What is Algae? Characteristics, Structure, Classes, Reproduction, Importance

What is Algae?

• The term algae is Used for a broad, informal grouping of organisms which perform photosynthesis but are not higher plants.
• The organisms are Predominantly eukaryotic, nucleus-bearing, although some definitions include prokaryotic cyanobacteria (“blue-green algae”) which also are Photosynthetic.
• The algae are Mostly aquatic organisms, living in fresh water, marine water, or sometimes in moist terrestrial habitats (on rocks, soil, etc.).
• True roots, stems, leaves, and vascular tissue are Absent in algae; their structure is simpler than in vascular plants.
• Photosynthetic pigments more various than those of typical plants are Present in algae; chlorophyll a is common, plus other accessory pigments give red, brown, golden or green coloration.
• Life cycles are Varied; both sexual and asexual reproduction are used, depending on species, which may show alternation of generations or simple cell division.
• The ecological role of algae is Fundamental; they serve as primary producers in aquatic ecosystems, producing oxygen and forming base of many food webs.
• The size and complexity of algae are Highly variable; single-celled microalgae exist, and also large multicellular macroalgae / seaweeds like kelp which may grow many meters long.
• The study of algae is Called phycology (or algology), considered a branch of biology that deals with taxonomy, physiology, ecology of algae.

Characteristics of Algae

• Cellular organization – Algae are Comprised of unicellular, colonial, filamentous or multicellular thalli which are not differentiated into true tissues, such as roots, stems, leaves.
• Photosynthetic pigments – Chlorophyll a is present, plus accessory pigments like carotenes, xanthophylls, sometimes phycobilins which give variation in colour, depending on species.
• Non-vascular nature – Specialized vascular tissues (xylem, phloem) are Absent.
• Thalloid body – The body (thallus) is Usually flat or simple, lacking organs; complex thalli occur in larger macroalgae though without true tissue differentiation.
• Habitat – Aquatic environments are Occupied mostly; fresh water, marine water, or moist terrestrial surfaces.
• Reproduction
  • – Asexual forms are Used, e.g by fragmentation, spore formation, cell division etc.
  • – Sexual reproduction is Also present in many species; gametes are formed though organs are simple, often unicellular, and embryo formation is not typical.
• Ecological role – Primary production is Carried, oxygen is Produced, organic matter is Synthesized, food webs are Supported.
• Protective features – Mucilage is Produced by many algae, which protects thallus from desiccation and from epiphytic growth.
• Size variation – Microscopic forms and macroscopic forms exist; extremes may reach many meters in size.
• Life cycle complexity – Alternation of generations (haploid/diploid phases) occurs in some; simple life cycles in others.¹²³⁴

Habitat of Algae

The Habitat of algae is described as highly diverse, because algae are of universal occurrence and are found in water, land, snow, hot springs, inside plants/animals, and even in extreme saline or aerial conditions.

The Aquatic algae are predominant in occurrence, and they are present in fresh water, brackish water, and marine water.

