Algae; Characteristics, Occurrence and Thallus Organization

Algae are autotrophic thallophytes, found in simple and more complex forms, belonging to both prokaryotic and eukaryotic categories.

They exhibit a broad spectrum of structures, ranging from single-celled to multi-celled arrangements, and they utilize autophytic nutrition (able to produce their own food).

This group belongs to Thallophyta, the earliest and simplest plant category. The systematic and organized study of algae is known as Phycology, coined from the Greek words “Phycos” meaning seaweeds, and “logos” meaning study or discourse.

The term “algae,” derived from Latin and meaning seaweeds, was first introduced by Linnaeus in 1753. This encompass a diverse collection of plants, varying in habitat, size, structure, physiology, biochemistry, and reproduction.

General Characteristics of Algae

• They are chlorophyll-bearing autotrophic thalloid plant body.
• Almost all the them are aquatic.
• The plant body may be unicellular to large robust multicellular structure.
• The multicellular complex thalli lack vascular tissue and also show little differentiation of tissues.
• The sex organs are generally unicellular but, when multicellular, all cells are fertile and, in most cases, the entire structure does not have any protection jacket.
• The zygote undergoes further development either by mitosis or meiosis, but not through embryo formation.
• Plants having distinct alternation of generations. Both gametophyte and sporophyte generations — when present in the life cycle are independent.

Occurrence of Algae

They have a widespread presence, being found in various locations such as freshwater and marine environments, soil, rocks, and as epiphytes or parasites on plants and animals.

They also inhabit diverse places like hot springs, deserts, and permanent snow-fields. However, their primary habitat is aquatic environments.

Based on habitat the they may be categorized as:

• Aquatic algae
• Terrestrial algae 
• Algae of remarkable habitats

Aquatic Algae

Aquatic forms exhibit a duality, with some thriving in freshwater environments (with salinity levels as low as 10 parts per million) and others flourishing in marine settings (with salinity ranging from 33% to 40%). Additionally, some specific species can adapt to brackish water, which is less salty than seawater but not suitable for drinking.

Aquatic algae can broadly be divided into two type viz, Freshwater and Marine.

Freshwater Algae

• Freshwater algae can be classified as planktonic when they grow and stay suspended in the upper water column (e.g., Volvox, Diatom), whereas benthic algae reside at the bottom.
• They commonly proliferate in various locations such as ponds, lakes, tanks, and ditches.
• Frequently encountered examples include Chlamydomonas, Volvox, Ulothrix, Chara, Oedogonium, Spirogyra, Nostoc, and Oscillatoria.

Marine Algae

• They which grow in saline water, can be categorized as supralittoral or sub-aerial, growing above the water level and within the spray zone.
• Intertidal forms flourish at a depth that exposes them periodically due to tidal fluctuations.
• Additionally some are sublittoral, meaning they are consistently submerged at depths as considerable as 30-60 meters.
• Most widely recognized examples are Sargassum, Laminaria, Ectocarpus, Polysiphonia, Caulerpa, Bangia, and Padina.

Terrestrial Algae

• They are found growing in terrestrial environments such as soil, rocks, and logs.
• They thrive to grow on the surface of soil are referred to as saprophytes.
• Several blue-green algae grow beneath the soil’s surface and are termed cryptophytes.
• Species that inhabit desert soil can be categorized as endedaphic (living within the soil), epidaphic (dwelling on the soil surface), hypolithic (growing on the underside of stones on soil), chasmolithic (occupying rock fissures), and endolithic (able to penetrate rocks).
• The most common example belonging to terrestrial form are Oscillatoria sancta, Vaucheria geminata, Chlorella lichina, Euglena sp., Fritschiella sp., and Phormidium sp.

