Cyanobacteria – common microorganisms that can be dangerous

Cyanobacteria (cyanobacteria) are ancient prokaryotic organisms (having no cell nucleus). Traces of their presence have even been discovered in Precambrian sediments dating back 3.5 billion years. Cyanobacteria cells are photoautotrophic, meaning that they produce organic compounds through photosynthesis. Photosynthesis, of which oxygen is a byproduct – so-called oxygenic photosynthesis – is believed to have first appeared precisely in cyanobacteria, which is why these organisms are classified as pioneers supplying the Earth’s atmosphere with this gas. To date, about 2,000 species of cyanobacteria have been described, however, given their cosmopolitan lifestyle, new ones are still being discovered.

Cyanobacteria – brief characteristics

Cyanobacteria are organisms that were originally aquatic, but now occupy other ecological niches, but are still most commonly associated with water. Their activity is recorded in both fresh and salt water, solid and flowing. On land, they are found in soil environments, on tree bark and other plant tissues (such as leaves). There are species whose cells can float in the air (aerophytic), while others, such as the genus Nostoc, enter into symbiosis with vascular plants (e.g., ferns or sago) and provide their partner with nitrogenous compounds, which they convert to a plant-available form (ammonium ion) using molecular atmospheric nitrogen.

The ability to fix atmospheric nitrogen is a feature possessed not only by symbiotic cyanobacteria. Some free-living ones are also able to do this, using an enzymatic nitrogenase complex contained in special cells (heterocytes) where anaerobic conditions prevail. This is an extremely interesting metabolic arrangement – in part of the cyanobacterial cells, oxygen is released through photosynthesis, and next door, in heterocytes, nitrogenase works in the absence of oxygen. Cyanobacteria have a diverse structure – there are spherical forms, which are shaped like a sphere (e.g. Microcystis sp.) and filamentous forms (e.g. Anabena sp.). For the most part, they take on a characteristic blue-gray coloration, resulting from the presence of the pigment phycocyanin.

Positive and negative effects of cyanobacteria

In recent years, cyanobacteria have increasingly been the focus of research groups around the world. The reason is, among other things, their ability to produce compounds that can find applications in medicine, pharmacy or cosmetology. Unfortunately, their presence can also pose a serious problem in natural ecosystems, as their excessive growth – the so-called bloom – carries a number of negative consequences. Reports of cyanobacterial blooms in recent decades have been appearing regularly and with continuously increasing frequency. This is due to the continuous discharge of municipal, industrial, as well as agricultural pollutants into waters (e.g., surface runoff of fertilizers), the presence of which in reservoirs results in the occurrence of high concentrations of biogenic elements (mainly nitrogen, carbon and phosphorus) – the so-called eutrophication.

Water containing a large amount of biogenic elements, provides an excellent medium for the growth of cyanobacteria, which in extreme cases can dominate other microorganisms in the water body, covering it with a green sheepskin of their cells. This contributes to a greater turbidity of the water, which ultimately reduces the access to light necessary for aquatic plants and causes their death.

Although cyanobacteria themselves produce oxygen, their cells also consume it, and as a result of the putrefactive processes that develop during a bloom, so-called anoxia, a condition in which the oxygen content of a body of water is insufficient, quickly develops. A frequent consequence of this phenomenon is the death of animals such as crustaceans, mollusks and fish. Not insignificant in the appearance of cyanobacterial blooms are also climatic changes, which cause conditions conducive to the excessive growth of these microorganisms in places where previously blooms were rare.

Toxicity of cyanobacteria

Cyanobacteria blooms can disrupt ecological relationships in a water body by producing and releasing toxic secondary metabolites of cyanobacteria – so-called cyanotoxins (cyanobacterial toxins) – into the water. These compounds are one of the most dangerous groups of poisons synthesized in nature. The toxicity of some is comparable to the most dangerous snake venoms or botulinum toxin. It is customary to divide cyanobacterial toxins into four groups, based on the consequences of their effects in vertebrate organisms. The earliest known are those that cause liver cell dysfunction, or hepatotoxins (e.g., microcystin and nodularin).

