ALGAE – DEFINITION, CHARACTERISTICS, CLASSIFICATION, EXAMPLES​

Algae and Fungi were first included in the group cryptogamia, established by Linnaeus. Later, this group was divided into Thallophyta, Bryophyta and Pteridophyta. Of these, Thallophyta included Algae and Fungi.

The group Thallophyta shows following characteristics.

• Absence of differentiation into stem, root and leaves.
• Sex organs generally unicellular, if multicellular, sterile envelope or jacket is absent.
• Zygote does not develop into multicellular embryo while inside the female sex organ.

Thallophyta is one of the largest group of Plant Kingdom. The members of this group are found in almost all the types of habitats. Though algae and fungi are customarily placed under Thallophyta, they are very different from one another, in their morphological features, metabolism, reproduction and life histories.

Algae and fungi differ from one another mainly due to presence of chlorophyll in the former and its absence in the latter. This makes algae autotrophic and fungi remains heterotrophic. Reserve food in Algae is the form of starch while fungi store glycogen. Algal cell walls are made of cellulose and those of fungi are composed of chitin.

The present day delimitation of algae is due to A.L. de lussieu (1789). Algae is mostly found in water though some are terrestrial and some are even parasitic (Cephaleuros virescence, a green alga causes Red Rust of Tea). They are as small as bacteria and as large as Macrocystis (a brown alga, 196 feet long). Algae show a great range of thallus structure-as simple as a single cell of Chlamydomonas and as complicated as an internally differentiated kelp (brown algae).

The algal cells are similar to those of higher plants. The characteristic colour of algae is due to specific pigments present in plastids.

Algae reproduce by vegetative, asexual and sexual methods. Sexual reproduction varies from simple isogamy to advanced oogamy. Few groups of algae also exhibit a distinct and well-defined isomorphic and heteromorphic alternation of generations.

Algae have been variously classified by numerous phycologists, their views always differing. The most simple and practical classification was proposed by British phycologist F.E. Fritsch in 1935.

CLASSIFICATION OF ALGAE​

DISTINGUISHING CHARACTERS OF ALGAE TAXA ​

SUB-DIVISION :- ALGAE

• Thallus simple
• Chlorophyll present
• Cell wall of cellulose

CLASS I. CHLOROPHYCEAE

• Grass green plastids
• Starch is reserve food
• Flagella of reproductive structures equal in length

Order 1. VolvocaIes

• Thallus with motile flagellated cells.

Family 1. Chlamydomonadaceae

• Thallus unicellular
• Contractile vacuoles present
  Example. Chlamydomonas

Family 2. Volvocaceae

• Thallus colonial
• Cells in a colony forming a flat plate
  Examples :- Pandorina, Eudorina, Pleodorina, Volvox

Order 2. Chlorococcales

• Cells single and non-motile
• Cells uninucleate
• Reproduction by zoospores or autospores

Family 1. Chlorellaceae

• Cells single, if united do not form a definite colony
• Reproduction by autospores
  Example :- Chlorella

Family 2. Hydrodictyaceae

• Cells united to form coenobe
• Reproduction by zoospores and biflagellate gametes
  Examples :- Pediastrum, Hydrodictyon

Family 3. Coelastraceae

• Reproduction by autospores
•  Autospores apposed to one another at the time of liberation
  Example :- Scenedesmus

Order 3. Ulotrichales

• Thallus simple or a branched filament
• Cells uni-or multinucleate
• Single chloroplast with 1 or more pyrenoids

Family 1. Ulotrichaceae

• Unbranched filaments
• Cell walls not articulated
• Cells uninucleate
• Example :- Ulothrix

Family 2. Ulvaceae

• Thallus expanded, 1 or 2 cells thick
• Cells uninucleate with laminate cup-shaped chloroplast
  Examples :- Ulva, Enteromorpha

Order 4. Cladophorales

• Branched or unbranched filaments
• Cells cylindrical and multinucleat

Family 1. Cladophoraceae

• Cells more than eight times longer than broad
• Chloroplasts do not form distinct transverse bands
  Example :- Cladophora

Order 5. Chaetophorales

• Plant body heterotrichous
• Hair or setae present

Family 1. Chaetophoraceae

• Filaments branched, branches free from one another or pressed together forming pseudoparenchymatous thallus
• Terminal cells modified into a long colourless setae (hair)
• Cells uninucleate with a single laminate and parietal chloroplast.
  Examples :- Draparnaidiopsis, Fritschiella