Algae of Remarkable Habitat

• Halophytic Algae: They grow in the highly concentrated salt lakes, and include Chlamydomonas ehrenbergli, Dunaliella and Stephanoptera sp.
• Symbiotic Algae: They grow in association with fungi, bryophytes, gymnosperms or angiosperms. Most of them are found in association with fungi are Nostoc, Gloeocapsa, Rivularia; the members of Cyanophyceae and Chlorella, Cytococcus, Pleurococcus; the members of Chlorophyceae.
• Cryophytic Algae: They grow on ice or snow provides attractive colours to snow-covered mountains. Scotiella nivalis and Raphidonema brevirostri cause black colouration of snow, whereas Ancyclonema nordenskioldii is responsible for brownish purple colouration.
• Thermophytes: This group is found occurs in hot water springs (50- 70°C) where normal life is not possible. Many blue-greens (e.g., Oscillatoria brevis, Synechococcus elongates, Heterohormogonium sp.) are grown in such hot springs.
• Lithophytes: They grow on the moist surface of stones and rocks, e.g., Nostoc,. Gloeocapsa, Enteromofpha, Batrachospermum etc.
• Epiphytic Algae: They grow on other plants including other algal members as on bryophytes as Nostoc, Oscillatoria, Diatoms etc, on angiosperms like Cocconis, Achnanthes etc.
• Epizoic Algae: They grow on animals like fish, snail etc. are called as epizoic, e.g., Stigeoclonium are found in the gills of fishes.
• Endozoic Algae: They grow in the tissues of animals, e.g., Zoochlorella sp. is found in Hydra viridis.
• Parasitic Algae: Some of them grow parasitically on different plants and animals. For example Rhodochytrium (Chlorophyceae) grows on ragweed (Ambrosia) leaves. Phyllosiphon (Chlorophyceae) grows on the leaves of Arisarum vulgare. Ceratocolax (Rhodophyceae) grows in Phyllophora thallus.

Thallus Organization of Algae

Algal thalli exhibit various levels of organization, ranging from single-celled forms to intricately structured multicellular habits.

In these habits, the plant-like body is distinguished into structures resembling roots, stems, and leaves, creating a resemblance to higher plants. It may vary in size, spanning from a few microns to several meters in length.

Based on their organization, algal thalli can be grouped under types;

Unicellular Algae

Unicelular algae are single-celled algae, known as acellular algae, operate independently as complete organisms. They are commonly found in all algal groups except Rhodophyceae, Phaeophyceae, and Charophyceae. These single cells can be capable of movement or motile.

Unicellular motile forms move either through rhizopoda or flagella. Rhizopodial types lack a rigid cell wall and use cytoplasmic projections for amoeboid motion, as in Chrysamoeba and Rhizochloris. Unicellular non-motile cells can be in the form of spiral filaments, as in Spirulina sp.

Multicellular Algae

Multicellular algae are found in different forms;

Colonial Form

Colonial behavior occurs when cells loosely aggregate within a shared mucilage.

• Coenohium Form: A coenohium forms when a colony consists of a specific number of cells arranged in a defined shape, such as in motile examples like Volvox and Pandotina, or non-motile forms like Scenedesmus and Hydrodictyon.
• Aggregated Form: In aggregated forms, cells irregularly gather into colonies of varying sizes and shapes. These can be Palmelloid, where cells remain embedded in an amorphous gelatinous or mucilaginous matrix as seen in Tetraspora, Phaeocystis, and Microcystis. Dendroid colonies exhibit a mucilaginous thread at the base of each cell, indicating polarity, as observed in Parsinocladus and Chrysodendron. Rhizopodial types unite cells through rhizopodia, characteristic of organisms like Chrysidiastrum.

Filamentous Form

The filamentous structure of plants forms due to continuous cell divisions in a single plane and direction, with cells remaining closely interconnected end-to-end, forming chains or threads. These filaments can either be unbranched or branched.

• Unbranched form: This type of filament can either float freely, like Spirogyra, or be attached to a surface, as seen in Ulothrix and Oedogonium. Free-floating unbranched filaments lack differentiation between basal and apical ends. All filament cells are generally identical except for the basal attachment cell, which is specially modified for anchorage, referred to as a holdfast or rhizoidal cell.
• Branched form: Filaments exhibit branching when divisions occur occasionally in a second plane. There are two types: falsely branched and truly branched. In Falsely branched forms, trichomes can break due to the death or decay of intercalary cells. These broken ends protrude from the mucilaginous sheath as branches but do not arise as lateral outgrowths (e.g., Scytonema). In truly branched forms, filaments occasionally undergo division in a second plane, resulting in true branches that arise as lateral outgrowths of the main filament. Examples include Cladophora (simple filament), Fritschella (heterotrichous), and Batrachospermum (Pseudoparenchymatous).
• Parenchymatous form: This occurs when filament cells divide in multiple planes, leading to the formation of a parenchymatous thallus. Over time, this structure can become foliose and flat, as seen in Ulva, tubular as in Enteromorpha, or complex as in Sargassum.