Introductory Chapter: Cyanobacteria – An Overview

1.1 General characteristics

Cyanobacteria do not have internal cell membranes that delimit the cell nucleus from other organelles and are therefore classified as prokaryotic organisms, which microscopically distinguishes them from algae, microalgae, and plants. They can be found as unicellular, colonial, or multicellular organisms and inhabit the most diverse environments; they can be planktonic (suspended in water), benthic (attached to surfaces), or metaphytic (attached to macrophytes or other surfaces submerged in water) [4].

They are photosynthetic organisms and therefore have a pigment called chlorophyll-a. However, due to the absence of internal cell membranes, cyanobacteria do not have chloroplasts, and, therefore, chlorophyll is found inside simple thylakoids, where light-dependent photosynthetic reactions take place. Cyanobacteria also have carotenoids (whose main function is photoprotection) and accessory pigments, such as phycobilins (e.g., phycocyanin and phycoerythrin), which do more than provide the characteristic cyanobacterial color: phycocyanins and phycoerythrins absorb some wavelengths of active radiation and transfer the absorbed light energy to chlorophyll-a in photosystem II [5].

Cyanobacteria reproduce by asexual reproduction, the division of vegetative cells. In unicellular species, cells divide completely, and some can form colonies, an aggregate of single cells in a mucilaginous matrix. When their cell division occurs in a single plane, they can form pseudofilaments, linear colonies formed by unicellular cyanobacteria. In true filamentous cyanobacteria, the cells remain connected after division, forming structures called trichomes or filaments. The filaments divide through a single plane of division that can also project in multiple directions, forming false-branched filaments, or they can divide through multiple planes of cell division, forming true-branched filaments [6].

The cellular morphology of these organisms is quite diverse and can be used to identify different species and taxonomic groups. The cell shape of cyanobacteria can be spherical, ellipsoid, cylindrical, conical, or discoid. Their size also varies greatly, from the so-called picobacteria, spherical cyanobacteria with a very small cell diameter (0.2 μm), to filamentous forms that can reach up to a few millimeters [7].

Despite not having flagella, many cyanobacteria have mobility mechanisms, although they are not very well elucidated. In fact, some filamentous species can develop hormogonia, reproductive and mobile units, which are formed by the fragmentation of filaments and then released from the parental filament. Hormogonia perform gliding movements until they develop into a new trichome [8].

Some specialized cellular structures are also found in some cyanobacterial species. Aerotopes, for example, are structures formed by cylindrical proteins that form air vesicles. These vesicles are filled with air, which diffuses into their interior, making the cyanobacterial cells less dense than water and allowing them to float or emerge. Aerotopes are refractory to light in microscopy techniques and are therefore used to differentiate taxa in microscopic analyses [9].

Not only specialized cellular structures can be identified in cyanobacteria, but there are also entire specialized cells that are morphologically distinct from vegetative cells, such as heterocytes and akinetes cells. The first are cells that enable nitrogen fixation, a process called diazotrophy, where nitrogenase enzymes reduce nitrogen to ammonium; therefore, heterocytes have an extra cell envelope to maintain the cell interior in anoxic conditions, and they do not have a complete photosynthetic mechanism, as this could damage nitrogenases with the production of oxygen. Akinetes are spore-like cells, larger than vegetative cells, with a multilayered cell wall and glycogen and cyanophycin granules. In general, the formation of heterocytes and akinetes is closely related to environmental conditions [10].

1.2 Taxonomy of cyanobacteria

The criteria for classifying cyanobacteria are phylogenetic relationships that indicate their grouping into taxa that share a common evolutionary ancestor, as is the case for all other living organisms. Initially, the taxonomic classification of cyanobacteria, as well as several other organisms, was based solely on morphological characteristics, that is, cellular properties observed through microscopic techniques. However, molecular techniques are now being used to elucidate the correct taxonomy of species, including cyanobacteria. Thus, many classifications have been revised, regrouped, and even renamed, and many of them are probably not definitive and may change as more research is carried out at the molecular level [11].