• Fresh water algae –
  • They inhabit ponds, pools, tanks, rivers, streams, and lakes.
  • Reported from both tropical waters (Cyclotella, Orthoseira, Volvox, Nostoc, Microcystis) and temperate waters (Pediastrum, Fragillaria, Nitzschia).
  • These may occur as:
    • Planktonic algae – Floating freely at water surface. Euplanktons are free-floating always (Volvox, Cosmarium, Chlamydomonas, Scenedesmus). Tychoplanktons detach later from substratum (Zygnema, Oedogonium, Cladophora).
    • Benthic algae – Bottom dwellers in shallow pools. Includes unicellular, filamentous or colonial forms. They may be epizoic (Cladophora on snail), epilithic (Ulothrix tenuissima, Batrachospermum), epipelic (Cosmarium, Oedogonium), epiphytic (Vaucheria, Ulothrix).
    • Neustonic algae – Growing at air-water interface, for example Botrydiopsis, Nautococcus, Chromatophyton.
• Marine algae (seaweeds) –
  • They dominate supralittoral, littoral, and sublittoral zones.
  • Supralittoral – Above water level on rocks dampened only by wave splashes (Prasiola stipitata, Ulothrix flacca).
  • Littoral – Exposed periodically to tides, examples are Dictyota, Rhodymenia, Laminaria, Fucus, Porphyra, Gracilaria.
  • Sublittoral – Below water level, often permanently submerged, examples include Laminaria and Chondrus crispus.
• Aerophytic algae –
  • Growing on moist surfaces like tree bark, leaf surfaces, walls, flowerpots, fencing wires.
  • They absorb moisture from dew, rainfall, and atmospheric humidity.
  • Examples are Trentepholia, Scytonema, Pleurococcus, Cephaleuros.
• Cryophytic algae –
  • Found on snow in polar or mountain regions, giving distinct coloration to snow.
  • Red snow due to Chlamydomonas nivalis, brown/purple snow by Acyclonema, green by Euglena or Mesotaenium, yellowish snow by Nostoc, black coloration by Raphidonema.
• Endozoic algae –
  • Growing inside bodies of aquatic animals and vertebrates.
  • Examples are Zoochlorella in Hydra viridis, Zooxanthella in sponges, Oscillatoria and Simonsiella in several vertebrates.
• Epizoic algae –
  • Growing on external body surfaces of animals.
  • For example, Cladophora crispata on snail shells, Stigeoclonium on fish gills, Oscillatoria on turtles.
• Epiphytic algae –
  • Growing on other aquatic plants but not host specific.
  • Examples are Coleochaete on Chara, Rivularia on Scirpus, Chaetophora on Nelumbo leaves.
• Halophytic algae –
  • Adapted to waters with very high salinity, sometimes 70–80 ppt.
  • Examples are Dunaliella, Stephanoptera, Chlamydomonas ehrenbergii, Ulothrix.
• Parasitic algae –
  • Living as parasites or semi-parasites on higher plants or algae.
  • Cephaleuros virescens causes red rust of tea and coffee, Rhodochytrium parasitizes Ambrosia leaves, Polysiphonia fastigata grows on Ascophyllum.
• Symbiotic algae –
  • Cyanophycean algae often form associations with plants.
  • Nostoc with Anthoceros, Anabaena cycadaceae with coralloid roots of Cycas, Anabaena azollae with Azolla.
• Terrestrial algae –
  • They grow on soils, moist logs, and rocks.
  • Examples include Vaucheria, Botrydium, Fritschiella, Oedocladium, Nostoc, Anabaena.
• Thermophytic algae –
  • Growing in hot springs, even at 85 °C or more.
  • Cyanidium caldarium in acidic springs, and others such as Synechococcus, Synechocystis, Phormidium, Scytonema.

By These broad adaptations, algae are found practically everywhere, and therefore they are considered as cosmopolitan group of organisms which thrive in ordinary as well as in extreme environments.⁵⁶⁷

