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In the summer of 2007, about 2 million residents in Wuxi, China were left when one of the largest freshwater lakes in the country was enveloped with a massive cyanobacterial bloom. In Aug. 2014, the city of Toledo, Ohio issued a for its residents when Lake Erie was covered with a thick blanket of cyanobacteria. On Aug. 6, 2021, Spokane, Wash. reported the barely 20 minutes after they went swimming in water contaminated with toxigenic cyanobacteria. Such cases of cyanobacterial blooms, many of which are harmful for people, animals and the environment, have increased in frequency in both space and time in recent years.

Cyanobacteria are photosynthetic bacteria found in water and moist soil. They use sunlight to convert carbon dioxide (CO₂) in the atmosphere into sugars and oxygen, which is released back into the atmosphere, in a process called CO₂ fixation. It is not wrong to say that humans owe their existence to cyanobacteria; about 3 billion years ago, the photosynthetic activity of cyanobacteria gave rise to an oxygenated atmosphere that still supports much of the life on Earth. Cyanobacteria are commonly known as blue-green algae, but are not true eukaryotic algae. Their distinct blue-green (cyan) hue comes from the accessory pigment, phycocyanin, although some species exhibit a vast array of colors including green, red, brown or yellow.  By themselves, cyanobacteria are not pathogenic, but they can become a reason for concern when they produce toxins. Moreover, overgrowth of cyanobacteria, or "cyanobacteria bloom," results in concentrations of toxins that are harmful for the environment and people.

What Causes Cyanobacterial Blooms?

Cyanobacteria grow rapidly in stagnant water and warm and nutrient-rich (high in nitrogen and phosphorous) environments, forming blooms across the water’s surface. Some of the common genera that form blooms include Microcystis, Nodularia, Dolichospermum and Trichodesmium. The blooms typically look like a green soup floating on the water’s surface. Sometimes, they can also appear as scums, mats or foams, although their colors and appearances vary depending on the species. Presently, there isn’t a consensus in the scientific community regarding the characterization of blooms. Different parameters are used to quantify them, for instance, amount of biomass, concentration of photosynthetic pigments or the measure of their harmful impacts. Although cyanobacterial blooms have been around for millions of years, they have increased substantially in the last century owing to many factors, including .

Eutrophication, or excessive abundance of nutrients in water bodies, from agricultural sources (e.g., nitrogen and phosphorus fertilizers), industrial and domestic waste is a major environmental contributor to cyanobacterial blooms.

Another factor responsible for bloom expansion is climate change, including global warming and rising CO₂ concentrations. The growth rate of eukaryotic algae decreases in warmer temperatures, favoring the growth of cyanobacteria. Warmer temperatures also lead to water stratification, the separation of water into layers with different densities. This enables many buoyant cyanobacteria with gas vesicles to float upwards and gain better access to sunlight for photosynthesis compared to non-buoyant algal counterparts. Surface blooms of cyanobacteria may further increase the temperature of water locally by absorbing light energy via their photosynthetic pigments, building a positive feedback loop to ensure their growth over eukaryotic algae.

Rising levels of CO₂ in the atmosphere are predicted to further escalate blooms. Many cyanobacteria have evolved sophisticated mechanisms to increase cellular CO₂ concentration in microcompartments called carboxysomes. Increased carbon dioxide concentration increases the efficacy of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO), an enzyme involved in  CO₂ fixation.

Why Are Some Cyanobacterial Blooms Harmful?

Many bloom-forming cyanobacteria produce cyanotoxins, secondary metabolites that are toxic to people, animals and/or the environment. Such blooms are called cyanobacteria harmful algal blooms and can be found in fresh water, estuaries or marine ecosystems. Cyanotoxins are among the most potent toxins known and come in a variety of chemical structures.

Microcystins are one of the most powerful and are produced by many freshwater cyanobacteria including Microcystis spp. and Planktothrix spp. The city of Toledo, Ohio succumbed to unsafe levels of microcystin in Lake Erie in Aug. 2014, which forced it to issue the "Do not drink" advisory to its residents. Nodularin is another cyanotoxin that is structurally similar to microcystin and is produced by the brackish (i.e., somewhat salty) water species Nodularia spumigena.  Both microcystins  and nodularins are cyclic peptides that inhibit eukaryotic protein phosphatases and primarily cause liver damage.

Species including Anabaena and Dochilospermum produce alkaloid neurotoxins, including anatoxins and saxitoxins. In fact, anatoxin-a is called the "," as mice injected intraperitoneally with anatoxin-a die within 2-5 minutes from muscle paralysis and resulting respiratory failure. Amino acid cyanotoxins include the neurotoxin β-N-methylamino-L-alanine (BMAA), which is thought to be widespread in many cyanobacteria. They are , including amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. In milder cases, direct skin contact with structural components of the cyanobacterial membrane (including lipopolysaccharides) may promote inflammation and cytokine production leading to irritation of the exposed body part.

Managing Cyanobacterial Blooms

Researchers have applied several biological, chemical and engineering-based approaches to either prevent or suppress cyanobacterial blooms, but one solution may not fit all. To control the nutrient run-off in water bodies, regulations exist on the use of nitrogen and phosphorus fertilizers, including banning the use of phosphorus fertilizers on urban lawns in 11 states in the United States. Such prevention strategies may take too long to show positive effects considering the drastic rate of other environmental determinants, like climate change. Other include hydrological interventions to reduce water stratification (e.g., artificial mixing via aeration/circulation), desiccation to remove blooms below the surface, algaecides (like copper sulfate and chlorine) and coagulation and/or flocculation to promote sedimentation of cyanobacteria to the bottom of the water column, which is deprived of oxygen. However, such strategies do not offer long-term solutions and are riddled with side effects. For instance, many chemical controls may lead to cell lysis that promotes the release of cyanotoxins, degrades water quality and may also impact other aquatic life.

Human actions are influencing the earth’s atmosphere and hydrosphere for the worse. Perhaps in a bid to survive, the innocuous cyanobacteria that once paved the way for humans to live by oxygenating the atmosphere are now aggregating into harmful blooms that threaten the planet’s delicate balance and existence. Hopefully, future research to understand the mechanism of cell division and bloom formation in cyanobacteria, along with actions and policies at the local, national and global scales, will help in the prevention and control of harmful cyanobacterial blooms.