Cell Structure of Algae; Prokaryotic & Eukaryotic

Cell structure of algae reveals distinct differences between prokaryotic and eukaryotic forms. Prokaryotic algae consist of incipient nucleus and lacks cytoplasmic cell organelles whereas eukaryotic algae consist of membrane bound nucleus and contain certain cytoplasmic organelles.

Hence, Algae exhibit two distinct types of cell structures, allowing them to be categorized into two groups: Prokaryotic algae and Eukaryotic algae

Cell Structure of Algae – Prokaryotic Algae

Cell structure of algae in prokaryotic form reveals the presence of a incipient nucleus which lacks a nuclear membrane, and the DNA is dispersed within the nucleoplasm.

The cytoplasm does not contain key organelles such as mitochondria, endoplasmic reticulum, or chloroplasts.

Prokaryotic algae include the organisms previously known as blue-green algae, formerly classified as Cyanophyceae or Myxophyceae, but now referred to as Cyanobacteria due to their prokaryotic cell structure.

Cell structure of algae in prokaryotes consist of cell wall, cell sheath, photosynthetic lamellae, granular inclusions in cytoplasm, gas vesicles, nucleoplasm, plasmids etc.

Cell Wall and Cell Sheath

Cyanobacterial cells are surrounded by a gelatinous sheath and a distinct cell wall located outside the plasma membrane.
Chemical analysis reveals that it consists of a mucopolysaccharide (peptidoglycan), similar to bacterial cell walls.
Structurally, it is complex, composed of a polymer of N-acetylmuramic acid and N-acetylglucosamine, cross-linked by peptides and other compounds.
The wall typically has at least four layers, with the outermost layer containing lipopolysaccharides and proteins.

In many cyanobacteria, the cell wall is further surrounded by a gelatinous mucilage.
This mucilage may be thin and colorless in planktonic forms, while in subaerial forms, it is thick, firm, and often yellow or orange-brown in color, with multiple layers.
Certain aquatic species, such as Scytonema and Petalonema, also exhibit multilayered and colored sheaths.

Photosynthetic Lamellae

Cyanobacteria lack chloroplasts, but their cells contain pigmented membranes located in the peripheral region, known as the chromatoplasm.
This region houses photosynthetic lamellae or thylakoids, which are folded double membranes embedded with photosynthetic pigments such as chlorophyll a and various carotenoids.

Rows of granules called phycobilisomes are found on the surface of the thylakoids.
These granules contain phycocyanin, allophycocyanin, and sometimes phycoerythrin, pigments characteristic of cyanobacteria.
Additionally, thylakoids are known to harbor enzymes essential for respiration.

cell structure of algae; prokaryotic algae — Fig: Cell Structure of Algae; Prokaryotic Algae

Granular Inclusions of Cytoplasm

The ultrastructure of the cyanobacterial cytoplasm contains several types of granules.
Glycogen appears as granules of varying sizes located between the thylakoids.
Protein granules, known as cyanophycean granules, are composed of a polymer of two amino acids, aspartic acid and arginine, and serve as nitrogen storage.
Another common granule, found in algae thriving in phosphate-rich waters, is the polyphosphate granule, which stores phosphate.
Some cyanobacteria also contain large crystals of poly-beta-hydroxybutyrate.
A unique feature of cyanobacteria is the presence of polyhedral crystalline granules called carboxysomes.
These granules are composed of the enzyme ribulose-biphosphate carboxylase (Rubisco), which plays a crucial role in the photosynthetic fixation of carbon dioxide.
Like all bacterial cells, cyanobacteria contain ribosomes essential for protein synthesis.
These ribosomes are scattered throughout the cytoplasm and are of the 70S type, characteristic of prokaryotes, as opposed to the 80S type found in eukaryotic cells.

Gas Vesicles

Many planktonic cyanobacteria, such as Microcystis, have elongated, cylindrical structures in their cells called gas vesicles, which may occur singly or in bundles.
These vesicles enable the cells to float on the water’s surface. When the gas inside escapes, the vesicles collapse and flatten, causing the cells to sink to the bottom.
The vesicle walls are composed of a single layer of protein molecules, which are permeable to gases but impermeable to water.