Classes of Algae

• Chlorophyceae (Green Algae)
  • Green pigmentation is Presented by chlorophyll a and b, plus carotenoids and xanthophylls.
  • Reserve food is Stored mostly as starch.
  • Structure is Unicellular, colonial or multicellular forms, flagella are Present in motile stages.
  • Habitats are Fresh water and marine environments.
• Xanthophyceae (Yellow-Green Algae)
  • Pigments are Present such as chlorophyll a, chlorophyll c, β-carotene, xanthophylls, but fucoxanthin is Absent, which gives lighter colour.
  • Reserve food is Stored mostly as oils or fats rather than starch.
  • Flagellar stages are Used (motile cells) and non-motile forms are Also present.
• Chrysophyceae (Golden Algae)
  • Golden coloration is Exhibited due to accessory pigments.
  • Many species are Unicellular; colonial or simple filamentous forms are Rare.
  • Storage product is Lipid/fat or other non-starch materials.
• Bacillariophyceae (Diatoms)
  • Silica in cell walls (frustules) is Used, ornamented patterning is Common.
  • Unicellular mostly; sometimes aggregate to form chains or colonies.
  • Habitats in marine, fresh water; major component of plankton.
• Cryptophyceae
  • Pigments such as phycobilins are Present.
  • Two flagella are used in motile form.
• Dinophyceae (Dinoflagellates)
  • Distinctive armored plates or coverings (theca) are Present in many.
  • Two flagella (often at different positions) are Employed for mobility.
  • Mixotrophic, heterotrophic or phototrophic nutrition is Used depending on species.
• Phaeophyceae (Brown Algae)
  • Fucoxanthin is Used as a major accessory pigment giving brown colouration.
  • Multicellular large seaweeds (e.g kelp) are Represented.
• Rhodophyceae (Red Algae)
  • Phycoerythrin and phycocyanin are Used, which give red / purplish coloration.
  • Absence of flagella is Typical, in most species.
• Cyanophyceae / Myxophyceae (Blue-Green Algae / Cyanobacteria)
  • Prokaryotic organization is Exhibited, no true nucleus, chloroplasts etc.
  • Pigments include chlorophyll a, phycobilins; photosynthesis is Performed by them.⁸⁹

Structure of Algal Cell

It may be Prokaryotic or it may be Eukaryotic, and this classification is Based on the presence or absence of membrane-bound nucleus and organelles.

Prokaryotic algal cell (Cyanobacteria / Blue-green algae)

• Outer cellular covering – Outside of the cell wall, a mucilaginous sheath or slime layer is Found, which is a diagnostic character for all cyanobacteria, and it provides protection against desiccation and mechanical damages.
• Cell wall – A rigid wall is Present, composed of mucopeptide. Four distinct layers (L1, L2, L3, L4) are Recognized, and they give stability to the cell.
• Plasma membrane – Beneath the wall, the plasma membrane is Situated. It is a lipid bilayer, and its invaginations are Used as the loci for different biochemical activities like respiration and photosynthesis.
• Cytoplasm – The cytoplasm is Divided into two parts, chromoplasm and centroplasm.
  • – Chromoplasm is Composed of photosynthetic lamellae/thylakoids, which contain chlorophyll a, carotenoids, and phycobilins. In this region, many metabolic functions are Performed.
  • – Centroplasm is Represented as the central colorless area, composed of chromatin/DNA material. Histone proteins are Absent, and additional DNA elements such as plasmids or transposons are Present.

Eukaryotic algal cell (All algae except cyanobacteria)

• Cell wall – A wall is Surrounding the cell, usually made of cellulose. In many algae, an outer pectose layer is Found. In brown algae, alginic acid may be Present, while in diatoms, a silicified wall exists. Red algae have cell walls with xylan, agar, carrageenin.
• Plasma membrane – Located just below the wall, the membrane is Composed of lipid-protein bilayer, and it separates cytoplasm from wall.
• Cytoplasm – Dense cytoplasm is Present inside, and it contains eukaryotic cell organelles like mitochondria, ER, Golgi bodies. The cytoplasm usually has chloroplasts, generally one per cell, although sometimes multiple are Present.
      - The chloroplasts may be Cup-shaped, parietal, discoid, lobed, star-shaped, spiral, barrel-shaped, or girdle-shaped. Their diversity is Recognized as significant.
• Nucleus – A well-defined nucleus is Present, with nuclear membrane, pores, and chromatin bound to histone proteins, and this nucleus is Similar to higher plants.
• Flagella – In motile forms, flagella are Seen emerging from basal granules (blepharoplast). They pass through a narrow canal in the cell wall. Internally, the flagellum is Arranged in 9+2 fibrillar pattern, with two central singlet fibrils surrounded by nine doublets. The flagella are Anchored to cytoplasm by pores in the wall.

Algae Pigments

The pigments of algae are Considered as primary components for capturing light energy, and they are Distributed in chloroplasts or in thylakoids, depending on cell type.

In many algae, the combination of pigments is Found complex, and the variation in pigment composition is Used as a basis for classification.

