One of the most prevalent genera of cyanobacteria found in freshwater environments is Microcystis, a genus of colonial, spherical cells. Microcystis can produce toxins; they are poisonous and can cause blooms. Their colonies can be any shade of green, from deep olive to vivid blue-green. Microcystis colonies expand swiftly and have a milliliter cell density of over 1 million ^[11][12][13][14,15,16]. They prefer to form mats on the water’s surface along with other species like Anabaena, Oscillatoria, or Aphanizomenon and are photosynthetic organisms ^[14][15][16][17,18,19].

Moving water like rivers or streams and standing water like ponds, lakes, or reservoirs can harbor Microcystis. Because of the lower levels of oxygen and warmer temperatures throughout the summer, blooms are more likely to happen. Due to decreased oxygen levels and the development of chemicals that can harm humans and plant life, the blooms may result in drop-down water quality ^[17][18][20,21].

Anabaena inhabits environments in freshwater. These cyanobacteria are particularly important for their capacity to fix nitrogen since they include heterocyst cells capable of doing so. Anabaena also has akinetes cells, which act as protective spores and can withstand harsh environmental conditions ^[19][20][22,23].

The filaments that make up an Anabaena comprise individual trichome cells, which are one cell thick. Each trichome contains photosynthetic cells with a thick cell wall and an external peptidoglycan coating. The nucleoids, thylakoids, and carboxysomes of the cells are home to the enzymes involved in photosynthesis. Along the length of the trichome, heterocysts act as the central locations for nitrogen fixation ^[20][21][22][23,24,25]. Anabaena has undergone extensive research due to its ability to produce diazotrophic endospores that can be used to study other cyanobacterial species further. It is commonly used in aquatic systems to increase their nutritional content because of its ability to fix atmospheric nitrogen ^[22][23][25,26]. By doing so, it can supply other species with fixed nitrogen in settings where it is scarce or absent.

Oscillatoria is a widespread genus of freshwater cyanobacteria, the bacteria that cause bodies of water to get murkier and greener. It has a slimy, thin filamentous structure. Inhabiting shallow and deep regions of nutrient-rich water, such as seas, rivers, and lakes, Oscillatoria prefers to coil in an oscillatory motion ^[24][25][27,28].

It belongs to the cyanobacteria family and is not heterocytic, meaning it does not have heterocysts, a specific cell that fixes nitrogen in plants’ roots. Oscillatoria is frequently grouped with the Chroococcales family regarding their growth and physiological circumstances ^[26][29].

Under some circumstances, these mobile bacteria can create floating mats of colonies on the water’s surface. Oscillatoria can withstand dry seasons’ droughts thanks to the viscous slime that coats its cell walls. Furthermore, by utilizing nitrates to generate energy, it can survive in low light or even complete darkness because of its unique photosynthesis style ^[25][26][27][28,29,30].

A filamentous and mobile genus of cyanobacteria called Cylindrospermopsis makes toxins. Freshwater systems like lakes and rivers and artificial reservoirs like ponds and canals frequently contain it. In temperate areas, this genus displays sporadic flowers in the summer ^[28][31].

Cylindrospermopsis spreads swiftly and takes over a system due to its motile habit, which makes it tough to eradicate. Despite being filamentous creatures, they can develop both deep blooms that can extend several meters below the surface of the water and enormous mats close to the water’s surface ^[29][30][32,33].

In addition to hepatotoxins (cyanotoxins), neurotoxins (ciguatoxins), and components of bacterial cell walls (lipopolysaccharides or LPS), Cylindrospermopsis species also produce toxins. Scientists must watch for these species in freshwater systems because these cyanotoxins are very hazardous to people and animals, even at low concentrations ^[29][30][31][32,33,34].

The genus Aphanizomenon of motile, free-floating filamentous cyanobacteria is widely found in freshwater bodies of water. They are known as nitrogen fixers because they can convert atmospheric nitrogen into a form that plants can utilize. As a result, they contribute nutrients to the food chain and maintain high water quality, making them vital to aquatic environments ^[32][33][35,36]. Certain Aphanizomenon species can harm humans and other animals when present in more significant proportions than usual. To further comprehend the potential risks these cyanobacteria pose, it is essential to frequently sample water and evaluate the levels of cyanobacterial metabolites ^[34][37]. Aphanizomenon is usually found in blossoms; therefore, that should also be considered. This indicates that an increase in their population may be caused by their quick growth, spurred by an abundance of nutrients or higher temperatures. It can help to prevent possible damage from increased toxin production or other negative impacts on aquatic plants and fauna by routinely checking for bloom indications ^[35][36][38,39].

Numerous distinct species of cyanobacteria that have evolved to flourish in freshwater ecosystems comprise the Planktothrix genus. Planktothrix agardhii, P. rubescens, and P. limnetica are the most prevalent species. Due to their preference for nutrient-rich waters, these organisms are frequently found in eutrophic lakes with high levels of phosphate and nitrogen ^[37][38][39][40,41,42].

Phytotoxicity diffusion is the process of producing toxic exudates on the cell walls of Planktothrix species due to dense blooms that decompose. Due to the pigments (such as carotenoids) they produce, Planktothrix blooms can also cause water to become discolored ^[40][43]. In addition, Planktothrix is renowned for its capacity to multiply quickly and, under the right circumstances, to create dense aggregates or mats at the water’s surface. Optimal temperatures for Planktothrix development and survival in freshwater systems are between 15 and 25 °C and photosynthetically active radiation (PAR) ^[40][41][43,44].

