Algae are the aquatic eukaryotic organisms that have chlorophyll and carry out oxygen-producing photosynthesis. The term algae mean Aquatic Plants but differ from the plants in lacking a well-organized vascular conducting system and in having very simple reproductive structures. The study of algae is called Phycology or algology.

There are different types of algal classification based on their characteristic. Basically, they are classified into seven divisions based on their cellular properties belonging to two different kingdoms (Plantae and Protista).

Some properties are following

  1. Cell wall chemistry and morphology
  2. Chlorophyll molecules and accessory pigments
  3. Flagella number and the location of their insertion in motile cells
  4. Morphology of the cells and/or body (thallus)
  5. Habitat
  6. Reproductive structures
  7. Life history patterns
Division    Common NameKingdom
Chrysophyta(yellow-green and golden-brown algae; diatomsProtista
Euglenophyta(photosynthetic euglenoid flagellatesProtista
PyrrhophytaDinoflagellatesProtista
CharophytaStonewortsProtista
Chlorophytagreen algaeProtista
Phaeophytabrown algaePlantae
Rhodophytared algaePlantae

Molecular classification systems have placed some of the classical algae with plants (green algae); some as a separate lineage (red algae); some with the stramenopiles (golden-brown and yellow-green algae, brown algae, and diatoms); some with the alveolates (dinoflagellates); and still others with some protozoa (euglenoids). The alveolates and stramenopiles have been created recently on the basis of rRNA comparisons and ultrastructural studies.

Alveolates have mitochondria with tubular cristae and subsurface alveoli or sacs. Dinoflagellates, ciliate protozoa, and the apicomplexan protozoa are alveolates.

Stramenopiles:  strameopiles contains mitochondria with tubular cristae and hollow hairs that give rise to a small number of fine hairs.

The Photosynthetic forms of stramenopiles often have chlorophylls a and c. Palinid protozoa, oomycetes, diatoms, brown algae or phaeophytes, chrysophytes, and xanthophytes are stramenopiles.

Summary of Some Algal division Characteristics

DivisionNumber of SpeciesCommon NamChlorophyllsPhycobilins (Phycobiliproteins)CarotenoidThylakoids per Stack in Chloroplast
Chlorophyta7,500Green algae (Chlamydomonas)a, b­_β-carotene, ±α-carotene, xanthophylls3-6
Charophyta250Stoneworts or brittleworts (Chara)a, b_α-, β-, τ-carotene, xanthophyllsMany
Euglenophyta700Euglenoids (Euglena)a, b_β-carotene, xanthophylls, ±τ-carotene3
Chrysophyta6,000Golden-brown, yellow-green algae; diatoms (Cyclotella)  a, c1/c2, rarely d_α-, ß-, ε-carotene, fucoxanthin, xanthophylls3
Phaeophyta1,500Brown algae (Sargassum)a, c_β-carotene, fucoxanthin, xanthophylls3
Rhodophyta3,900Red algae (Corallina)a, rarely dC-phycocyanin, Allophycocyanin, phycoerythrinXanthophylls, (β-carotene, zeaxanthine, ±α-carotene)1
Pyrrhophyta1,100Dinoflagellates (Gymnodinium)a, c1, c2_β-carotene, fucoxanthin, peridinin, dinoxanothin3

Chlorophyta (Green Algae)

Chlorophyta is a heterogeneous group of photoautotrophic protoctists (a phylum) comprised of green algae that live in fresh and saltwater, in soil, on other organisms, and within other organisms and have wide variability of shape, size, and habit. It is highly diverse in the terms of morphology, ranging from microscopic unicells to macroscopic multicellular algae also exhibits a wide diversity of body forms, ranging from unicellular to colonial, filamentous, membranous, or sheetlike, and tubular types.

The predominant pigment is chlorophyll a and b with specific carotenoids. According to the molecular classification, Green algae is associated with plants kingdom and have mitochondria with lamellar cristae.

Green algae reproduce both sexually and asexually (Chlamydomonas reproduces asexually by producing zoospores through cell division) and involve the formation of flagellated spores non flagellated spores.

This phylum contains nine classes According to Hoek, Mann and Jahns system (an older taxonomic classification of algae)

  1. Chlorodendrophyceae (46 species)
  2. Chlorophyceae (3046 species)
  3. Ulvophyceae (1610 species)
  4. Trebouxiophyceae (672 species)

Chlamydomonas is a representative unicellular green alga, has two flagella of equal length at the anterior end by which they move rapidly in water. Each cell of Chlamydomonas contains a nucleus a large chloroplast, a conspicuous pyrenoid, and a stigma (eyespot). Stigma helps in the phototactic responses.

The presence of two small contractile vacuoles at the base of the flagella function as osmoregulatory organelles that continuously remove water. The alga reproduces sexually and 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. At the end of this resting phase, Meiosis occurs that produces four haploid cells that give rise to adults.

Charophyta (Stoneworts/Brittleworts)

The stoneworts are abundant in fresh to brackish waters, grow as macrophytes and have a worldwide distribution. Common names of stoneworts or brittleworts of Charophyta are due to that some species precipitate calcium and magnesium carbonate from the water to form a limestone covering.

They have calssified reproductive organs (oospores) called gyrogonites are the reproductive organs. Charophytes are a major source of food for the invertebrates and have the ability to form low-growing meadows of vegetation and they appear as a dense covering on the bottom of shallow ponds.

