Charophyta – General Characteristics, Occurrence, Morphology, Cell Structure And Life-cycle of Chara

What is Charophyta?

• Charophyta, commonly referred to as charophytes, represents a distinct group of freshwater green algae. This group is sometimes categorized as a division, superdivision, or unranked clade within the broader classification of algae. Importantly, the Charophyta are recognized as the closest relatives of land plants, with the Embryophyta, or true terrestrial plants, having evolved from unicellular charophytes. This evolutionary connection underscores the significant role that charophytes play in the understanding of plant evolution.
• The classification of charophytes aligns them with the group known as Streptophyta, which encompasses all land plants. Within the Charophyta, the sister group is Chlorophyta, another major lineage of green algae. Certain charophyte groups, such as Zygnematophyceae (previously known as Conjugatophyceae), exhibit unique reproductive strategies; for instance, these algae lack flagella, and their sexual reproduction does not involve free-swimming flagellate sperm. Conversely, in the orders Charales (stoneworts) and Coleochaetales, flagellate sperm are present, establishing a link to land plants where flagellate sperm are found, except in conifers and flowering plants.
• Fossil evidence suggests that charophytes have existed since at least the early Devonian period. Fossilized remains of stoneworts, resembling modern forms, have been discovered in locations such as the Rhynie chert of Scotland. Additional fossils from the Late Devonian, including species of Octochara and Hexachara, represent some of the earliest documented charophyte structures, providing valuable insights into their evolutionary history.
• Charophyta derives its name from the genus Chara, a key representative within this group. However, despite its historical significance, the classification of Charophyta has not been narrowed to include only this genus. Instead, it encompasses a broader spectrum of algae, including the Charophyceae, which forms a major branch of charophytes. Zygnematophyceae, in contrast, typically features two prominent chloroplasts in each cell, as opposed to the numerous discoid chloroplasts found in other green algae. Reproduction in these forms can be asexual, involving septum formation, or sexual, through the conjugation of entire cell contents between two cells.
• In various environments, particularly non-calcareous, acidic waters found in oligotrophic lakes, members of the Zygnematophyceae thrive. This includes desmids, which can be unicellular or filamentous and are often dominant in these pristine water bodies.
• Another significant group within the Charophyta is the Klebsormidiophyceae, typified by the genus Klebsormidium. This group is characterized by simple filamentous forms featuring circular, plate-like chloroplasts. Reproduction can occur through fragmentation or by producing isogamous gametes, where male and female gametes are morphologically indistinguishable.
• The Charales, also known as stoneworts, consist of freshwater and brackish algae, characterized by their slender green or grey stems. The grey appearance in some species results from the deposition of calcium carbonate on the cell walls, which obscures the green chlorophyll. The structure of the Charales includes lateral branchlets arranged in whorls and attached to the substrate by rhizoids. Their reproductive systems include specialized organs known as antheridia and oogonia, which differ significantly from similar structures in other algal groups. Following fertilization, a protonema develops, eventually leading to the formation of sexually reproducing algae.
• A notable addition to the understanding of Charophyta is the discovery of a new genus, Streptofilum, found in sandy soils in the Czech Republic. This genus may occupy a unique phylogenetic position, potentially warranting its own class. Interestingly, Streptofilum lacks a traditional cell wall; instead, its cells are enveloped by a multilayered membrane composed of specific scales. This filamentous, unbranched algae is typically surrounded by a mucilaginous sheath, which can break down into smaller units, facilitating its reproductive process.

General Characteristics of Charophyta

Below are the key characteristics that define Charophyta:

• Macroscopic Plant Body: The overall structure of Charophyta is macroscopic, allowing for visibility and identification in aquatic environments.
• Differentiation into Nodes and Internodes: The plant body is organized into distinct nodes and internodes. Nodes bear whorls of branches that display limited growth, resembling the leaves found in angiospermic plants. This structural organization aids in the functional distribution of resources and growth.
• Cell Structure: Cells within Charophyta are prokaryotic, elongated, uninucleate, and characterized by discoid chloroplasts. Each cell is enclosed by a cellulosic cell wall, providing structural support and facilitating photosynthesis.
• Central Vacuole: A prominent central vacuole occupies the core of internodal cells, causing the cytoplasm to be located peripherally. This vacuole plays a critical role in maintaining turgor pressure and storage of nutrients.
• Nuclear Position: The nucleus is centrally positioned within each cell, contributing to efficient cellular functions and regulation.
• Reproductive Strategies: Charophyta employs both vegetative and sexual reproduction mechanisms to propagate.
  • Vegetative Reproduction: This occurs through various structures, such as:
    • Amylum Stars: These star-shaped aggregates develop from the lower nodes and are densely filled with starch, serving as energy reserves.
    • Bulbils: These structures are food-storing bodies that form on rhizoids, facilitating survival in adverse conditions.
    • Protonema: Outgrowths resembling protonema develop from the nodes, contributing to vegetative growth.
• Absence of Asexual Reproduction via Spores: Notably, Charophyta does not reproduce asexually through spore formation, which differentiates them from other algal groups.
• Sexual Reproduction: The sexual reproduction in Charophyta is strictly oogamous, meaning it involves the fusion of specialized gametes. The male gametes are produced in complex structures called antheridia (globules), while the female gametes are formed in oogonia (nucules). This complexity underscores the evolutionary advancement of charophytes compared to simpler algae.
• Diversity of Genera and Species: As of now, there are seven recognized living genera within Charophyta, comprising approximately 294 species. These include:
  • Chara (27 species)
  • Tolypella (3 species)
  • Nitella (37 species)
  • Nitellopsis (1 species)
  • Lychnothaminus (1 species)
  • Lamprothamnium
  • Protochara

Occurrence of Charophyta

Charophyta, particularly the genus Chara, is prevalent in various aquatic environments, primarily thriving in freshwater ecosystems. Understanding the occurrence of charophytes provides insight into their ecological roles and adaptability to diverse habitats. The following points outline the key occurrences and characteristics of Chara species:

• Freshwater Habitats: The majority of Chara species, approximately 180 in total, are predominantly found submerged in shallow waters of ponds, lakes, and tanks. This habitat preference emphasizes their reliance on aquatic ecosystems for survival and growth.
• Tolerance to Environmental Conditions: While most Chara species inhabit freshwater environments, some have adapted to more extreme conditions. For example, C. baltica is known to grow in alkaline waters, and C. fragilis can thrive in hot springs. These adaptations highlight the ecological versatility of charophytes.
• Calcium and Magnesium Carbonate Encrustation: Species of Chara that inhabit heavy water environments tend to accumulate calcium and magnesium carbonate deposits on their bodies. This phenomenon results in their common name, “stoneworts.” The encrustation not only provides structural support but also contributes to the rough and brittle texture of these plants.
• Distribution in India: In India, Chara is represented by about 30 species, including C. braunii, C. gracilis, C. gymnoptis, and C. zeylanica, which are commonly found in various freshwater habitats. This diversity underscores the ecological importance of Chara within Indian aquatic ecosystems.
• Physical Strengthening: The accumulation of calcium and magnesium carbonate on Chara species enhances their physical resilience, allowing them to withstand various environmental stresses. Consequently, these adaptations enable charophytes to flourish in conditions that may be challenging for other aquatic plants.

Thallus Structure of Charophyta/Morphology of Charophyta

The thallus structure of Charophyta, particularly in the genus Chara, is a complex and highly organized system that plays a crucial role in the plant’s survival and adaptation to its aquatic environment. The thallus is primarily differentiated into two main components: the rhizoids and the main axis. Below are detailed descriptions of these structural features.