1. Chlorophyll pigments
   • Chlorophyll a is Present in all groups of algae, and it is Essential for photosynthesis.
   • Chlorophyll b is Found in green algae (Chlorophyceae), while it is Absent in red and brown algae.
   • Chlorophyll c is Located in brown algae (Phaeophyceae), diatoms, and dinoflagellates.
   • Chlorophyll d is Reported in red algae (Rhodophyceae).
   • Chlorophyll e is Rarely found, but its occurrence is Recorded in some groups of algae.
2. Carotenoids
   • These pigments are Commonly Present as accessory pigments.
   • Carotenes and xanthophylls (like lutein, zeaxanthin, violaxanthin) are Included in algae.
   • Fucoxanthin is Observed in brown algae and diatoms, and it gives characteristic brown coloration.
3. Phycobilins
   • These pigments are Soluble in water, and they are Present in phycobilisomes attached on thylakoid membranes.
   • Phycoerythrin gives red color and is Abundant in red algae.
   • Phycocyanin and allophycocyanin are Found in blue-green algae (cyanobacteria) and in some red algae

Diversity of pigments – With different pigment combinations, algae are Capable of absorbing wide range of light wavelengths, and thus photosynthesis is Conducted even in habitats where light is very low or filtered.

The variation of pigments is Responsible not only for photosynthesis but also for the specific colors which are Displayed by algal classes, such as green, brown, golden, red, or blue-green.

By their ecological importance, the algal pigments are Understood as essential for primary production, and they also are Utilized in industries for natural dyes, food additives, fluorescence markers etc.

Thallus Organization of Algae

The Thallus of algae is represented by a wide range of organizational patterns, which is considered from simple unicellular forms to highly evolved multicellular structures.

In size, the algal thalli are observed to vary from microscopic bodies to macroscopic plants, sometimes even attaining lengths of several meters. Micromonas pusilla is noted as the smallest algae with only 1 μm (0.00004 in.), while on opposite side giant kelps have thalli extended up to 60 m (200 ft) long.

A unicellular thallus is generally capable of performing all vital life functions, since each cell is physiologically and genetically complete. They may be solitary or organized into colonies within a gelatinous matrix, forming transitional stages between unicellular and multicellular structures.

In many instances, intermediate stages of organization have been recognized, such as palmella, dendroid, coccoid, filamentous, siphonaceous, heterotrichous, uniaxial, and multiaxial types.

Unicellular forms

• The unicellular forms are grouped mainly into rhizopodial, flagellates, coccoid, and spiral forms.
• Rhizopodial forms – A rigid cell wall is absent, and instead the cell surface is covered by a periplasmic layer. Naked protoplast is present and shape alterations are permitted. Pseudopodia or rhizopodia are used for locomotion in amoeboid fashion. Examples are Chrysamoeba and Rhizochloris.
• Flagellates – The vegetative body is a motile unicell with one, two, or more flagella. Flagella are unequal, equal, tinsel or whiplash type. Shape of the cell may be round, elongated, ovoid or cylindrical. Examples include many chlorophytes.
• Coccoid forms – These are non-motile with rigid cell walls, and the flagella are absent. They dominate in Xanthophyceae (about 70%) and are represented by Prochloron, Synechococcus.
• Spiral form – Thallus is unicellular but arranged in spiral filamentous manner. Spirulina is common example.