The most prevalent genus of cyanobacteria is Synechococcus. It can be single-celled or colonial, consisting of one or more cells that can cluster and form filaments or colonies containing single or multiple cells. These colonies typically have a spherical form, a single gas exchange aperture, and a polysaccharide wall ^[42][43][45,46].

Synechococcus is renowned for its capacity to endure harsh conditions, including acidic and hypersaline waters. Due to its tolerance for high salt concentrations, it is also a significant species in coral reefs and soils, both aquatic environments. Its capacity for photosynthesis has also made it a vital component of the oxygen cycle in aquatic settings ^[44][45][46][47,48,49].

Rivularia create colonies that resemble bubbles, with each cell living inside a separate, protective “home”. Several characteristics set Rivularia apart from other cyanobacterial taxa. First, unlike most other cyanobacteria, Rivularia colonies are relatively solid in the water. The individual cells each have a distinct shape, with flat bottoms and rounded tops ^[47][48][49][50,51,52].

Additionally, depending on their environment, Rivularia colonies can appear very varied. When exposed to large amounts of light or oxygen, the colonies turn a deeper shade of green or even blue-black. However, colonies deprived of oxygen and light become paler and more transparent ^[50][51][53,54].

The Gloeotrichia genus is distinguished by its distinctive branching, which gives its members the appearance of a starburst or a dandelion gone berserk. Due to their characteristic morphology, Gloeotrichia members are frequently observed in freshwater or brackish water habitats. They are anoxygenic phototrophs, producing sulfur rather than needing oxygen to use the light energy from photosynthesis ^[52][53][55,56].

These creatures’ internal sheaths have a spiral arrangement of cells that facilitates their movement through their environment, improving nutrition intake and mobility throughout your aquascape. The cell’s structure and surface area influence its interaction with its surroundings ^[53][54][55][56,57,58].

Most organisms have a variety of photosynthetic pigments in their photosynthetic system, allowing them to utilize light from locations where it is most abundant efficiently. Additionally, they have a variety of flagella to assist them in moving around in water deficient in nutrients or containing a lot of dissolved materials, such as salts and metals ^[55][56][58,59].

Cyanobacteria belonging to the genus Scytonema have filaments that are erratically branched. When growing in colonies, cells can form cords, tufts, and even mats and are typically oriented spirally. The second name of this particular cyanobacterium species refers to the color of its distinctive brilliant reddish-brown spores. It can be found on mosses and lichens and often grows on damp surfaces ^[57][58][60,61].

Scytonema stands out from other cyanobacteria in several ways. It does not require light for photosynthesis, enabling it to flourish in dark places like caves or deep-sea cracks. A bud forms from the side of the bacteria cell and splits off to create a new organism, which is how they mainly reproduce ^[58][59][61,62].

They can also continue for extended periods without water because they enter a latent stage when they do not require oxygen. On the other hand, several species have unique pigments that enable them to absorb various light wavelengths, allowing them to adapt to multiple habitats ^[58][59][60][61,62,63].

Because of its pseudo-parenchymatous filaments, Tolypothrix can be identified. As a filamentous cyanobacterium that resembles a thread, Tolypothrix can join with other Tolypothrix cells to form lengthy chains of cells. This genus is extensively distributed worldwide and can be found in fresh and saltwater settings ^[61][64].

Typically comprised of four to six cells, Tolypothrix cells are protected from external challenges like UV radiation and drying out by a rigid coating consisting of glycoproteins and polysaccharides. The glycoprotein sheath keeps Their cell walls together, making it easier for them to build long chains ^[61][62][63][64,65,66].

Contrary to other cyanobacterial genera, such as rivularia or Gloeotrichia, Tolypothrix filaments rely on robust glycoprotein sheaths to make these connections rather than a visible sheath or stalk-like structure. This distinctive structure provides them a distinct edge when adapting to various settings, which explains why they are such a widespread genus globally ^[62][64][65,67].

Merismopedia, a common genus of cyanobacteria, has a distinctive cubic packet-shaped cell and can split up to four times before finally dispersing. Merismopedia cells typically come in two shapes: flattened and cubed, ranging in size from 2 to 6 m ^[65][66][68,69]. When these cells cluster together in their surroundings, their morphology enables them to create a slimy, jelly like substance. This characteristic slimy mass typically comprises numerous layers with various types of Merismopedia cells ^[67][70].

Although it can also be found in wet soil and other moist habitats, Merismopedia is primarily found in oceans, seas, and other aquatic environments. Additionally, Merismopedia is frequently linked to cyanobacterial mats because of its ability to adapt to different light, temperature, and salt concentrations ^[68][69][71,72].

Merismopedia needs specific minerals like potassium and nitrogen to thrive appropriately and does it best in direct sunshine. It can grow rather quickly under ideal circumstances, such as temperatures between 25 and 35 °C and pH levels between 6 and 8.1. It is crucial to understand where Merismopedia is typically located and how to recognize it because this tiny but mighty genus of cyanobacteria can create toxins that can be dangerous to people if swallowed or even inhaled ^[68][69][70][71,72,73].