ANABAENA: CHARACTERISTICS, HABITAT, REPRODUCTION AND NUTRITION - BIOLOGY - 2025

Anabaena is a genus of prokaryotic photosynthetic cyanobacteria, that is, they are unicellular, without a defined nucleus, with genetic material, but dispersed in the cytoplasm. They are plantonic organisms of shallow waters, with barrel shapes and can form colonies.

Cyanobacteria, including Anabaena, are also called blue-green algae, although they are not related to the kingdom Plantae. They are so called, in addition to their coloration, because of the ability of some species to fix nitrogen and others because they present chlorophyll and photosynthesize.

Cyanobacterium species Anabaena circinalis. Taken and edited from: Bdcarl Nabaenas are the subject of much scientific attention, since they are one of the few groups of cyanobacteria that can fix atmospheric nitrogen, which has made them of particular interest in genomic, evolutionary, biochemical, and pharmaceutical studies among much others.

characteristics

They are prokaryotic organisms. They present a peptidoglycan cell wall, with a structure very similar to that of bacteria that react negatively to Gram staining (Gram negative).

They generally measure about 2 to 10 micrometers, although some species can measure up to 20 micrometers. There are free-living species and most are filamentous (with unbranched trichomes).

They have chlorophyll a, so they can photosynthesize. Filamentous cells have specialized cells called heterocysts that lost the ability to photosynthesize, but instead allow them to fix atmospheric nitrogen with the help of an enzyme called nitrogenase.

Heterocystic cells are formed within the filament and / or in terminal areas of it. For physiological and chemical reasons, these cells form a thicker cell membrane than the other cells in the colony.

The function of this membrane is to create an anaerobic microenvironment and thus be able to capture and fix atmospheric nitrogen, since the nitrogenase enzyme is inactivated in the presence of oxygen.

Nabaenas, like other cyanobacteria that have heterocysts, can carry out nitrogen fixation processes in the absence or presence of light; even when grown in the absence of nitrogen, they capture carbon dioxide and photosynthesize.

Some species generate bloom or blooms, others have the capacity to produce bioluminescence and certain species can become toxic.

Taxonomy and phylogeny

Anabaena belongs to the Bacteria domain, proposed by Carl Woese in 1990. This domain, according to Woese, is a sister group to Archea (another group of prokaryotes more closely related to eukaryotes) and Eukarya (eukaryotic organisms).

They are within the kingdom Bacteria and the phyllum Cyanobacteria. The taxonomic arrangement of cyanobacteria is currently subject to controversy, their classification is quite complex and scientists are not entirely in agreement with the current taxonomic arrangements.

However, within the group of cyanobacteria there is some scientific acceptance of the phylogenetic relationships that exist between the Nostocal and Stigonematal orders, both groups with heterocystic cells typical of the Anabaena, Nostoc and Cylindrospermum genera.

Some phylogenetic studies reveal that within the Nostocales, the Anabaena and Nostoc genera are more related to each other than to Cylindrospermum. At present, more than 170 species of the genus Anabaena have been described.

Habitat

They are common organisms inhabiting shallow freshwater bodies, some species are from marine environments and others have even been reported in humid terrestrial environments.

Marine species can live in different salinity conditions. Regarding temperature, certain species are present in temperate zones in summer, they are tolerant to temperature variations and can even develop in environments with temperatures of more than 70º Celsius.

Being mainly freshwater waters, they tolerate certain levels of acidity, however there are species that also live in alkaline thermal environments, that is, warm environments with high pH (basic).

Symbiotic associations

Several species of Anabaena have been found living in mutualistic symbiotic associations with algae and phanerogamic plants. They inhabit the body of their host and provide nitrogen in exchange for protection against predators.

Reproduction

These cyanobacteria reproduce asexually, that is, they do not require the presence of male or female gametes. Instead they employ other reproductive mechanisms.

In Anabaena reproduction occurs by fragmentation; these microorganisms form filamentous-type colonies. Fragmentation occurs when a section of the filament (hormogonia) detaches from the rest of the colony.

As detachment occurs, the smallest part that was fragmented will slide or float in the water column. Later it will begin to form its own colony.

Nutrition

The genus Anabaena is an autotrophic nutritional taxon, that is, its representatives are organisms that produce their own food from inorganic compounds or elements. Anabaena have two types of autotrophic nutrition:

Photosynthesis

Photosynthesis is a chemical process that occurs in some organisms such as plants and some bacteria, in which carbon dioxide and water form sugars in the presence of light and oxygen is released as a by-product.

Anabaena has chloroplasts with the photosynthetic pigment chlorophyll a, which is what allows it to absorb light energy and transform it.

Fixation of nitrogen

Many bacteria are nitrogen-fixing heterotrophs. However, most of these do it in the absence of sunlight and in anoxia, or low oxygen concentrations.

As we have previously described, Anabaena is one of the few groups that have specialized cells called heterocysts. These allow them to fix atmospheric nitrogen in the presence of sunlight and oxygen, and thus obtain the necessary nutrients for the development of the colony.

Heterocysts in cyanobacteria of the genus Anabaena. Taken and edited from: Berkshire Community College Bioscience Image Library.

Toxicity

The genus Anabaena is also known to present species that produce toxins. When the habitat conditions are favorable, a proliferation or flowering (Bloom) of these species can occur.

During these blooms, water becomes a very dangerous toxic agent for the organisms that drink it, due to the presence of cyanobacteria. It is known of poisoning of cattle, birds, fish, pets and even human beings for this cause.

Toxic species produce a neurotoxin (eg, anabazines) that affects the central nervous system of organisms that ingest it. This toxin causes dementia similar to Alzheimer's, symptoms similar to Parkinson's disease, among others.

In some acute cases, death of patients may ensue. There is no known cure for this toxin, treatment is symptomatic.

References

1. Anabaena. Recovered from britannica.com
2. Anabaena. Requested from wikipedia.org
3. M. Burnat & E. Flores (2014). Inactivation of agmatinase expressed in vegetative cells alters arginine catabolism and prevents diazotrophic growth in the heterocyst-forming cyanobacterium Anabaena. Microbiologyopen.
4. Anabaena. Recovered from bioweb.uwlax.edu.
5. Anabaena. Recovered from wildpro.twycrosszoo.org.
6. N. Rosales Loaiza, P. Vera, C. Aiello-Mazzarri, E. Morales (2016). Comparative growth and biochemical composition of four strains of Nostoc and Anabaena (Cyanobacteria, Nostocales) in relation to sodium nitrate. Colombian Biological Act.