Algae – Definition, Classification and Examples | Biology Ideas

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Introduction

“The term Phycology or Algology is nothing but the study of algae.”

Algae are described as eucaryotic organisms that have chlorophyll and carry out oxygen-producing photosynthesis.

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Algal cell lacks a well-organized vascular conducting system and they have very simple reproductive structures.

Algae do not represent a monophyletic group (a group composed of a collection of organisms).

Algae is simply a group of related eucaryotic organisms that share some morphological, reproductive, ecological, and biochemical characteristics.

 

Distribution

Algae are most commonly occur in water (fresh, marine, or brackish) in which they may be suspended (planktonic) or attached and living on the bottom (benthic).

“A few algae live at the water-atmosphere interface so, these algae are termed as a neustonic.”

Plankton is consists of free-floating, and they are mostly microscopic aquatic organisms.

Phytoplankton is made up of algae and small plants; whereas Zooplankton consists of animals and non-photosynthetic protists.

Some algae grow in moist rocks, wood, trees, and moist surfaces.

Algae also live as endosymbionts (“organisms that form a symbiotic relationship with another cell”) in various protozoa, mollusks, worms, and corals.

Several algae grow as endosymbionts within plants, some are attached to the surface of various structures, and a few lead to a parasitic existence.

Algae also associate with fungi to form lichens which is a very sensitive form of organisms.

 

Ultrastructure & Nutrition

The eucaryotic algal cell is surrounded by a thin, solid cell wall.

Some algae have an outer matrix lying outside the cell wall of algal cells.

This usually is flexible and gelatinous (thick and like jelly), similar to bacterial capsules.

The flagella are the locomotor organelles in algae.

The nucleus has a typical nuclear envelope with pores; within the nucleus are a nucleolus, chromatin, and karyolymph.

The chloroplasts have membrane-bound sacs called thylakoids that carry out the light reactions of photosynthesis.

These starch organelles are embedded in the stroma where the dark reactions of carbon dioxide fixation take place.

In the dense proteinaceous areas, the pyrenoid that is associated with the synthesis and storage of starch may be present in the chloroplasts.

Mitochondrial structure varies greatly in the algae.

  • some algae (euglenoids) have discoid cristae
  • some, lamellar cristae (green and red algae)
  • remaining, (golden-brown and yellow Mitochondrion green, brown, and diatoms) have tubular cristae

Algal nutrition may be either be autotrophic or heterotrophic but Most are photoautotrophic; they require only light and CO2 as their principal source of energy and carbon.

Chemoheterotrophic algae require external organic compounds as a source of carbon and source energy.

 

Structure of algal thallus

The vegetative body of algae is called the thallus.

It varies from the relative simplicity of a single cell to the more striking complexity of multicellular forms, such as the giant kelps.

Single-cell algae can be as small as bacteria, and kelp can reach a size of more than 75 feet [75 m] in length.

Algae are unicellular, colonial, filamentous, membranous, and bladelike, or tubular.

 

algal reproduction

Asexual Reproduction

There are three basic types of asexual reproduction:

  1. Fragmentation
  2. Spores
  3. Binary fission

In fragmentation, the thallus breaks up and each fragmented part grows to form a new thallus.

Spores can be formed in ordinary vegetative cells or in specialized structures termed sporangia.

Flagellated motile spores in algae are called zoospores.

Non-motile spores produced by sporangia are termed Aplanospores.

In some unicellular algae, binary fission may occur (nuclear division followed by division of the cytoplasm).

Sexual Reproduction

Eggs are formed within vegetative cells called oogonia that function as female structures.

Sperm are produced in special male reproductive structures called antheridia.

As we know that gametes fuse to produce a diploid zygote.

 

Classification of Algae

According to the five-kingdom system of Whittaker, the algae belong to seven divisions distributed between two different kingdoms.

Classification is based on cellular properties :

  • Cell wall (if present) chemistry and morphology
  • A form in which food or photosynthesis products are stored
  • Chlorophyll molecules and accessory pigments that contribute to photosynthesis
  • Flagella number and the location of their placing in motile cells
  • Morphology of the cells and/or body (thallus)
  • Habitat
  • Reproductive structures
  • Life history patterns

 

Division Chlorophyta (Green algae)

Green algae

 

The common name of division Chlorophyta is nothing but the Green Algae.

This division is an extremely varied division.

Green algae grow in fresh and salt-water, in soil, on other organisms, and within other organisms chlorophylls a and b along with specific carotenoids, they store carbohydrates as starch.

Many have cell walls of cellulose.

They exhibit a wide diversity of body forms, ranging from unicellular to colonial, filamentous, membranous or sheetlike, and tubular types.

Both asexuals, as well as sexual reproduction, occur in green algae.

In molecular classification schemes, green algae are associated with the Tand plants and have mitochondria with lamellar cristae.

Individuals have two flagella of equal length at the anterior end by which they move rapidly in water.

Each cell has a single haploid nucleus, a large chloroplast, a conspicuous pyrenoid, and a stigma (eyespot) that aids the cell in phototactic responses.