The ability of Charophytes to produce repellent (allelopathic) materials exclude certain limnetic species of invertebrates and phytoplankton. Their primary storage product is starch. Members of Charophyta can b unicellular, filamentous, colonial or multicellular.

Example of charophyta is following

  • Desmids (Unicellular)
  • Spirogyra (Filamentous)

Euglenophyta (Euglenoids)

Euglenophyta occurs in fresh, brackish, and marine waters and on moist soils; they often form water blooms in ponds and cattle water tanks. They have 40 genera and 1000 species. Euglenophyta have chlorophyll a and b in their chloroplasts like the Chlorophyta and Charophyta.

Euglenophyta is the primary producer and heterotrophs of both bacteria and other eukaryotes.  Some euglenids form a symbiotic relation with metazoans. They have 3 to 4 flagella per cell. In molecular classification schemes, euglenoids are associated with the amoeboflagellates (flagellated protozoa) and kinetoplastids because all members have related rRNA sequences and mitochondria with discoid cristae at some stage in their life cycle.

The primary storage product is paramylon (a polysaccharide composed of β-1,3 linked glucose molecules), which is unique to euglenoids and is deposited as granules in the cytoplasm.  The euglenoids are of different shapes such as ovoid, spindle-shaped, or flattened single cells (unicells), of various transverse shapes depending on the genus or species.

Euglena is the representative genus of Euglenophyta. A typical Euglena cell is elongated and bounded by a plasma membrane; contains a structure called the pellicle, which is composed of articulated proteinaceous strips lying side by side. This pellicle enabling the turning and flexing of the cell.

The stigma is located near an anterior reservoir. Contractile vacuoles inside the cell regulate the osmotic pressure within the organism by continuously collecting water from the cell and empties it into the reservoir.

Chrysophyta (Golden-Brown and Yellow-Green Algae; Diatoms)

Chrysophyta is a common component of the of the plankton in oligotrophic lakes thus found in fresh water although a few species are found in brackish or marine waters. They may be the primary source of food for zooplankton.

They are not true autotrophes because some species become facultatively heterotrophic in the absence of adequate light, or in the presence of plentiful dissolved food.  The storage polysaccharide in chrysophytes is chrysolaminarin (a polysaccharide storage product composed principally of β-1,3 linked glucose residues, which is dissolved in special vacuoles). Chrysophyta have 200 genera and 1000 species

The major photosynthetic pigments are usually chlorophylls a and c1/c2, and the carotenoid fucoxanthin. The cells have a golden-brown color when fucoxanthin is the dominant pigment.

Reproduction usually is asexual but occasionally sexual. Most Chrysophyta are unicellular or colonial but there are also some multicellular species.

The division is further sub divided into three major classes:

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

Phaeophyta (Brown Algae)

The Phaeophyta arises from Greek word “phaeo” means brown. Brown algae are the most complex form of multicellular algae mostly present in the sea.  Phaeophyta includes no single-celled species; the simplest brown algae consist of small openly branched filaments; the larger, more advanced species have a complex arrangement.

They contain the brown pigment fucoxanthin which gives it a brown color. The other accessory pigments are chlorophylls a and c, carotene, and violaxanthin. Laminarin is the storge product of Phaeophyta. The size of Phaeophyta ranges from a microscopic length to several meters.

Phaeophyta consist nine orders of 240 genera and over 1,500 species.

The order of Phaeophyta are following

  • Ectocarpales e.g., Ectocarpus, Haiothrix.
  • Tilopteridales e.g., Ptilopteris.
  • Cutleriales e.g., Cutlria.
  • Sporochnales e,g. Sporochnus.
  • Desmarestiales e.g., Desmarestia.
  • Laminariales e.g., Laminaria.
  • Sphacelariales e.g., Sphacelaria.
  • Dictyotales e.g., Dictyota.
  • Fucales e.g., Sargassum.

 Rhodophyta (Red Algae)

The rhodophyta arises from Greek word “rhodon” which means rose mostly include seaweeds. Mainly Rhodophyta species are filamentous and multicellular but few reds are unicellular. They store carbohydrates as Floridian starch (composed of β-1,4 and β-1,6 linked glucose residues). Rhodophyta contains the red pigment phycoerythrin along with blue pigment phycocyanin.

These pigments help in the survival of this at depths of 100 m or more. The cell walls include a rigid inner part composed of microfibrils and a mucilaginous matrix. The matrix is composed of sulfated polymers of galactose (source of galactan) called agar, funori, porphysan, and carrageenan. These four polymers give the red algae their flexible, slippery texture. Agar is used extensively in the laboratory as a culture medium component for the cultivation of bacteria etc.

It is consisting of about 831 genera and over 5,250 species. Porphyridium is est studied and a particular source of sulfated polymers of galactose

Pyrrhophyta (Dinoflagellates)

Most dinoflagellates are marine, but some live in freshwater.  Most dinoflagellates have chlorophylls a and c and carotenoids and xanthophylls. As a result, they usually have a yellowish-green to brown color. Their mitochondria have tubular cristae.

Sana Riaz
Author

Sana has just completed her MPhil in Microbiology. She loves reading books and the latest discoveries in sciences.

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