Chara

Chara algae, belonging to the Characeae family, is a multicellular green algae resembling land plants with stem-like and leaf-like structures. They thrive in freshwater, especially in limestone regions of the northern temperate zone, where they attach to muddy bottoms.

1.0Structure of Chara

Chara comprises 188 species of freshwater green algae thriving in ponds, lakes, and slow-moving water. Chara is found primarily in hard fresh water, rich in organic matter and calcium, and deficient in oxygen. Chara plants are often encrusted with calcium carbonate and are called stonewort. Chara constantly emits a disagreeable onion-like odour due to the presence of sulphur compounds. 

Chara's thallus is multicellular, branched, and macroscopic, typically 20-30 cm tall but occasionally up to 90 cm to 1 m. Some species, like C. hatei, are 2-3 cm smaller. Resembling Equisetum, it's nicknamed aquatic horsetail. Thallus differentiation includes rhizoids and the main axis. In cell structure, nodal cells are isodiametric with dense cytoplasm, few chloroplasts, and absent central vacuole. Internodal cells are elongated with multinucleate cytoplasm, discoid chloroplasts, and streaming movements. Porous cell walls aid cytoplasmic continuity between nodal and internodal cells.

Rhizoids: Rhizoids of Chara are multicellular, white, and thread-like, emerging from rhizoidal plates at the base of the main axis or peripheral cells of lower nodes. They feature oblique septa and minute solid particles at their tips, acting as statoliths. Rhizoids exhibit apical growth, aiding in attachment to substrates like mud or sand, mineral absorption, and vegetative propagation through bulbil and secondary protonema formation.

Main Axis: The main axis of Chara is tall, erect, and branched, with distinguishable nodes and internodes. Internodes are elongated or oblong, often surrounded by a single-celled cortex in corticate species. Nodes contain a central pair of small cells and 6-20 peripheral cells originating from a single nodal initial cell. Four types of appendages develop at nodes:

• Limited Growth Branches: Arising in whorls from peripheral node cells, these branches, also known as branchlets or primary laterals, cease growth after forming 5-15 nodes. Stipulodes and reproductive structures form on their nodes.
• Unlimited Growth Branches: Emerging from branchlet axils, these long laterals resemble the main axis and continue growth indefinitely, bearing primary laterals at nodes.
• Stipulodes: Short, unicellular oval outgrowths develop at the basal node of limited growth branches. In species like C. burmanica, stipulode numbers are twice that of primary laterals, categorised as bi-stipulate. Chara can be further classified based on stipulode arrangement into Haplostephanous (single row) and Diplostephanous (two rows).
• Cortex: in many species, the internodal cells of the main axis are en-sheathed by cortex cells. Such species are called corticated species. The cortex consists of vertically elongated narrow cells. The species in which cortical cells are absent are called ecorticate species. 

Structure of Cell

The main axis of chara consists mainly of two types of cells. 

The nodal cells are smaller in size and are isodiametric. The cells are dense cytoplasmic and uninucleated with few small ellipsoidal chloroplasts. The central vacuole is not developed; instead, many small vacuoles may be present. The cytoplasm can be differentiated into outer exoplasm and inner endoplasm. 

The inter-nodal cells are much elongated. The cytoplasm is present around a large central vacuole. The cells are multinucleated and contain many discoid chloroplasts. The cytoplasm is also differentiated into outer exoplasm and inner endoplasm. The endoplasm shows streaming movements. The cell walls between the nodal and internodal cells are porous to help in cytoplasmic continuity between cells.

2.0Diagram of Chara

3.0Classification in Chara

4.0
Reproduction in Chara 

Vegetative Reproduction

The common methods of vegetative reproduction in Chara are as follows : 

• Bulbils: a small tuberous oval-shaped body formed either on rhizoid or in stem gets detached and germinates into the new plant. 
• Amorphous bulbils: These clusters consist of numerous irregularly shaped cells, forming on the lower node of the main axis or on rhizoids. Known as amorphous bulbils, these structures serve as perennating bodies. When the main plant dies in adverse conditions, the bulbils endure, ready to regenerate Chara plants when conditions become favourable.
• Amylum stars: a multicellular star-shaped body is formed on the lower node of the main axis. Amylum starch is present in these cells; due to this, their name is amylum stars. 
• Secondary protonema:  these are the filamentous-like structures that are formed in the basal cells of rhizoids. These develop into the new plant.

