GREEN ALGAE: CHARACTERISTICS, HABITAT, TYPES AND PROPERTIES - BIOLOGY - 2025

The green algae are a group of organisms belonging to the subkingdom Viridiplantae, consisting of 10,000 species that live primarily in inland waters. These organisms have pigments and reserve substances similar to those of higher plants, which is why they are considered their ancestors.

The presence of chlorophyll pigments a and b are responsible for its characteristic greenish hue. In addition, they have some auxiliary pigments such as carotenoids and xanthophylls, as well as starch molecules that are stored as reserve substances in plastids.

Green algae. Source: pixabay.com

The vast majority of green, unicellular or multicellular algae, free-living or sessile, live in fresh waters, with only 10% of the species being marine. They are also located in humid and shady terrestrial environments, snow banks, trees, rocks, or even on animals, or in symbiotic association forming lichens.

The form of reproduction is very varied from one class to another, presenting both sexual and asexual reproduction. Sexual reproduction includes isogamy and oogamy, and in asexual reproduction zoospores and planespores predominate.

Today algae, including green algae, are one of the most widely used organisms for various purposes. The algae are used as food for human consumption and as a nutritional supplement for animals; They are used as biofuel, in the cosmetic and pharmaceutical industry. They are also a determining factor in the preservation of marine ecosystems.

General characteristics

Morphology

Morphological variability, functional organization, metabolic flexibility and development have favored the broad level of colonization of green algae on various habitats. In fact, there are microscopic and macroscopic species, adapted to live in freshwater aquatic environments, and even in brackish and salty waters.

The microscopic species are unicellular, spherical or elongated (eg Volvox), with one or more flagella, or absent flagella, covered by special or smooth scales. The multicellular species of filamentous form are constituted by chains of tubular cells (eg Spirogyra), or sometimes particular cells outlined at the ends.

Spirogyra. Source: Bob Blaylock at English Wikipedia

The tissues show great variability in terms of their levels of organization, observing species with reserve, photosynthetic or filling parenchyma, or pseudoparenchyma. The cells of most green algae are uninucleated, but species such as Caulerpa have plurinucleated cells or coenocytic cells.

The cell wall of green algae is mainly made up of cellulose, including some structural polymers. In some cases it is calcified. On the other hand, the structures that provide the characteristic color or photosynthetic pigments are chlorophylls a and b, b-carotene, and some carotenoids.

Taxonomy

- Kingdom: Plantae.

- Subkingdom: Viridiplantae.

Green algae are considered to have evolved into two major divisions or clades: Chlorophyta and Charophyta.

- The UTC clade Chlorophyta (chlorophyte) includes flagellate unicellular algae (Chlamydomonas) and colonies (Volvox). As well as filamentous marine algae (Codium, Ulva), soil algae (Chlorella), phycobionts (Trebouxia) and epiphytes (Trentopohlia).

- Clade C Charophyta (charofíceas) comprises a group of algae that inhabit fresh waters, soils, aerial environments and terrestrial plants. Examples of this clade are unicellular algae (Micrasterias), filamentous (Spirogyra) or species with parenchymal thallus (Chara).

Phylogenetic tree of the Viridiplantae. Source: Judd et al. (2002) Drawing and Spanish translation: User: RoRo

Habitat and distribution

Green algae inhabit coastal marine environments, subject to the seabed or forming part of the nanoplankton of coastal waters. Likewise, they are abundant in fresh water, both in stagnant waters or water courses, such as lakes, ponds, streams, rivers or wells.

In addition, they are also located in land environments with high humidity, such as rocky areas, tree trunks and flooded or muddy soils. Likewise, certain species live in extreme conditions of temperature, salinity or electrical conductivity, and Hydrogen potential (pH).

The Extremophilic species Dunaliella acidophila lives in extremely acidic conditions, with pH levels less than 2.0. Likewise, the Dunaliella salina species used to obtain carotenes, develops in hypersaline waters with more than 10% of dissolved salts.

Certain species thrive on desert soils where they tolerate long dry periods and high temperatures. In fact, other species (psychrophiles) develop at temperatures below 10ºC.

Green algae also establish symbiotic associations with other species, such as fungi. In this case, the fungus is the mycobiont and the algae constitute the photosynthetic phase or phycobiont of lichens.

Types

Chlorophyta

Known as chlorophytes, chlorophytes or green algae, they are aquatic organisms that contain chlorophylls a and b, β-carotene and in their plastids they store starch as a reserve substance. They comprise about 8,000 photosynthetic eukaryotic species of aquatic habits.

In this group, unicellular and multicellular species are located, of sexual reproduction by isogamy or oogamy, and asexual reproduction by spores or cell division. Similarly, flagellate or motility-displaced organisms are found.

They inhabit freshwater ecosystems or marine environments, as well as terrestrial environments with high humidity, on rocks, logs or under snow. Its life cycle is haplodiplont, characterized by a haploid and a diploid phase.

Chloriphytes are classified into Prasinophytina, which includes the Mamiellophyceae, Nephroselmidophyceae, and Pyramimonadophyceae. As well as the Chlorophytina, which groups the Chlorodendrophyceae, Chlorophyceae, Pedinophyceae, Trebouxiophyceae and Ulvophyceae.

