Algae – Introduction and Classification

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What is Algae ? What are the types of Algae ? How do we classify algae ?
Gelatinous red alga on the coral reef

The land plants evolved from algae about 500 million years ago. Algae is distinguished from other plants by the lack of true roots, stems or leaves. The terrestrial plants have specialized cells for absorbing and moving nutrients and for reproduction. Algae are thus simple plants that range from microscopic microalgae (Diatoms) to macroscopic algae (Kelp). They are very diverse and found every where on planet. Algae are aquatic photosynthetic organisms containing cells with unique features, not found among plants or animals.There are a large number of algae found across the world. Scientists have been doing research from a long time,to identify the types of algae found across the world.Many types have been discovered but still many are left to be discovered.

Basic Characteristics of Algae

  • Algae has the ability to perform photosynthesis with the production of molecular oxygen, which is associated with the presence of chlorophyll a, b or c
  • They do not have specialized transport tissues or organs consisting of interconnected cells that move nutrients and metabolites among different sites within the organism
  • Algae Reproduce sexually or asexually to produce gametes that are generally not surrounded by protective multi-cellular parental tissues.

Classification of Algae

The major algal groups are distinguished on the basis of :

  • Pigmentation and Photosynthetic apparatus
  • Products stored
  • Flagella characteristics
  • Method of propagation
  • Cell wall composition
  • Nuclear organization

Pigmentation and Photosynthetic Apparatus

Green algae Chara

The pigments used in the photosynthetic apparatus of algae help in separating algae on the basis of their color. Algae has three basic pigments :

  • Chlorophylls : The types of chlorophyll present in algae are chlorophyll a, b, c, d and e. Chlorophyll-a is present in all classes of algae.
  • Carotenoids : Algae contains two kinds of carotenoids which are xanthophylls (20 types) and carotenes (4 types). 
  • Biloproteins : These are water soluble pigments of algae (phycocyanin, phycoerythrin and allophycocyanin).

Chloroplasts of algae have an arrangement of stacks in their granum called thylakoids which contain these pigments and are known as photosynthetic apparatuses.

Products Stored

All algae groups have the same photosynthetic primary products but the nature of insoluble reserved foods may differ due to the life span of the algae. Starch is often considered for the distinction of different groups of algae.

Flagella Characteristics

Chlamydomonas reinhardtii : Single Cell Green Algae with Long Flagella

Classes of algae can be separated on the basis of type, number and position of flagella. Though the basic flagellar structure is similar (9+2 – component fibrils pattern) but sometimes it is completely absent.

Method of Propagation

The method of propagation which is sexual leads to isogamy, anisogammy and oogamy or asexual results in different life cycles of algae.

  • Haplontic Life Cycle
  • Diplontic Life Cycle
  • Triphasic Life Cycle
The presence or absence of sexual reproduction, complexity of reproductive organs, method of sexual reproduction becomes an important criteria of algal classification.
Single cell algae with cell wall made up of silica : Diatom

Cell Wall Composition

The cell wall in algae is generally made up of poly-saccharides. In some cases lipids and proteins are also present in them. The inner layer of cell wall in algae is generally made up of cellulose, which is an insoluble poly-saccharide. The outer layer of algae is generally made up of pectic substances. In some algae, the cell walls are calcified or silicified, while some algae have alginic acid or fucinic acid in the cell wall instead of cellulose.

Nuclear Organization

Acetabularia

On the basis of nuclear organization, algae can be prokaryotic or eukaryotic. DNA fibrils are free in nucleoplasm and are not associated with any Histones. Cell division by mitosis and meiosis is not found. If the membrane bound cell organelles like chloroplast, mitochondria and ER are absent then the algae is prokaryotic. Eukaryotic algae have a well differentiated nucleus, mitochondria, chloroplast and endoplasmic reticulum in their cell structure.

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Types of Algae

The size range of algae varies, and forms the basis of division in groups including the basic cellular characteristics. Many types of algae consist of only one cell, while the largest of algae has millions of cells. In a large, macroscopic algae, groups of cells are specialized for specific functions, such as anchorage, transport, photosynthesis, and reproduction. Such specialization indicates a measure of complexity and evolutionary advancement.

  • Blue-green algae (Cynobacteria)

    Crystal structure of Cynobacteri

The Cynobacteria are also referred to as blue-green algae because they are capable of conducting oxygen-producing photosynthesis and live in the same environments as eukaryotic algae. The cyanobacteria are gram-negative bacteria, and therefore are prokaryotes. They are also capable of independently conducting nitrogen fixation viz. the process of converting atmospheric nitrogen to usable forms of the element such as ammoniaThe prefix “cyano” in cynobacteria means blue because the bacteria has pigments that absorb specific wavelengths of light and give them their characteristic colors. Many cyanobacteria have the blue pigment phycocyanin, a light-harvesting pigment (it absorbs red wavelengths of light). All Cyanobacteria  have some or the other form of the green pigment chlorophyll, which is responsible for harvesting light energy during the photosynthetic process. Some also have the red pigment phycoerythrin, which absorbs light with the green region and gives the bacteria its characteristic pink or red color.

