base via the stalk cell. It produces 64 pear-shaped biflagellate antherozoids.
    5. Detachment of Antheridia: Once mature, the antheridia detach from the stalk and exit the conceptacle through an opening called the ostiole.
    6. Oogonial Initials: Any cell within the fertile layer of the female conceptacle can serve as an oogonial initial. This initial undergoes transverse division to form a small lower stalk cell and a larger upper oogonial cell.
    7. Oogonia Development: The oogonial cell gradually transforms into a spherical shape filled with dense cytoplasm. The diploid nucleus within the oogonial cell undergoes meiotic division (reduction division) followed by two mitotic divisions, resulting in eight haploid nuclei. Only one of these nuclei becomes the egg nucleus, while the others degenerate.
    8. Mature Oogonia Release: Mature oogonia are expelled from the conceptacle but remain tethered to the conceptacle wall via a long gelatinous stalk.
    9. Fertilization Process: A large number of antherozoids cluster around the oogonium, attaching to its wall with their anterior flagella. Among them, only one antherozoid penetrates the oogonial wall.
    10. Zygote Formation: The fusion of male and female nuclei leads to the formation of a diploid zygote.
    11. Germination of Zygote: The zygote germinates immediately, first dividing transversely to produce a lower cell and an upper cell. The lower cell develops into rhizoids, while the upper cell eventually forms a new diploid thallus through anticlinal and periclinal divisions.
    12. Life Cycle Characteristics: The life cycle of Sargassum is classified as diplontic, with no alternation of morphological generations.