Family 2. Coleochaetaceae

• Vegetative cells with setae
• Sexual reproduction oogamous
  Example. Coleochaete

Order 6. Oedogoniales

• Filaments branched or unbranched
• Cell division resulting in ‘cap’ formation
• Chloroplast reticulate
•  Zoospores and antherozoids multiflagellate

Family 1. Oedogoniaceae (Single family)
Example :- Oedogonium

Order 7. Zygnematales

• Absence of flagellated reproductive cells
• Sexual reproduction by conjugation

Family 1. Zygnemataceae

• Filaments unbranched
• Chloroplast parietal and ribbon-shaped or single or two axial chloroplasts.
  Examples :- Spirogyra, Zygnema

Family 2. Desmidiaceae

• Cells composed of two semi-cells
• Conjugating cells have chloroplast escaping from surrounding walls as they unite to form zygospores
  Example :- Cosmarium

Family 3. Mesotaeniaceae

• Cells made of single piece and without pores
• Conjugating cells do not transfer contents from one cell to another
  Example :- Netrium

Order 8. Siphonales

• A single, multinucleate and tubular cell (coenocyte) represents the thallus
• Chloroplasts many and discoid

Family 1. Caulerpaceae

• Thallus differentiated into rhizome, rhizoids and aerial folliar shoots, macroscopic
• Internally shows the presence of trabeculae
  Example :- Caulerpa

Family 2. Codiaceae

• Thallus freely branched and tubular
• Sexual reproduction anisogamous and gametangia distinct
  Example. Codium

Order 9. Charales

• Thallus differentiated into nodes and internodes
• Characteristic sex organs-globule and nuclei

Family 1. Characeae (Single family)
Examples :- Chara, Nitell

CLASS II XANTHOPHYCEAE

• Chromatophores yellow-green
• Photosynthetic reserves-oil droplets
• Motile cells with unequal flagella

Order 1. Heterosiphonales

• Thalli multinucleate, unicellular and siphonaceous.

Family 1. Botrydiaceae

• Thallus unicellular, multinucleate, vesicular
• Zoospores biflagellate
•  Sexual reproduction isogamous
  Example :- Botrydium

Family 2. Vaucheriaceae

• Thallus branched, coenocytic, tubular and filamentous
• Zoospores multiflagellate
• Sexual reproduction oogamous
  Example :- Vaucheria

CLASS III BACILLARIOPHYCEAE

• Chromatophores golden-brown or yellow with or without pyrenoids
• Cell wall made of two silicified overlapping halves
• Food reserve oil
• Reproduction mostly by simple cell division

Order 1. Centrales

• Valves circular, ornamentation radial or concentric
• Statospores or microspores formed
• Auxospores formed by oogamy
  Example :- Melosira

Order 2. Pennales

• Valves bilaterally symmetrical, ornamentation bilateral
• Valves always with raphe
• Statospores or microspores never formed
• Auxospores formed by oogamy
  Example :- Pinnularia

CLASS IV. PHAEOPHYCEAE

• Yellowish-brown chromatophores
• Laminarin and mannitol are reserve food products
• Reproductive cells with two unequal, lateral
  flagella

Order 1. Ectocarpales

• Thallus filamentous
• Growth trichothallic
• Reproductive organs-unilocular and plurilocular sporangia
• Isomorphic alternation and generation

Family 1. Ectocarpaceae

• Thallus monoaxial, branched; branches uniseriate
• Uni-and plurilocular sporangia, terminal or intercalary
  Example :- Ectocarpus

Order 2. Laminariales

• Sporophytes large, parenchymatous
• Sporangia in sori, on stipe and blade
• Gametophytes microscopic and dioecious

Family 1. Laminariaceae

•  Sporophytes differentiated into holdfast, stipe and blade
• Blade simple or digitate
• Paraphyses hyaline or with colourless appendages
• Sporangia on both the surfaces of blade
  Example :- Laminaria

Order 3. Fucales

• Plants parenchymatous with complex morphological and anatomical differentiation
• Medulla filamentous
• Asexual reproduction absent
• Sex organs in conceptacles

Family 1. Fucaceae

• Axes subterate to alate with midrib but not foliar
• Vesicle when present inercalary
• Oogonia with eight oospheres
  Example :- Fucus