Two small vacuoles with low levels of flagella activity as osmoregulatory organelles continue to remove water.

Chlamydomonas reproduce asexually by producing zoospores through cell division.

The alga also reproduces sexually when some products of cell division act as gametes and fuse to form a four flagellated diploid zygote that ultimately loses its flagella and enters a resting phase.

Meiosis occurs at the end of this resting phase and produces four haploid cells that give rise to adults.

From organisms like Chlamydomonas, several distinct lines of evolutionary specialization have evolved in the green algae.

The first line contains nonmotile unicellular green algae, such as Chlorella -widespread both in fresh and salt-water and also in soil.

It only reproduces asexually and lacks flagella, eyespots, and contractile vacuoles; the nucleus is very small.

Motile colonial organisms such as Volvox represent the second major line of evolutionary specialization.

E.g. Chlamydomonas.

 

Division Charophyta

The common name of division charophyte is Stoneworts or Brittleworsts.

This division s abundant in fresh to brackish waters and have a worldwide distribution.

They appear as a dense covering on the bottom of shallow ponds.

Some species precipitate calcium and magnesium carbonate from the water to form a limestone covering, thus giving the Charophyta their common names of stoneworts or Brittleworts.

 

Division Euglenophyta

Euglenoids

The common name of Euglenophyta is Euglenoids.

Euglenophyta have the presence of chlorophylls a and b in their chloroplasts.

The primary storage product in Euglenoids is paramylon (a polysaccharide composed of B-1,3 linked glucose molecules) uníque to euglenoids.

They occur in fresh, brackish, and marine waters and on moist soils; they often form water blooms in ponds and cattle water tanks.

Euglenoids may be found to be associated with the amoeboflagellates (flagellated protozoa) and kinetoplastids.

In the Euglena cell pellicle is composed of articulated proteinaceous strips lying side by side. They provide elasticity enough to enable turning and flexing of the cell.

Several chloroplasts contain chlorophylls a and b together with carotenoids.

Euglenoids Stigma is located near an anterior reservoir.

A large contractile vacuole near the reservoir continuously collects water from the cell and empties it into the reservoir – regulates the osmotic pressure within the organism.

2 flagella are present in Euglenoids but only one actively beats to move the cell.

Reproduction in euglenoids is by longitudinal mitotic cell division.

 

Division Chrysophyta

Chrysophyta is diversified with respect to pigment composition, cell wall, and type of flagellated cells.

The division is divided into three major classes,

  1. Golden-brown algae
  2. Yellow-green algae
  3. Diatoms

Pigments chlorophylls a and c1/c2, and the carotenoid fucoxanthin (gives golden brown color) chrysolaminarin (a polysaccharide storage product composed principally of B-1,3 linked glucose residues) are present.

Some lack cell walls and instead have patterned coverings external to the plasma membrane, such as scales, walls, and plates.

Reproduction usually is asexual but occasionally sexual.

Blooms of some species produce unpleasant odors and tastes in drinking water.

 

Diatoms

Diatoms

Diatoms are photosynthetic, circular, or oblong chrysophyte cells with a two-piece wall of silica frustules composed of two halves or thecae that overlap like a Petri dish. A large part is an epitheca, and a small part is a hypotheca.

Diatoms grow in freshwater, saltwater, and moist soil.

Chloroplasts contain chlorophylls and as well as carotenoids.

Some diatoms are facultative heterotrophs and can absorb carbon-containing molecules through the holes in their walls.

The vegetative cells of diatoms are diploid.

Diatoms have a single large nucleus and smaller plastids Reproduction consists of the organism dividing asexually, with each half then constructing a new theca within the old one.

Because of this mode of reproduction, diatoms get smaller with each reproductive cycle when they diminish to about 30% of their original size, sexual reproduction usually occurs.

Diploid vegetative cells that pass through meiosis form gametes, which are then shortened to produce a zygote.

The zygote develops into an auxospore, which increases in size again and forms a new wall

The mature auxospore eventually divides.

Diatoms lack flagella (non-flagellated).

Shape exists as unicellular, colonial, or filamentous.

New vegetative cells with normal frustules.

 

Practical importance of Diatoms

Diatoms are the most important source of food for fish and other marine animals in these regions.

When diatoms die their frustules tend to accumulate at the bottom of aquatic environments.

These form deposits of material called diatomaceous earth -an active ingredient in many commercial preparations like detergents, fine abrasive polishes, paint removers, decolorizing and deodorizing oils, and fertilizers is used extensively as a filtering agent, as a component in insulating (firebrick), and soundproofing products as an additive to paint to increase the night visibility of signs and license plates.

Use of diatoms as indicators of water quality and of pollution tolerance.

Specific tolerances for given species to various environmental parameters (concentrations of salts, pH, nutrients, nitrogen, temperature) a diatomaceous earth product to control insects, called INSECTO, has been introduced.