Multicellular Forms

• Multicellular thalli are observed in microscopic as well as macroscopic algae. They may be colonial, filamentous, siphonocladous, siphonous, parenchymatous, pseudoparenchymatous, uniaxial or multiaxial.
• Colonial aggregation – Colonies are produced by mucilaginous secretion which binds separate but structurally similar cells. These colonies can be differentiated into:
  • Coenobium – Colony has fixed number of cells arranged in determined pattern, like Volvox and Hydrodictyon.
  • Palmelloid – Non-motile cells embedded within amorphous mucilage, variable in size and shape. Cells are physiologically independent. Example Palmella.
  • Tetrasporal – Colony has cubical or plate-like arrangements, with pseudocilia in mucilage. Examples Merismopedia, Halopedia.
  • Dendroid – Cells connected in branching manner with mucilage, colony resembles miniature tree. Example Chrysodendron.
  • Rhizopodial colonies – Cells interconnected through rhizopodia, as in Chrysidiastrum.
• Filamentous forms – Thalli appear as rows of cells joined end to end.
  • Unbranched filament: Spirogyra, Zygnema, Oedogonium.
  • Branched filament: Cladophora, Pithophora. Branching may be true or false.
    • False branching occurs when fragmentation of trichome causes outgrowth resemblance of branching (e.g. Scytonema).
    • True branching results by transverse divisions of lateral outgrowths.
  • Heterotrichous type: A prostrate base with an erect branched system, considered advanced type, seen in Stigeoclonium and Ectocarpus.
  • Parenchymatous type: Division of cells occurs in all planes, filamentous appearance lost, seen in Ulva and Porphyra.
  • Pseudoparenchymatous type: Formed from aggregation of branched filaments. May be uniaxial (Batrachospermum) or multiaxial (Nemalion).
• Siphonocladous organization – Filaments are composed of multinucleated (semi-coenocytic) cells. In some cases, like Valonia, the thallus appears as giant spherical vesicles (up to 10 cm), considered as largest plant cells.
• Siphonous organization – Thallus consists of coenocytic, multinucleated structure, without septation. It develops due to nuclear division without cytokinesis.
  • Saccate form (Botrydium).
  • Uniaxial form (Vaucheria, Bryopsis).
  • Multiaxial form (Codium).

Reproduction in Algae

Reproduction is Carried by three major ways, namely vegetative, asexual, and sexual reproduction. The method is Selected depending on species and on environmental conditions.

Vegetative reproduction

The Vegetative reproduction in algae is described as a simple process where a portion of thallus gets separated from the parent and gives rise to a new thallus.

In this method, newly formed individuals are genetically identical to parent, and therefore no genetic variation is introduced.

By Various specialized or unspecialized parts of thallus, vegetative propagation is carried out in algae.

1. Budding –
   • Some small vesicles are separated from the parent by septum formation.
   • Each vesicle is capable of germinating into new thallus.
   • Example is Protosiphon, where such buds are formed naturally.
2. Cell Division –
   • The simplest method, since unicellular algae divide by binary fission.
   • Each cell divides into two equal daughter cells that function as new organisms.
   • Examples include Chlamydomonas and Diatoms.
3. Fragmentation –
   • Filamentous thallus is broken accidentally or by development of separation discs.
   • Each broken fragment develops into a complete new filament.
   • Examples include Spirogyra, Nostoc, Oscillatoria.
4. Bulbils –
   • Tuber-like outgrowths are produced, mostly at rhizoidal tips.
   • In Chara, bulbils also appear at nodes, and after detachment they germinate into new plant.
   • This method allows the plant to multiply even underground.
5. Hormogones –
   • Short, many-celled fragments of filaments are produced within parent sheath.
   • They are commonly seen in members of Cyanophyceae.
   • By separating from parent, each hormogone grows into a new filamentous plant.
6. Amylum Stars –
   • In Chara, star-shaped starch aggregations appear at lower nodes.
   • These serve as vegetative reproductive units, which later develop into new thalli.

Asexual reproduction

The Asexual reproduction in algae is mainly carried out by the formation of different types of spores, and these spores are usually produced in large numbers for ensuring survival and rapid multiplication.

The Process involves the development of specialized or unspecialized spores inside sporangia or directly from vegetative cells, and after liberation, they germinate into new plants without sexual fusion.