There is no precise definition of species for the taxonomic classification of cyanobacteria, as this would require obtaining pure, axenic cultures, which is very difficult in the case of cyanobacteria. Furthermore, there are two different nomenclature systems for these organisms: the International Code of Nomenclature for Algae, Fungi, and Plants (ICN) and the International Code of Nomenclature for Bacteria (ICNB). Thus, depending on the scientific vision and knowledge of the taxonomist, a given species can be identified in different ways, confusing the literature [3]. Some authors argue that the description of cyanobacterial taxa would be more appropriate using the bacteriological code since it is already known that cyanobacteria are a monophyletic branch in the bacterial phylogenetic tree [2].

2.1 Bioenergy

The depletion of the world’s oil reserves and the environmental impact caused by the emission of polluting gases during fuel combustion have forced humanity to look for sustainable energy alternatives [12, 13, 14]. Cyanobacteria emerge as a promising energy source, primarily due to their significant lipid accumulation potential, which can subsequently be converted into biodiesel through transesterification [15]. Furthermore, cyanobacteria have high growth rates, high photosynthetic capacity, low nutritional requirements, and do not need fertile land. However, its growth and lipid accumulation are affected by several factors, such as light intensity, temperature, pH of the medium, the availability of macro and micronutrients in the medium, and cultivation systems, such as closed photobioreactors or open raceway ponds [16].

The concentrations of phosphorus and nitrogen are decisive in the accumulation of lipids by cyanobacteria, which can accumulate up to 50% of their weight in lipids. For example, the cyanobacterium Microcystis protocystis accumulated around 42% of its weight in lipids under controlled nitrogen and phosphorus conditions. Other cyanobacteria, such as Synechocystis sp. PCC 6803 and Synechococcus elongatus, can accumulate up to 20% of lipids of their weight [15].

On the other hand, cyanobacteria can also be used to produce high-energy molecules such as ethanol. For example, Dexter & Fu [17] developed a mutant cyanobacterium Synechocystis sp. PCC 6803 that can photoautotrophically convert CO₂ into bioethanol. The transformation was carried out using a double homologous recombination system to integrate the genes for pyruvate decarboxylase (pdc) and alcohol dehydrogenase II (adh) from the obligate ethanol-producing Zymomonas mobilis into the cyanobacterial chromosome. Thus, Liang et al. [18] used a Synechocystis PCC 6803 mutant to achieve ethanol concentrations of up to 900 mg/L ethanol.

Cyanobacteria can also produce other energetic molecules, such as methane (CH₄). According to Bižić et al. [19], some cyanobacteria living in marine, freshwater, and terrestrial environments produce methane at substantial rates under light, dark, oxic, and anoxic conditions.

2.2 Biofertilizer

The frequent application of chemical or synthetic nitrogen-based fertilizers changes the composition and structure of the soil, in addition to negatively affecting the microbial flora [20, 21, 22]. Therefore, nitrogen sources that are less aggressive to the environment are needed. One of the alternatives is biological nitrogen fixation, which is a process generally developed by microorganisms that convert atmospheric or inorganic nitrogen into a form of nitrogen that can be used by plants. According to Rashid et al. [23], employing nitrogen-fixing microorganisms presents an economically appealing and environmentally friendly alternative. Among the various microorganisms capable of fixing atmospheric nitrogen, cyanobacteria are particularly noteworthy.

Cyanobacteria are considered one of the most promising microorganisms for sustainable agricultural development due to their high nitrogen fixation capacity. According to Joshi et al. [21] and Song et al. [24], cyanobacteria such as Diazotrophs are potentially useful microorganisms for the production of biofertilizers. Cyanobacteria such as Anabaenas can fix up to 60 kg of atmospheric nitrogen/ha in the soil. Other microorganisms such as Anabaena variabilis and Nostoc linkia also have a significant ability to fix nitrogen at around 25 kg nitrogen/hectare [21].