• General Characteristics:
  • The thallus of Chara is macroscopic, multicellular, and typically exhibits a profuse branching pattern.
  • The height of the thallus generally ranges from 20 to 30 cm, although some species can reach lengths of up to 90 cm to 1 meter. In contrast, smaller species such as C. hatei may only measure 2-3 cm.
  • Morphologically, the thallus bears resemblance to Equisetum, commonly leading to the designation of Chara as aquatic horsetail.
• Rhizoids:
  • The rhizoids are characterized as thread-like, white, multicellular, uniseriate, and branched structures. They can develop from rhizoidal plates located at the base of the main axis or from peripheral cells of lower nodes.
  • These structures possess oblique septa and display apical growth, which contributes to their elongation.
  • Functionally, rhizoids serve multiple purposes: they attach the plant to the substrate (such as mud or sand), absorb minerals, and facilitate vegetative reproduction by forming bulbils and secondary protonema.
  • The tips of rhizoids contain minute solid particles that function as statoliths, aiding in the plant’s response to gravity.
• Main Axis:
  • The main axis of Chara is erect, elongated, and branched, differentiated into nodes and internodes.
  • Internodes: Internodal cells are typically elongated and cylindrical. In some species, these cells may be surrounded by a layer of cortical cells, classifying them as corticated species. In species lacking this cortical layer, they are termed ecorticated.
  • Nodes: Each node consists of a pair of small central cells encircled by 6-20 peripheral cells. The cells at the node arise from a single nodal initial cell.
• Appendages from Nodes:
  • Nodes develop four types of appendages: branches of limited growth, branches of unlimited growth, stipulodes, and the cortex.
  • Branches of Limited Growth: These branches arise in whorls of 6-20 from peripheral cells at the nodes of the main axis or from branches of unlimited growth. They are also known as branchlets or primary laterals. Their growth ceases after forming 5-15 nodes and internodes.
    • Reproductive structures and stipulodes are produced at the nodes of these branches.
  • Branches of Unlimited Growth: These branches emerge from the axils of branches of limited growth and are also referred to as axillary branches or long laterals. They are similar in structure to the main axis and can grow indefinitely, forming their own nodes and internodes.
  • Stipulodes: At the basal nodes of the branches of limited growth, unicellular outgrowths called stipulodes develop. The number of stipulodes can vary: in some species, it is equal to the number of primary laterals (unitipulate), while in others, it is double (bi-stipulate). The arrangement of stipulodes can be classified as haplostephanous when present in one whorl and diplostephanous when in two whorls.
  • Cortex: In certain species, internodal cells of the main axis are enveloped by cortex cells, making them corticated species. The cortex consists of vertically elongated narrow cells, with filaments from upper and lower nodes meeting at the midpoint of the internode. Species without a cortex are referred to as ecorticated.

Cell Structure of Charophyta

The cell structure of Charophyta, particularly in the genus Chara, exhibits specialized adaptations that enable efficient functioning in aquatic environments. The main axis of Chara comprises two principal types of cells: nodal cells and internodal cells. Understanding the distinctions and functions of these cell types is crucial for grasping the overall physiology of Chara.

• Nodal Cells:
  • Nodal cells are relatively smaller in size and isodiametric, meaning they have a uniform diameter.
  • These cells are densely cytoplasmic and uninucleate, containing a limited number of small ellipsoidal chloroplasts.
  • The central vacuole is typically underdeveloped in nodal cells; however, multiple small vacuoles may be present.
  • The cytoplasm can be differentiated into two distinct regions:
    • Exoplasm: The outer region of the cytoplasm.
    • Endoplasm: The inner region adjacent to the vacuole.
  • The structure of nodal cells contributes to their role in supporting the plant and facilitating nutrient transport.
• Internodal Cells:
  • In contrast to nodal cells, internodal cells are significantly elongated, which allows for increased surface area and volume.
  • These cells contain a large central vacuole, providing turgor pressure that maintains cell rigidity.
  • Each internodal cell is multinucleate, which is beneficial for regulating cellular functions across the elongated structure.
  • Numerous discoid chloroplasts are present, facilitating photosynthesis and energy production.
  • Like nodal cells, the cytoplasm in internodal cells is also differentiated into:
    • Exoplasm: The outer layer of the cytoplasm, which interacts with the cell wall.
    • Endoplasm: The inner layer, which contains the vacuole and organelles.
  • The endoplasm of internodal cells exhibits streaming movements, which are crucial for the distribution of nutrients and cellular components.
    • This cytoplasmic streaming is facilitated by the alternating contraction and expansion of protein fibrils fixed to the cell wall. Only the endoplasm is capable of this streaming, ensuring continuous movement up and down along the longitudinal plane of the cell.
• Cell Wall Characteristics:
  • The cell walls between nodal and internodal cells are porous, promoting cytoplasmic continuity between adjacent cells. This permeability allows for the efficient transfer of materials and signals across the plant structure, enhancing the overall function of Chara.