• Akinetes –
  • Thick walled resting spores with plenty of food reserves are produced.
  • They are resistant to unfavorable conditions like drought.
  • On arrival of favorable conditions, akinetes germinate into new plant.
  • Examples are Nostoc, Gloeotrichia, Ulothrix, Cladophora, Pithophora.
• Autospores –
  • Non-motile spores identical to parent plant are formed inside sporangium.
  • They are thick-walled and full of stored food material.
  • They germinate into new thallus without variation.
  • Seen in Scenedesmus, Chlorella.
• Aplanospores –
  • Non-motile spores are produced singly or in multiples by sporangium.
  • They are common during drought conditions.
  • Example: Ulothrix.
• Bispores –
  • When only two spores are formed inside a sporangium, they are called bispores.
  • The sporangium containing them is known as bisporangium.
  • Reported in Grateloupia filicina, Porphyra, Lithophylum littorale.
• Carpospores –
  • Produced during triphasic life cycle in red algae (Rhodophyceae).
  • They are diploid and formed from zygote inside carposporangium.
  • Example: Polysiphonia, Gracilaria, Grateloupia.
• Endospores –
  • Formed by successive internal divisions of cell contents inside sporangium.
  • All spores are formed before sporangium ruptures to release them.
  • Example: Dermocarpa clavata.
• Exospores –
  • Formed externally in Cyanophycean members.
  • Sporangium bursts at the apex and spherical spores are released one after another.
  • Example: Chamaesiphon, Stichosiphon.
• Hypnospores –
  • Aplanospores that become very thick-walled with abundant food reserves.
  • They germinate directly, or their protoplast may divide again to produce zoospores.
  • Example: Pediastrum, Sphaerella.
• Monospores –
  • A single spore formed inside a sporangium is termed monospore.
  • Very common in red and brown algae.
  • Considered the most frequent type of red algal asexual spores.
• Neutral Spores –
  • In red algae, sometimes vegetative cells are directly transformed into spores.
  • Such spores are termed neutral spores.
  • Example: Bangia.
• Paraspores (Polyspores) –
  • When more than four spores are produced after reduction division, they are called paraspores.
  • Found in red algae such as Palmaria elegans, Ceramium.
• Statospores –
  • Thick-walled, smooth or ornamented resting spores are produced.
  • Characteristic of Bacillariophyceae, Chrysophyceae, Xanthophyceae.
  • Example: Chaetoceros.
• Tetraspores –
  • Produced in groups of four inside a tetrasporangium.
  • They are common in red algae like Porphyra, Gracilaria, Grateloupia.
• Zoospores –
  • Naked, motile spores with flagella are known as zoospores.
  • They may be haploid or diploid, produced inside zoosporangia.
  • Biflagellate zoospores occur in Chlamydomonas, Ulothrix.
  • Quadriflagellate spores are found in Ulothrix.
  • Multinucleate and multiflagellate ones, called synzoospores, are produced in Oedogonium and Vaucheria.

Sexual reproduction

The Sexual reproduction in algae is found in almost all members except cyanophyceae, and it always involves the fusion of two gametes for the formation of a diploid zygote.

The Zygote formed after gametic union generally acts as a resting spore and germinates during favorable conditions, so survival as well as variation are ensured.

On Basis of structure and behavior of gametes, sexual reproduction in algae has been classified into different types.

1. Isogamy –
   • Fusion takes place between gametes which are morphologically and physiologically similar.
   • They are known as isogametes.
   • After fusion, a diploid zygote is formed which undergoes resting stage.
   • Example: Ulothrix, Chlamydomonas.
2. Anisogamy –
   • In this type, the gametes are morphologically and physiologically different.
   • Larger gamete is passive while smaller gamete is motile and active.
   • A special case is physiological anisogamy where gametes appear similar in shape but differ physiologically, as reported in Spirogyra.
   • Example: Chlamydomonas braunii.
3. Oogamy –
   • This is considered the advanced form of sexual reproduction.
   • Male gametes (antherozoids) are usually motile, and female gamete is large, non-motile and produced in oogonium.
   • Exception occurs in Rhodophyceae where both gametes are non-motile, and transport of male gametes to female is carried out by water currents.
   • Examples include Oedogonium, Chara, Polysiphonia, Porphyra, Chlamydomonas.
4. Autogamy –
   • Fusion occurs between gametes that are produced inside the same mother cell.
   • Such gametes are haploid and after union they form diploid zygote.
   • No genetic variation is introduced in this type, therefore offspring remains identical in character.
   • It is commonly found in Diatoms.