Nucleoplasm

The central region of the cell, commonly called the nucleoplasm, contains the genetic material (DNA), which is functionally equivalent to the nucleus in eukaryotes.
This DNA appears as a network of fibrils and, like bacterial DNA, exists as a long thread in the form of a circular chromosome.
A cell may contain multiple copies of this chromosome. Unlike eukaryotic cells, the DNA in cyanobacteria is not associated with histone proteins.

Plasmids

Similar to bacteria, cyanobacterial cells also contain DNA in the form of small, covalently closed circular molecules known as plasmids.
Plasmids are not a permanent feature of the cells; they can be lost and regained over time.
Additionally, they are capable of multiplying within the host cells.

Heterocysts

Heterocysts are thick-walled cells found in filamentous cyanobacteria, either positioned between vegetative cells (intercalary) or at the ends of a filament (terminal).
Their primary function is the fixation of atmospheric nitrogen, as they possess the enzyme system nitrogenase required for this process.
A heterocyst has a thick wall composed of three distinct layers. The innermost layer contains glycolipids, which make the heterocyst impermeable to oxygen.
This is crucial because oxygen would otherwise inhibit the nitrogenase enzyme, preventing nitrogen fixation.
Additionally, heterocysts have numerous photosynthetic lamellae, although these are less densely packed compared to those in vegetative cells.

Akinetes

These thick-walled cells, also called spores, serve as structures for perennation.
In a filament, all vegetative cells or only specific cells, such as those near a heterocyst, can develop into spores.
Akinetes have thick walls and are typically light brown, dark brown, or black in color.
The cell contents are highly granular, containing glycogen, but lacking polyphosphate.
Akinetes can endure extended periods of desiccation and, when exposed to favorable conditions, can germinate to produce new filaments.

Cell Structure of Algae – Eukaryotic Algae

Eukaryotic algae are classified into several divisions, each characterized by distinct cell structures and unique traits.

Common features across all groups include a membrane-bound nucleus, chromosomes, plastids, mitochondria, Golgi bodies, and 80S type of ribosomes.

In addition to cell division through mitosis, many groups also reproduce sexually, involving gamete fusion and meiosis (reduction division).

Fig: Cell Structure of Algae; Eukaryotic Algae

Cell structure of algae in eukaryotes consist of cell wall, plasma membrane, plastids, pyrenoids, nucleus, mitochondria, golgi body, endoplasmic reticulum, vacuole, flagella, eye spots etc.

Cell Wall

The cell wall of algae is primarily composed of cellulose, with additional compounds added during development.
In brown algae, substances such as hemicelluloses, alginic acid, fucan, and fucoidin are also present.
In diatoms, the cell wall is primarily made of silica.
The cells of Division Chrysophyta lack a true cell wall, instead being covered by silica scales (e.g., Mallonionas).
In coccolithophorids, intricate scales made of calcium carbonate (calcite) are present.
Red algae have a cell wall that contains polysulfate esters of carbohydrates, in addition to cellulose and pectin.
Calcium carbonate deposits are found on the surfaces of algae from various groups of marine seaweeds, known as calcareous algae.
Examples include Neomeris and Udotia (green algae), Corallina (red algae), Padina (brown algae), and the freshwater alga Chara.

Plasma Membrane

The cell membrane, also known as the plasma membrane, safeguards the protoplast and its internal components from the external environment.
It consists of lipids and proteins arranged in a fluid mosaic model.
This thin, elastic membrane is selectively permeable, allowing it to regulate the movement of substances into and out of the cell.

Plastids

All photosynthetic algae contain plastids, or chloroplasts, whose basic structure is similar to that of higher plants.
The shape and position of chloroplasts in algae vary by species.
When chloroplasts are positioned at the center of a cell, they are referred to as axile, while those located near the cell’s edge are called parietal.
The number of chloroplasts also varies, ranging from one to several, but remains consistent within a species.
Under a microscope, chloroplasts can be identified by distinct shapes such as cup-shaped (Chlamydomonas), girdle-shaped (Ulothrix), spiral band-shaped (Spirogyra), and stellate (star-shaped, e.g., Zygnema).

Pyrenoids

The plastids of many green algae contain distinct proteinaceous granules known as pyrenoids, around which starch is accumulated.
In many cases, photosynthetic thylakoids can be observed either passing through the pyrenoid matrix or closely associated with it.
When the chloroplasts divide, the pyrenoids also divide, leading to the formation of new pyrenoids.