Reproduction of Charophyta

The reproduction of Charophyta, particularly in the genus Chara, involves both vegetative and sexual means, with a notable absence of asexual reproduction. This dual reproductive strategy allows for effective colonization and adaptation in aquatic environments. Understanding the mechanisms of reproduction in Chara is essential for comprehending its life cycle and ecological roles.

• Vegetative Reproduction:
  • Chara employs various structures for vegetative reproduction, enabling the formation of new plants from existing tissues:
    • Bulbils:
      • These are small oval or spherical bodies that develop on stem or root nodes.
      • Bulbils can be found on the roots of C. aspera and the stems of C. baltica.
      • Once detached, bulbils germinate and give rise to new plants.
    • Amorphous Bulbils:
      • Amorphous bulbils are small cells that aggregate at the nodes.
      • They are observed in species such as C. fragilis and C. baltica.
      • Upon detachment from the mother plant, these bulbils also germinate, leading to the formation of new individuals.
    • Amylum Stars:
      • These structures are multicellular aggregations resembling stars, filled with amylum starch.
      • Amylum stars develop at the nodal cells of the basal region, such as in C. stelligera.
    • Secondary Protonema:
      • Secondary protonema are thread-like structures that develop from the primary protonema or the basal cell of the rhizoid.
      • They serve as another source for new plant development.
• Sexual Reproduction:
  • The sexual reproduction in Chara is characterized as an advanced oogamous type, featuring large and macroscopic sex organs.
    • Male Sex Organ (Globule):
      • The male sex organ is spherical, typically yellow to red in color, and is referred to as the globule.
      • Globules develop at the nodes of branches with limited growth, interspersed with secondary laterals.
    • Female Sex Organ (Nucule/Oogonium):
      • The female organ is more or less oval and green, known as the nucule or oogonium.
      • In homothallic species, the nucule is always positioned singly above the globule.
  • Most Chara species are homothallic or monoecious, meaning both male and female sex organs are present on the same plant. However, some species are heterothallic or dioecious, such as C. wallichii.
• Structure of Mature Globule:
  • Mature globules are spherical and exhibit a yellow to red coloration.
  • Each globule is composed of eight curved plates known as shield cells located on the outer side.
  • From the inner side of each shield cell, a centrally placed rod-like structure called the manubrium is formed.
    • At the distal end of each manubrium, one or more globose cells develop, referred to as primary capitula.
    • Each primary capitulum gives rise to two or more secondary capitula.
    • Ultimately, each secondary capitulum can develop 2-4 long antheridial filaments.
      • Each antheridial filament comprises 25-250 cells, with each cell known as an antheridium.
      • Each antheridium produces a biflagellate, coiled, and uninucleate antherozoid, leading to the potential production of 20,000 to 50,000 antherozoids from a single globule.
• Structure of Mature Nucule or Oogonium:
  • The nucule of Chara is oval with a short stalk, similar to the globule in terms of development.
  • It is also formed at the node of primary laterals, positioned just above the globule in homothallic species.
  • The nucule consists of a central cell, a stalk, and a large egg at the top, covered at the base by five spirally twisted tube cells, except at the apex where they form a crown of five corona cells.
  • The jacket of the nucule bears similarity to the neck cells of the archegonium found in Bryophytes.
• Development of Sex Organs:
  • The globule develops at the node of branches with limited growth. The single peripheral cell functions as the antheridial initial, undergoing transverse division to form two cells: a lower pedicel cell, which becomes the stalk, and an upper antheridial mother cell.
  • The antheridial mother cell undergoes multiple divisions, resulting in an octant stage with eight cells. These cells continue to divide, producing outer shield cells, a manubrium, and primary capitula, which further develop into secondary capitula and antheridial filaments.
  • The nucule development starts with the oogonial initial from the peripheral nodal cell. This initial undergoes transverse divisions, forming a three-celled stage. The lower cell becomes the pedicel, while the middle cell produces sheath initials surrounding a central cell.
  • The oogonial mother cell divides to form a stalk cell and an egg, which elongates and develops a receptive spot. The sheath initials differentiate into crown-like corona cells at the top and spirally twisted tube cells for protection.
• Fertilization:
  • During fertilization, tube cells separate slightly, creating openings for the entry of antherozoids. Typically, only one antherozoid approaches the receptive spot of the egg, fusing to form an oospore (2n).
• Oospore:
  • The oospore is a hard, spherical to ellipsoidal structure, varying in color from light yellow to brown, red, or black.
  • It possesses four-layered walls, with the outer two colored and the inner two colorless.
• Germination:
  • Upon germination, the nucleus of the oospore migrates to the upper region, undergoing meiotic division to form four haploid nuclei.
  • The oospore divides into two unequal cells: a smaller upper lenticular cell containing one nucleus and a larger lower basal cell containing three nuclei, which gradually degenerate.
  • The lenticular cell projects outward, rupturing the oospore wall, and divides mitotically to form a larger protonemal initial and a smaller rhizoidal initial.
  • The protonemal initial differentiates into nodes and internodes, forming the upper part of the plant body, while the rhizoidal initial develops into rhizoids. Secondary rhizoids may also form from the lower node of the protonemal filament.