Perennation or Survival Strategies in Algae

In Freshwater and sub aerial algae, the unprotected thalli are frequently exposed to desiccation and temperature extremes, so special survival strategies are adopted.

In Marine algae, environmental variations are less common, but tidal fluctuations are frequent, therefore survival is maintained by secretion of mucilage that prevent dehydration during low tides.

The Perennation is defined as a temporary resting stage in algae, where metabolic activities are suspended until the arrival of favorable conditions.

By Production of several resistant structures, algae are enabled to tide over unfavorable periods and continue into next growing season.

• Akinetes –
  • Specially modified vegetative cells are developed with thick, resistant wall and abundant food reserves.
  • They are produced when thallus dries under drought, while the akinetes remain viable.
  • With return of favorable conditions, akinetes germinate into new plants.
  • Examples are Ulothrix, Cladophora, Pithophora, Nostoc.
• Aplanospores –
  • Thick-walled, non-motile spores are produced by rounding off cytoplasmic contents.
  • They are liberated after parent wall separates, and they rest for some time.
  • With favorable conditions, they develop directly into new individuals.
  • Reported in Chlamydomonas, Ulothrix, Coleochaete, Oedogonium.
• Palmella stage –
  • In Chlamydomonas, under adverse conditions vegetative cell divides repeatedly.
  • Daughter cells accumulate and remain embedded in mucilaginous envelope formed by gelatinization of parent cell wall.
  • This stage is short-lived and is considered as a mild type of perennation.
• Cysts –
  • Plant body in certain algae (Euglena, Vaucheria, Protosiphon) produce resistant cysts.
  • Each cyst separates and germinates into new plant when favorable season arrives.
  • In Vaucheria and Protosiphon, multinucleate segments form coenocysts.
• Oospore or Zygospore –
  • Zygotes produced by sexual fusion develop into thick-walled resting spores.
  • They store food reserves and remain dormant until favorable conditions come.
  • Therefore, zygote formation is also regarded as perennation strategy.
• Hormospores –
  • In Blue-green algae, hormogones separate from trichome tips.
  • These become rounded, develop thick wall, and survive as hormospores.
• Hypnospores –
  • Aplanospores sometimes secrete an additional thick wall for enduring winter or drought.
  • In quiescent state, they persist until conditions become favorable.
  • Example – Westiella (Cyanophyceae).
• Tubers –
  • Rhizoidal cells in Chara and Cladophora divide, become thick-walled, and accumulate food.
  • Produced from subterranean plant parts, they are able to survive drought and temperature extremes.

Through such multiple methods, perennation and survival of algae is ensured, even though unfavorable environments persist for long period, and hence growth is restarted once favorable season returns.

Ecological Importance of Algae

• Algae are Recognized as Primary Producers in aquatic ecosystems; organic compounds are Synthesized by algae via photosynthesis, and oxygen is Released which supports higher trophic levels.
• Large fraction (about 30-50 %) of the global oxygen supply is Produced by marine algae / phytoplankton.
• Carbon cycling is Greatly influenced by algae; carbon dioxide is Fixed by algae and some of that carbon is Sequestered in biomass or sediments.
• In many aquatic food webs, algae form the base of food chains; phytoplankton are Consumed by zooplankton, which are then Eaten by larger fishes etc, thus energy flow is Supported.
• Nutrient cycling is Assisted by algae; excess nutrients (e.g nitrogen, phosphorus) are Absorbed by algae which helps maintain water quality and avoid eutrophication.
• Habitats and ecological structure are Maintained by macroalgae (seaweeds etc); they Provide shelter, nursery grounds, breeding grounds for many marine organisms.
• Algae are used in bioremediation; pollutants are Removed from water by algal uptake and filtration processes.
• Climate regulation is Influenced by algae; sequestration of CO₂ (by algae) helps mitigate greenhouse gas levels, and thus global warming effects.
• Seasonal / polar ecosystems depend on ice- or snow-associated algae; when algae under ice or snow are Produced, they support higher trophic links during low sunlight, they also affect albedo (surface reflectance) when pigmented algae colonize ice/snow‐surfaces.^11121314