Nucleus and Chromosome

Many algae have a single nucleus per cell, but some green algae, such as Cladophora and Vaucheria (Xanthophyceae), are multinucleate, containing more than one nucleus.
Similar to the nuclei of eukaryotic plants and animals, the algal nucleus is surrounded by a distinct double membrane with pores.
During interphase (the resting, non-dividing phase), uncoiled chromatin threads are visible within the nucleus. Chromatin consists of DNA, histones, and non-histone proteins. The chromatin condenses to form chromosomes during cell division.
Many algal nuclei contain one or more globular nucleoli, sometimes attached to specific regions of a chromosome called the nucleolus organizer.
The nucleolus may disappear during cell division but reappears in interphase. It is involved in the synthesis of cytoplasmic ribosomes.
The structure of the nucleus in the algal groups Euglenophyta and Dinophyta is distinct and differs from that of other eukaryotes.
During interphase, these algae exhibit highly condensed chromosomes instead of uncoiled chromatin fibers. Additionally, unlike other organisms, their nuclei do not contain histone proteins.
The number of chromosomes in each genus or species of algae does not correlate with its systematic classification.
The smallest recorded number is n=2, while the highest can exceed 600.
The size of individual chromosomes also varies, with large chromosomes found in Oedogonium, Cladophora, and Chara.

Mitochondria

The number of mitochondria in algal cells can vary, ranging from just one in some flagellates to many in other algae.
Their size and shape also differ significantly. The ultrastructure reveals a double membrane, with the inner membrane folded inward to form cristae that extend into the lumen.
New mitochondria are formed by the division of existing mitochondria, similar to the process in plastids.
It is believed that mitochondria originated from endosymbiotic bacteria that adapted to live inside ancestral eukaryotic host cells.
Like chloroplasts, mitochondria contain circular DNA, RNA, 70S ribosomes, and the machinery necessary for protein synthesis.

Golgi Bodies

These structures, also known as dictyosomes, are commonly found in algal cells.
They consist of 2 to 20 stacked lamellae or membranes.
Dictyosomes play a crucial role in the formation of cell wall material, particularly in red algae.
In many other algae, they are also involved in secretory functions.

Endoplasmic Reticulum

The endoplasmic reticulum is a large, interconnected network of tubular membranes and flattened disc-like structures called cisternae.
It extends throughout the cytoplasm and connects to the nuclear membrane.
The cisternae are arranged parallel to each other and are interconnected.
Ribosomes are frequently attached to the cytoplasmic-facing side of the cisternae, although not all cisternae have ribosomes.
The ER is classified as rough or smooth based on the presence or absence of ribosomes.

Vacuole

Algae consist of one or more vacuoles, enclosed by a membrane called the tonoplast.
They serve specialized functions, contributing to the survival and adaptation of algae in aquatic environments. Algal vacuoles are of different types.
Contractile vacuoles, are small and regularly expand and contract to expel metabolic waste, regulate water content, and secrete excess water.
Complex vacuoles consist of a cytopharynx, other vacuoles, and a large reservoir. They are essential for osmoregulation and can store reserve food materials. These vacuoles are characteristic of Dinophyceae and Euglenophyceae.
Gas vacuoles are cylindrical, transparent stacks filled with gas. They provide buoyancy for planktonic algae and protect them from intense light.

Flagella

Flagella serve as the means of locomotion for the motile cells of algae and are present in all divisions except Rhodophyta.
Algae may be motile themselves, as seen in unicellular and colonial algae, or they may produce reproductive motile cells, such as zoospores and gametes, at certain stages of their life cycle.
The flagella of algae vary in number, length, appendages, and the point of attachment on the cell.
The surface of the flagellum may be smooth (acronematic) or may have one or more lateral hairs (pleuronematic).
When two flagella are present, they may be of equal length (isokont) or one may be shorter than the other (heterokont).

Eyespots

Motile cells of algae from groups such as Chlorophyta, Phaeophyta, Euglenophyta, and Chrysophyta contain orange-red colored eyespots.
In some algae, the eyespot may be part of the chloroplast and is located at the base of the flagellum, whereas in Euglena, it is distinct and separate from the chloroplasts.
The typical eyespot found in green algae, such as Chlamydomonas, consists of a row of orange-colored lipid granules, which are part of the thylakoids at the anterior portion of the chloroplast.
These granules contain carotenoids, with β-carotene being the most prominent.