Family 2. Sargassaceae

• Axes terete, bearing distinct foliar organs
• Vesicles usually present, lateral or immersed in the terminal branchlets
• Branching of the thallus radial to the central axis
  Example :- Sargassum

Order 4. Dictyotales

• Plants parenchymatous, less differentiated
• Branching often dichotomous.
• Growth by apical cells
• Asexual reproduction by tetraspores
• Isomorphic alternation of generation

Family 1. Dictyotaceae

• Gametophytes dioecious/monoecious
• Oogonia and antheridia in definite sori
• Sperms motile and eggs non-motile
• Sporophyte with tetrasporangia
  Example :- Dictyota

CLASS V. RHODOPHYCEAE

• Chromatophores pure red to dark purple
• Photosynthetic reserve-Floridian starch or floridoside
• Male gametes and female gametes nonmotile and non-flagellated
• Female reproductive organ with a receptive organ-trichogyne
• Post fertilization product is called cystocarp

Sub-class 1. Bangioideae

• Thallus simple, growth intercalary
• Pit connections not well marked
• Asexual reproduction by gonidia ormonospores
• Sexual reproduction mostly unknown

Sub-class 2. Florideae

• Thallus basically filamentous
• Pit connections distinct
• Cells with more than one chromatophore
• Carpogonium highly specialized

Order 1. Nemalionales

• Plants filamentous, corticated, uni- or multiaxial
• Cells uninucleate, chromatophores axial or lateral
•  Cystocarps superficial or embedded in the thallus
• Life cycle without free living tetrasporophyte

Family 1. Batrachospermaceae

• Fresh water members
• Thallus uniaxial
• Life cycle haplobiontic
  Example. Batrachospermum

Order 2. Gigartioales

• Plants filiform, fleshy-membranous or crustose; corticated and multiaxial
• Tetrasporangia scattered on plants.
• Spermatangia borne on surface at special points
• Carpogonia embedded in the cortex

Family 1. Gracilariaceae

• Branches terete and firm
• Outer cells not radially serrate
• Narrow, small celled assimilatory cortex bearing delicate, colourless hairs
• Medulla parenchymatous
• Tetrasporangia tetrapartite
  Example :- Gracilaria

Order 3. Ceramiales

• Thalli uni- to mutiaxial
• Filaments corticated, polysiphonous
• Spermatangia in cluster
• Presence of trichoblasts

Family 1. Ceramiaceae

• Thallus monosiphonous
•  Naked or corticated by secondary filamentsdeveloped at nodes
• Procarp exterior to the thallus
• Tetrasporangia usually tetrahedral and external
  Example :- Ceramium

Family 2. Rhodomelaceae

• Axes polysiphonous
• Axes naked, corticated or covered with branches
• Main axis surrounded with pericentrals
• Plants bushy, sparingly branched; branches delicate
  Example :- Polysiphonia

CLASS VI. MYXOPHYCEAE

• Plastids not organised
• Pigments blue-green in colour
• Photosynthetic reserve-cyanophycean starch or glycogen
• True nucleus absent, cells prokaryotic
• Sexual reproduction not known

Order 1. Chroococcales

• Plants unicellular or colonial
• No differentiation into base and apex
• Nannocytes often present

Family 1. Chroococcaceae

• Cells unicellular or forming colomes
•  Filaments no known
  Example :- Gloeocapsa

Order 2. Nostocales

• Thallus with trichomes
• Trichomes unbranched or with false branching
• Hoqnogones, heterocysts, exospores, endospores, etc. present

Family 1. Oscillatoriaceae

• Trichomes uniseriate, sometimes tapering a the ends
• Heterocysts and spores absent
• Sheath absent or diffluent
  Example :- Oscillatoria

Family 2. Nostocaceae

•  Trichomes simple, unbranched, uniseriate, undifferentiated
• Heterocysts and akinetes present
  Example :- Nostoc

Family 3. Scytonemataceae

• Trichomes uniseriate, without marked attenuation
• Filaments free, with false branching
• Sheath firm enclosing one or more trichomes
  Example :- Scytonema

Family 4. Rivulariaceae

• Trichomes markedly attenuated from base to apex
• Trichomes show false branching
•  Growth trichothallic with terminal hair
  Examples :- Rivularia, Gloeotrichia