Economic Importance of Algae

• Algae as Food Chain Base – Both in fresh and in salt water, algae are Constituting the fundamental primary link of food chains. Organic food is Synthesized by algae, and reserve substances like starch, oils, sugars or polysaccharides are Utilized by fish, which in turn are Consumed by higher animals and even man.
• Algae in Fish Culture – The algae are Fruitfully employed in aquaculture. Tilapia mossambica is Fed on filamentous algae, and cultures of Scenedesmus are Daily used as feed in its rearing.
• Algae in Recreation – In certain water bodies, algae are Cultivated together with fish, and the purpose is Recreational, such as in lakes or ornamental ponds.
• Sewage Treatment – Chlamydomonas, Scenedesmus, Chlorella and Euglena are Applied in sewage plants, where oxygen is Supplied by photosynthesis, which in turn Accelerates bacterial decomposition.
• Water Supply Effects – During summers, algal blooms are Frequently produced in reservoirs and lakes. The water becomes Cloudy, colored or foul-smelling, and toxins from blue-green algae are Responsible for mortality of cattle, sheep, and other animals after drinking contaminated water.
• Petroleum and Gas Origin – Fossil fuels have been Considered partly derived from ancient algal biomass. Marine phytoplankton captured sunlight, and later deposits were Transformed into oil and gas. Methane (CH₄) is Produced by bacteria upon organic matter.
• Limestone Formation – Algae, including blue-green and red, are Capable of depositing CaCO₃ in their walls or sheaths. Extensive limestone beds near hot springs and coral reefs have been Constructed by them.
• Space Research and Studies – Chlorella has been Experimented for keeping air pure in spacecraft. Through photosynthesis, CO₂ is Converted into O₂. Algae like Chlamydomonas and Acetabularia are Used in fundamental genetics and biochemistry research.
• Algae as Food – Many seaweeds such as Laminaria, Gracilaria, Porphyra, Ulva, and Chondrus are Consumed directly. Vitamins (A, E, C, D), minerals like iodine, and carbohydrates are Provided. Historical records in China and Japan confirm their dietary importance.
• Algae as Fodder – Rhodymenia, Alaria, and others are Fed to cattle, goats, and sheep, especially in coastal Europe. Kelp meal is Added to poultry diets due to its vitamin and mineral content.
• Fertilizers and Soil Enrichment – Seaweeds have Long been used to fertilize soils, and blue-green algae fix nitrogen in paddy fields. Species like Nostoc, Anabaena or Tolypothrix are Employed in increasing rice yields.
• Medicinal Uses – Laminaria, Sargassum, and Gelidium are Applied for treating glandular disorders, stomach diseases, and in obstetrics. Agar from red algae is Used as laxative, in dietetics, and in pharmaceuticals. Antibiotic chlorellin is Prepared from Chlorella.
• Industrial Uses of Algae
  • Kelp Industry – Burning kelps (Laminaria, Saccorhiza) was Historically done to produce ash rich in soda and potash, containing potassium chloride and iodine.
  • Algin Industry – Sodium alginate and related algins from brown algae (Macrocystis, Sargassum, Fucus) are Used as emulsifiers, stabilizers, and thickeners in food, paint, textiles, and cosmetics.
  • Agar Industry – Extracted from Gelidium and Gracilaria, agar is Used in bacteriological media, food jellies, dairy products, and in canning of fish/meat. It is also Applied in leather, photography, cosmetics, and even in lamp manufacturing.
  • Diatomaceous Earth Industry – Deposits from siliceous diatom walls are Used as abrasives, soundproofing, thermal insulators, filtering agents in sugar, petrol, and as fillers in paints, varnishes, phonograph records, and battery boxes.¹⁰