  • Euglenoids (Euglenophyta)

    Euglena garcilis under microscope

The euglenoids, or Euglenophyta, are single-celled, protozoan-like algae, mostly found in freshwater. Most euglenoids make their own food using light energy from the Sun but are capable of surviving in the dark if they are fed with organic materials. Some species are heterotrophic, meaning they do not produce their own food but feed on organic matter suspended in water. At the anterior end of the cell there is a gullet, through which one, two, or sometimes more flagella extend themselves. The contracting vacuoles and stigma are located on the sides of the gullet but in creeping forms the flagellum is rudimentary.  They reproduce by means of longitudinal splitting and under unfavorable conditions, some species discard their flagella and form protective spores (cysts).

  • Golden-brown algae (Chrysophyta)

    Diatoms with silica cell wall

The Chrysophyta, or golden-brown algae and diatoms, are named because of the yellow pigments that they possess. These are single-celled algae which can live both in freshwater as well as salt water. The cell walls of chrysophyta have no cellulose but are composed mostly of pectin, which is often filled with silica, a compound that makes the walls of this algae quite rigid. These algae store energy both as a carbohydrate and as large oil droplets. Diatoms have two glass shells made largely of silica that fit together like a pillbox and are exquisitely marked. Their species live in groups  of 40,000 to 100,000 algae. When they die, their shells help to form sediments on the sea bottom. This fine-grained sediment is often used for filtration in liquid purification systems.

  • Fire algae (Pyrrophyta)

    Dinoflagellates

Pyrrophyta are mostly unicellular microorganic Protists divided by botanists in two phyla, dinoflagellates and criptomonads. Fire algae, or Pyrrophyta, are single-celled algae and include the dinoflagellates  which have two flagella used for locomotion. Most of these microscopic species live in salt water, with some occurring in freshwater. Some species of dinoflagellates emit bright flashes of light when exposed to air, which at night look like fire on the ocean’s surface

  • Green algae (Chlorophyta)

    Sea lettuce

The green algae, or Chlorophyta, occur in freshwater, although some live in the sea as well. Most green algae are single-celled and microscopic in nature, forming the slimy green scum often found in stagnant ponds. Others are larger and more complex, forming spherical (round) colonies composed of many cells or as straight or branched filaments (long, thin series of cells). Green algae is believed to be in the evolutionary line, that gave rise to the first land plants of the planet earth. More than 6,500 species of green algae are classified as Chlorophyta and most of them live in the ocean, while there are around 5,000 species that live in the freshwater lakes and ponds.

  • Red algae (Rhodophyta)

    Dragon tongue Red algae

The red algae, or Rhodophyta, are marine plants that live mainly in shallow waters and deep tropical seas. A few types of red algae are found in freshwater as well. Their body has a range of single-celled as well as branched filaments. The larger species have filaments that are massed together and resemble the leaves and stems of plants. They have no flagella and typically grow on a hard surface or on other algae. Some species contain a red pigment also. The cell walls of Coralline red algae are heavily encrusted with minerals and help to cement and stabilize coral reefs.

  • Brown algae (Phaeophyta)

    Kelp forest

The brown algae also known as Phaeophyta, are shiny brown seaweeds which are found along rocky coasts. Brown algae are large in size and include the giant kelps, which are located along the Pacific coast and often form forests, that provide habitat to a wide range of marine life. Some species of brown algae have structures called holdfasts that anchor the algae to submerged rocks. Attached to the holdfasts are stemlike stalks that support wide leaflike blades of algae. These blades provide the major surface for nutrient exchange and photosynthesis and are lifted up toward the water’s surface by air bladders. Brown algae contain an accessory brown-colored pigment that gives the plants their characteristic dark color. Other well-known brown algae are the common rockweed Fucus and Sargassum, which floats in a thick, tangled mass through the Sargasso Sea(a huge area of slow currents in the mid-Atlantic Ocean that supports a variety of marine organisms)

  • Yellow-green algae (Xanthophyta)
Xanthophyta

The yellow-green algae, or Xanthophyta, primarily occur in freshwater lakes or ponds. They can be either single celled or form colonies. Their cell walls are made up of cellulose and pectin compounds that sometime contain silica. They can have two or more flagella for locomotion, and they store their energy as carbohydrates. They derive their yellow-green color from the pigments carotenoids and xanthrophyll.

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