Prasinophytina

It constitutes a group of microscopic green algae, unicellular and flagellate, of marine habits, currently considered as primitive organisms. The genus Ostreococcus is the most representative: it is made up of eukaryotic organisms with marine habits and a free life of just 0.95 μm.

These species present a simple cellular development, possess a single chloroplast and a mitochondrion, with a limited genome among eukaryotes. They are located mainly in marine environments, being of great interest for paleontology due to the large number of fossils found.

Pyramimonas sp. Source: Pyramimonas_sp.jpg: ja: User: NEON / User: NEON_jaderivative work: Addicted04

Chlorophytin

It represents a group of green algae characterized by multicellular organisms, which develop microtubules called phycoplasts during cytokinesis in mitosis.

Chlorophytins comprise a taxon generally considered at the subphylum taxonomic level, which gathers together chlorophytic green algae. This grouping is frequently designated as nuclear chlorophytes or UTC clades (acronyms derived from the initials Ulvophyceae, Trebouxiophyceae and Chlorophyceae).

Ulvophyceaes. Source: Fleliaer

Charophyta

Charofitas are a group of green algae that represent the closest ancestors to land plants. This group of algae is characterized by a multicellular thallus, with whorls of short leaves and gametangia surrounded by sterile structures.

The cells of the charphytes have cell walls composed of cellulose, calcified, with chlorophyll a and b, carotene, xanthophyll and reserve starch. They are organisms of aquatic life, which can be in fresh or brackish waters, at rest or with little circulation, fixed to the seabed or submerged.

Coleochaetales

Coleochaetales are a group of discoidal filamentous green algae, growing apically across the edges of the disc. They are species of aquatic habitats, which are located on submerged rocks or on stems and branches of aquatic plants.

Their reproduction is carried out asexually through zoospores, and sexually through oogamy. They are organisms closely related to green plants due to the formation of fragmoplasts, the presence of oxidase enzymes and antheridia or motile spermatozoa.

Coleochaete orbicularis. Source: Cooke, MC (Mordecai Cubitt), b. 1825

Chlorokybophyceae

Chlorokybophyceaes are represented by a unique species of unicellular green algae. In fact, the Chlorokybus atmophyticus is a species of terrestrial habits of the alpine areas.

Charophyceae

Known as caral algae, they are a type of green algae that contain chlorophyll a and b. They are free-living organisms in fresh water, being the only macroalgae of the Charophyta division that reach 60 cm in length.

This group is related to bryophytes, specifically in the configuration of archegonium and the functionality of the enzyme glycolate oxidase in the photorespiration process.

Chara globularis. Source: Christian Fischer

Klebsormidiophyceae

Klebsormidiales are a group of charophytic green algae consisting of three genera of multicellular organisms and filaments devoid of branches. The genera that make up this group are Entransia, Hormidiella and Klebsormidium.

Klebsormidium bilatum. Source: Katz Lab from Northampton, MA, USA

Mesostigmatophyceae

The mesostigmatophyceaes constitute a classification of carophyte green algae that is made up of a single genus of unicellular algae called Mesostigma. The only species in this genus is M. viride Lauterborn (1894), which lives in freshwater aquatic environments, and is phylogenetically related to the clade Streptophyta.

Zygnematophyceae

The green algae zygnematophyceae or conjugatophyceae are a group of unicellular or multicellular algae that live in fresh water and reproduce by isogamy or conjugation.

In this group are unicellular organisms (Desmidiales) or with branched filaments (Zygnematales). They are also genetically linked to land plants.

Conjugated algae. Source: Ernst Haeckel

Medicinal properties

The nutritional analysis of the algae has made it possible to determine that they contain low levels of calories; however, they are high in protein, fiber, minerals and vitamins. Likewise, they have essential amino acids, glycine, alanine, arginine and glutamic acid, as well as polyphenols, bioactive elements with a high oxidative capacity.

Algae have high nutritional value, being in many cases superior to land plants in this sense. They contain vitamins A, B ₁, B ₂, C, D and E, as well as the mineral elements calcium, phosphorus, iron, potassium, sodium and iodine.

In this regard, frequent consumption of seaweed makes it possible to regulate glucose levels in the blood, purify it and detoxify the digestive and lymphatic systems. Seaweed has anti-inflammatory, antiviral and immunological properties, preventing various diseases and strengthening the immune system.

The high level of iodine in seaweed is effective for treating problems related to the thyroid gland. The incorporation into the hormonal system of sufficient levels of iodine improves the immune system, stimulates the creation of proteins and improves hormonal reactions.

Green algae in marine environments. Source: pixabay.com

Algae are chelating elements, that is, they have the ability to absorb metals and toxins from the body and facilitate their expulsion. In addition, they act as satiating: their consumption gives a feeling of filling, helping to fight obesity and overweight.

The green algae Haematococcus is cultivated to obtain the carotenoid astaxanthin, widely used in medicine and as a dietary supplement in aquaculture. Indeed, astaxanthin has antioxidant properties similar to vitamin E, which regulates oxidative stress, making it ideal for healthy skin and eyesight.

In general, algae can regulate cholesterol levels, improve digestive and nervous system functions. Likewise, they act as diuretics, alkalizing, increase immunity and provide important nutritional elements during menopause.

References

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