Taxonomic Status of Chara

The taxonomic status of Chara, a genus of green algae commonly referred to as stoneworts, has been a subject of debate among scientists for many years. Various researchers have proposed different classifications based on a range of morphological, physiological, and reproductive characteristics. This overview synthesizes key perspectives from various taxonomists, illustrating the complexity of Chara’s classification.

• Historical Perspectives:
  • The systemic position of Chara has evolved through the contributions of several notable botanists:
    • Fritsch (1935) categorized Chara within the order Charales of the class Chlorophyceae, emphasizing the following criteria:
      • The cellulosic nature of the cell wall.
      • The presence of chlorophyll a and b as photosynthetic pigments.
      • Utilization of starch as a reserve food.
      • The existence of two flagella of equal length.
      • A life cycle pattern resembling typical chlophycean members.
• Smith’s Classification (1938, 1955):
  • Following Fritsch, Smith proposed placing the order Charales in a separate class called Charophyceae under the division Chlorophyta. His classification was based on distinct characteristics:
    • Commonly known as stoneworts.
    • Possession of an erect branched thallus, differentiated into nodes and internodes.
    • Each node supporting a whorl of branches of limited growth, with potential for unlimited growth in axillary branches.
    • Heavily calcified structures.
    • The female reproductive structures (nucule) are one-celled and surrounded by a sheath of sterile cells, consistently located on the leaves.
    • The antheridia are one-celled, organized into uniseriate branched filaments, enclosed by a common spherical envelope made up of eight cells.
• Prescott’s Contribution (1965):
  • Prescott reinforced Smith’s classification, continuing to place Chara within the Charophyceae of the phylum Charophyta.
• Bold and Wynne (1978):
  • Bold and Wynne expanded upon previous classifications by placing Chara and other members under the division Charophyta, notably opting not to include “phyco” in the division’s name due to uncertainty regarding their classification as true algae.
• Evolutionary Characteristics:
  • Chara exhibits advanced traits that suggest its evolutionary position is higher than that of typical algae and closer to the Bryophytes:
    • A highly developed aerial portion consisting of differentiated nodes and internodes, alongside a prostrate root-like system.
    • Complex sexual reproductive organs.
    • The presence of elongated biflagellate antherozoids.
    • An oogamous type of sexual reproduction, characterized by the production of large non-motile eggs.
    • Elaborate post-fertilization changes.
• Transition Between Groups:
  • The unique characteristics of Chara indicate its evolutionary significance:
    • The haploid nature of the plant body.
    • Although the nucule appears complex, it is fundamentally a simple unicellular structure typical of algae.
    • The diploid stage is limited to the zygotic phase.
• Cytological and Morphological Aspects:
  • The morphological and cytological traits of Chara, alongside its oospore ornamentation, led researchers such as Dr. P. Chatterjee, Dr. Samit Roy, and Dr. Ruma Pal to argue that Chara constitutes a specialized group within the order Charales, justifying its classification under Chlorophyceae.
• Indian Species of Chara:
  • Notable Indian species include:
    • Chara brachypus
    • C. fragilis
    • C. gymnopitys
    • C. zeylanica
    • C. braunii
    • C. benthamii