Insects have their soft body parts exposed but covered by a waxy film to prevent dehydration. When they contact the diatoms in INSECTO, the silica frustules break the waxy film on the insects, causing them to dehydrate and die.

The INSECTO is a physical control product to which insects cannot build up resistance. However, this product can be given to chickens, livestock, and pets without any side effects.

 

Phaeophyta (Brown Algae)

Brown algae

Brown algae consist of multicellular organisms that occur almost exclusively in the sea.
Seaweeds that are brown to olive green in color are found in the division Phaeophyta.

The simplest brown algae consist of small openly branched filaments; the larger, more advanced species have a complex arrangement.

Some large kelps are conspicuously differentiated into flattened blades, stalks, and holdfast organs that anchor them to rocks Sargassum, from huge floating masses that dominate the Sargasso Sea.

The color of these algae reflects the presence of the brown pigment fucoxanthin and chlorophylls a and c, ß-carotene, and violaxanthin The main storage product is laminarin, which is quite similar in structure to chrysolaminarin (polysaccharide storage product of Chrysophyta).

 

Division Rhodophyta (Red algae)

Red algae

Rhodophyta belongs to the kingdom Plantae.

Few Red algae are unicellular but most are filamentous and multicellular.

The stored food is the carbohydrate called Floridian starch (composed of ß -1,4 and ß -1,6 linked glucose residues).

They contain the red pigment phycoerythrin – a type of phycobilins and an accessory pigment is the blue pigment phycocyanin The cell walls of most red algae include a rigid inner part composed of microfibrils and a mucilaginous matrix The matrix is composed of sulfated polymers of galactose called agar, funori, porphyrin, and carrageenan.

These four polymers give the red algae their flexible, slippery texture.

Agar is widely used in the laboratory as a main culture medium component.

Many red algae also attach calcium carbonate to their cell walls and play a key role in the formation of coral reefs.

 

Division Pyrrhophyta (Dinoflagellates)

Dinoflagellates

Dinoflagellates are unicellular, photosynthetic alveolate algae.

Most Dinoflagellates are found in marine water, but some Dinoflagellates live in freshwater also.

Along with the chrysophytes and diatoms, the dinoflagellates make up a large part of freshwater and marine plankton and are at the base of many food chains species of Noctiluca, Pyrodinium, Gonyaulax, and other genera can produce light and are responsible for much of the luminescence (phosphorescence) seen in ocean waters at night.

They are armored or clad in stiff, patterned, cellulose plates or thecae, which may become encrusted with silica.

Most Pyrrhophyta have two flagella.

Most dinoflagellates have chlorophylls a and c, in addition to carotenoids and xanthophylls and they have a characteristic yellowish-green to brown color.

 

Toxic algal blooms

The poisonous and destructive red tides that occur frequently in coastal areas often are associated with population explosions, or “blooms,” of dinoflagellates.

Gymnodinium and Gonyaulax species are the dinoflagellates most often involved.

The pigments in the dinoflagellate cells are responsible for the red color of the water – dinoflagellates produce a powerful neurotoxin called saxitoxin

Toxin degrades respiratory muscles with multiple vertebrates by blocking the transport of sodium, which is essential for the functioning of their nerve cells.

This toxin does not harm the specific fish shellfish that feed on the dinoflagellates.

However, the shellfish do accumulate the toxin and are themselves highly poisonous to organisms, such as humans, who consume the shellfish, resulting in a condition known as paralytic shellfish poisoning or neurotoxic shellfish poisoning.

 

Important Related Terms

Phycology or Algology: Study of algae and algal cells.

Monophyletic group: A group composed of a collection of organisms.

Metonic: Algae that live at the water-atmosphere interface.

Phytoplankton: Organisms made up of algae and small plants.

Zooplankton: Organisms made up of animals and non-photosynthetic protists.

Endosymbionts: Organisms that form a symbiotic relationship with another cell.

Lichens: Microbial association of algae and fungi.

Thylakoids: The chloroplasts have membrane-bound sacs.

Thallus: Vegetative body of algae.

Zoospores: Flagellated motile spores in algae.

Aplanospores: Non-motile spores produced by sporangia.

Antheridia: Sperm are produced in special male reproductive structures.

Oogonia: Eggs are formed within vegetative cells.

Stoneworts or Brittleworsts: Common name of division charophyte.

Paramylon: A polysaccharide composed of B-1,3 linked glucose molecules.

Chrysolaminarin: A polysaccharide storage product composed principally of B-1,3 linked glucose residues

 

References

  1. Helfman, G. S., Collette, B. B., Facey, D. E., & Bowen, B. W. (2009). The Diversity of Fishes: Biology, Evolution, and Ecology. Oxford: Wiley-Blackwell.
  2. Kaiser, M. J., Attrill, M. J., Jennings, S., Thomas, D. N., Barnes, D. K., Brierley, A. S., & Hiddink, J. G. (2011). Marine Ecology: Processes, Systems, and Impacts. New York: Oxford University Press.
  3. McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  4. https://www.biologyonline.com/dictionary/algae

 

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