According to Song et al. [24], cyanobacteria play an essential role in maintaining and improving soil fertility, due to which these microorganisms contribute to the formation of porous soils and produce substances such as phytohormones (auxiana and gibberellins), as well as vitamins and amino acids that promote plant growth. Furthermore, cyanobacteria also improve water retention capacity due to their gelatinous structure [21, 25, 26].

2.3 Food supplement and pharmaceutical products from cyanobacteria

Cyanobacteria are considered foods and dietary supplements because they are a source of carbohydrates, proteins, peptides, essential amino acids, fibers, lipids, polyunsaturated fatty acids, minerals, vitamins, etc., compounds necessary for human and animal nutrition [27]. Some of these compounds have antioxidant, antimicrobial, anticancer, antimycotic, and antifungal properties, among others [28].

Some species of Spirulina are important sources of macromolecules, including mainly proteins. They have been sold as natural products, spreading worldwide popularity for being one of the most nutritious foods from an alternative source. Spirulina has the outstanding ability to accumulate protein and has already been used in meat substitutes, food products, animal feed, nutraceuticals, and pharmaceuticals [27].

Ahmed [29] evaluated the antimicrobial activity of methanolic strata of cyanobacteria such as Oscillatoria formosa, Nostoc caeruleum, Cylindrospermum majus, and Spirulina platensis against the bacteria Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and the fungi Trichophyton mentagrophytes, Candida albicans, and Aspergillus fumigatus. The results obtained showed that the alcoholic extract of S. platensis was the most effective against the microorganisms tested.

According to Vijayakumar & Menakha [30], cyanobacteria are also sources of bioactive secondary metabolites such as apratoxins, lynbyabellin, and curacin A, compounds that can be used to manufacture drugs against complex diseases such as cancer. Some freshwater cyanobacteria produce peptides with side chains that are effective against different enzymes such as microginin, aeruginosin, anabaenopeptin, etc. [31].

Additionally, bioactive compounds from some species of cyanobacteria have been explored for biomedical purposes. Polysaccharides are interesting sources due to their many physicochemical properties and biological roles. These biomolecules, especially exopolysaccharides, are extremely important for market purposes because they can be used as anti-inflammatory, immunomodulatory, antiglycemic, antitumor, antioxidant, anticoagulant, antilipidemic, antiviral, antibacterial and antifungal agents [32].

2.4 Cyanobacteria in bioremediation processes

Cyanobacteria play a significant role in bioremediation processes due to their unique capabilities. Bioremediation involves the use of living organisms to detoxify and eliminate pollutants from the environment. These microorganisms contribute to the biogeochemical cycles of carbon and nitrogen [31, 33]. Furthermore, cyanobacteria are potent bioremediation agents due to their ability to grow under extreme conditions and metabolize different metabolites. Compounds that have been bioremediated with cyanobacteria include pesticides, heavy metals, paints, and emerging contaminants such as hormones, pharmaceuticals, and others [31].

One of the most important xenobiotics removed by cyanobacteria are heavy metals such as Mn, Zn, Cu, Cd, and Pb. Some species of cyanobacteria can produce exopolysaccharides that are used to sequester xenobiotics [34, 35]. According to Potnis et al. [35], biofilms produced by Phormidium can sequester up to 99% Cu ions. Biofilms of Nostoc commune and Nostoc linckia can remove between 55% and 87% [36].

Cyanobacteria are utilized in the treatment of various types of wastes. They can effectively reduce organic pollutants and nutrients in wastewater, contributing to the purification of water before it is released back into natural ecosystems. In addition, cyanobacteria can fix carbon dioxide through photosynthesis. This capability is harnessed not only for potential biofuel production but also for CO₂ capture purposes. By converting CO₂ into organic matter, cyanobacteria can help mitigate greenhouse gas levels in the atmosphere [4].