Sexual Reproduction 

• Sexual reproduction in Chara is an oogamous type; it is an advanced type of sexual reproduction in which the Male gamete is small and motile or immotile while the female gamete is large and immotile. 
• The male sex organ is known as the antheridium or globule, and the female sex organ is the oogonium or nucule. The nucule arises above the globule, and the globule matures earlier than nucule. The species of Chara can either be homothallic or heterothallic. 

Homothallic: When the male and female sex organs are borne on the same nodes.

Heterothallic: When the male and female sex organs are borne by the different node. Most of the species are homothallic. 

5.0Structure of Globule 

Mature globules are spherical and range in colour from yellow to red. Each globule comprises eight shield cells forming curved plates on its outer surface. These shield cells enclose a centrally positioned rod-shaped structure called the manubrium. At the distal end of each manubrium, one or more globose cells form the primary capitulum, which then gives rise to two or more secondary capitula. Each secondary capitulum eventually develops 2-4 long antheridial filaments. These filaments consist of 25-250 cells, each containing a biflagellate, coiled, and uninucleate antherozoid.

6.0Structure of Nucule

The female sex organ (nucule) of Chara is initially large and ovoid, elongating as it matures. It contains an oogonium containing a single egg, surrounded by an envelope of five elongated tube cells. These cells spiral clockwise, forming a protective flask-shaped sheath around the oogonium. Each sheath cell ends in a small erect cell, collectively forming a crown of five closely fitting cells atop the mature oogonium, known as the corona.

Within the oogonium, the protoplast produces a large ovum rich in starch and oil, with a single nucleus positioned at its base. The apex of the ovum features a colourless, finely granular cytoplasm, constituting the receptive spot. The presence of the sheath around the oogonium distinguishes Chara from other green algae, leading to comparisons with the archegonium of bryophytes.

7.0Fertilisation in Chara

When the oogonium reaches maturity, the five tube cells separate, creating narrow slits between them. Antherozoids are attracted chemotactically towards the ovum. They enter through these slits and penetrate the gelatinized wall of the oogonium. While many antherozoids may enter the oogonium, only one fertilises the egg, resulting in the formation of a diploid zygote. The zygote envelops itself in a thick wall, forming an oospore.

Germination of Oospore

The oospore germinates under favourable conditions. The diploid nucleus in the apical colourless region undergoes meiosis, yielding four haploid daughter nuclei. At this point, a septum divides the oospore into two cells of unequal size. The smaller apical cell contains a single nucleus, while the larger basal cell contains three nuclei, which gradually degenerate.

The apical cell of the oospore divides longitudinally, giving rise to a rhizoidal initial and a protonemal initial. The rhizoidal initial exhibits positive geotropism, forming primary rhizoids, whereas the protonemal initial displays negative geotropism, generating primary protonema. The primary protonema differentiates into nodes and internodes.

Peripheral cells of the basal node produce rhizoids and secondary protonema, while those of the upper nodes give rise to lateral branches.

8.0Life Cycle of Chara

The life cycle of Chara is haplontic, their main plant body is gametophytic (haploid), and the haploid plant produce haploid gametes after the fertilisation a diploid oospore is formed which divide meiotically and haploid spore is formed, these spore germinate and haploid plant is formed. 

9.0Functions of Chara

• Photosynthesis : The main function of the thallus and its branches is to perform photosynthesis. Chara helps oxygenate the surrounding water by producing oxygen during photosynthesis. This is beneficial for other aquatic organisms in the freshwater environment.
• Ecosystem Support : Chara, with its dense structure, provides habitats for microorganisms and small aquatic creatures, contributing to the biodiversity of freshwater ecosystems.
• Carbonate Precipitation : The deposition of calcium carbonate by Chara aids in regulating the pH of the surrounding water and might contribute to the reduction of Carbon dioxide levels.
• Nutrient Cycling : Chara is involved in nutrient cycling in freshwater ecosystems, as it absorbs and recycles nutrients from the water.