Their action involves a number of processes, including inhibition of enzymes involved in phosphoric acid metabolism (phosphatases), disruption of lipid metabolism and the integrity of the cell cytoplasm. As a consequence, lipidosis, impaired functioning of bile ducts, bleeding occur in liver cells, which in extreme cases can lead to death. The second group of cyanotoxins are neurotoxins, which act in the central and peripheral nervous system (e.g. anatoxin-a, anatoxin-a(S), beta-N-methylamino-L-alanine). These compounds disrupt acetylcholine-dependent receptors (anaotoxin-a) or inhibit acetylcholinesterase . The result is a disruption of nerve impulse propagation, which is particularly dangerous in neuromuscular systems because it leads to permanent muscle contraction and so-called functional paralysis. If the receptors operating in the respiratory muscles are paralyzed, death by asphyxiation quickly follows.

The results of some studies suggest that neurodegenerative diseases such as ALS-PDC (amyotrophic lateral sclerosis-parkinsonism-dementia complex), which resemble Parkinson’s disease and amyotrophic lateral sclerosis in some symptoms, may develop due to the effects of certain cyanobacterial neurotoxins (beta-N-methylamino-L-alanine). The third group of cyanobacterial toxins are compounds that exhibit general cytotoxic properties and impair a number of processes in the cells of various organs. The best known cytotoxin is cylindrospermopsin (CYN). It is a very potent inhibitor of protein biosynthesis, which also aberrates antioxidant systems and consequently leads to oxidative stress. CYN has been shown to inhibit cell proliferation, often exhibiting genotoxicity. It also has carcinogenic properties and interferes with fertilization and maintenance of pregnancy. The last group of cyanotoxins consists of dermatotoxins (e.g., lipopolysaccharides, lyngbyatoxins), which cause irritation of the skin and mucous membranes.

Cyanotoxins lethal only sometimes

The danger of cyanobacterial toxins should not be marginalized, and when a bloom occurs, the water body in question should be immediately excluded from recreational or commercial use. To date, a number of cases of cyanotoxin poisoning have been described, some with very dramatic outcomes In 1978, 148 people, mostly children, were hospitalized in the Palm Island area (Queensland, Australia) after consuming water from a spring covered with a cyanobacterial bloom and contaminated with CYN. In 1996. At the Brazilian dialysis center in Caruaru, dozens of patients have died as a result of treatments carried out with microcystin-contaminated water. Most often, however, cyanotoxin poisoning does not have a radical course – its symptoms resemble the flu. For this reason, it is relatively difficult to link, for example, malaise, diarrhea or vomiting in people vacationing by the water with cyanobacterial activity.

An additional danger is generally the high chemical stability of cyanotoxins (e.g., CYN does not decompose at the boiling point of water), as well as the demonstrated toxicity of their decomposition products. The reasons for the synthesis of toxins by cyanobacteria have not been fully elucidated. The most commonly postulated hypothesis is that cyanotoxins are compounds, enabling them to interact with other microorganisms and gain an advantage in the ecosystem, such as through allelopathy.

Michal Adamski – PhD in biological sciences, assistant professor at the W. Szafer Institute of Botany of the Polish Academy of Sciences. Leader of a team working on compounds synthesized by cyanobacteria and algae. Co-author of dozens of scientific publications. He is currently pursuing a research grant under the Science for Society II program entitled: Can algae be a valuable component of the human diet? (NdS-II/SP/0234/2024/01).

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[1] Adamski, M., Wolowski, K., Kaminski, A., & Hindáková, A. (2020a). Cyanotoxin cylindrospermopsin producers and the catalytic decomposition process: A review. Harmful Algae, 98, Article 101894(https://doi.org/10.1016/j.hal.2020.101894)

[2] Adamski, M., Zimolag, E., Kaminski, A., Drukała, J., & Bialczyk, J. (2020b). Effects of cylindrospermopsin, its decomposition products, and anatoxin-a on human keratinocytes. Science of the Total Environment, 765, Article 142670(https://doi.org/10.1016/j.scitotenv.2020.142670)

[3] De La Cruz, A. A., Hiskia, A., Kaloudis, T., Chernoff, N., Hill, D., Antoniou, M. G., He, X., Loin, K., O’Shea, K., Zhao, C., Pelaez, M., Han, C., Lynch, T. J., & Dionysiou, D. D. (2013). A review on cylindrospermopsin: The global occurrence, detection, toxicity and degradation of a potent cyanotoxin. Environmental Sciences: Processes and Impacts, 15(11), 1979-2003(https://doi.org/10.1039/c3em00353a)

[4] Huisman, J., Codd, G. A., Paerl, H. W., Ibelings, B. W., Verspagen, J. M. H., & Visser, P. M. (2018). Cyanobacterial blooms. Nature Reviews Microbiology,16(8), 471-483(https://doi.org/10.1038/s41579-018-0040-1)