by Olivia Foster Rhoades

Green globs coat the shore and placards caution visitors from touching the water. First it was New Jersey’s Lake Hopatcong, then the Pacific Northwest, and most recently in Central Park. The culprit? An ancient and prolific family of microbes that have been shaping our world for millennia—cyanobacteria.” Cyanobacteria are bacteria that thrive in the same conditions that make algae flourish and are actually the source of toxicity in an otherwise innocuous algae bloom.

Sometimes referred to by the misnomer ‘blue-green algae’, cyanobacteria are bacteria that thrive in the same conditions that make algae flourish and  are actually the source of toxicity in an otherwise innocuous algae bloom. With warmer weather and rushes of nutrients from run-off, cyanobacteria grow exponentially with some very deleterious effects. Cyanobacteria blooms have impacted economies by hurting summer tourism, causing the U.S. fishing industry to lose $34 million a year, and contaminating drinking water,. With rising temperatures due to future climate change, we are expected to continue to see an increase in toxic blooms over the next decade. Understanding the science behind such events will be paramount in informing the best policies to protect both the environment and public health.

Cyanobacteria: the planet sculptors

Cyanobacteria are a family of single-celled microbes that derive their energy from sunlight and can be found in almost every environment on the planet. There are over 6,000 known species of cyanobacteria, such as spirulina (yes, the kind you eat). They can either exist individually and free-floating or form complex structures (Figure 1). Evolving over 2.7 billions years ago, cyanobacteria were the first lifeforms to absorb sunlight and water to create energy and oxygen gas through the process of photosynthesis. 

Around 3 billion years ago the Earth’s atmosphere had very little oxygen and was primarily composed of nitrogen and carbon dioxide. The emergence and success of early cyanobacteria corresponds roughly with the ‘Great Oxidation Event’, in which our planet’s atmosphere changed to be about 20% oxygen gas. The correlation of oxygenation and the rapid proliferation of cyanobacteria seen in fossilized stromatolites (Figure 1) suggests that these little green microbes helped shape the atmosphere and world as we know it.

Figure 1: Cyanobacteria. Cyanobacteria species exist in many different forms. Free-floating species like Microcystis have small pockets of air that, when inflated and deflated, cause them to move up and down in the water. Other species like Oscillatoria and Anabaena can have more complex morphologies, forming ribbon-like structures. Ancient cyanobacteria formed mineralized structures known as Stromatolites through building up layers over eons.

Cyanotoxins and the circle of life

Besides producing oxygen, many cyanobacteria are also adept chemists. Cyanobacteria produce a myriad of compounds as  byproducts of their metabolism. Oscillatoria boryana cyanobacteria, for example, produce a small molecule with therapeutic potential for cancer (Figure 1).  More frequently, however, cyanobacteria-derived compounds are toxic and consequently called cyanotoxins. Cyanotoxins can be produced under normal conditions, and it is not well understood why they are made. Studies suggest that some cyanotoxins may provide a competitive advantage to discourage grazing by predators, while other cyanotoxins may have a primary function unrelated to their toxicity.

Cyanotoxins are released into the environment by active secretion and through passive leakage as cyanobacteria die, disintegrate, and spill their internal contents. When cyanobacteria rapidly reproduce under favorable conditions, they inevitably exhaust nutrient sources and experience a steep die-off. These die-offs are behind the sudden release of cyanotoxins that poison the water. 

 In a toxic bloom, there can be multiple species of cyanobacteria, each producing different variants of cyanotoxins. These toxins are harmful to humans and pets when contaminated water is swallowed (either intentionally or during recreation). The three major classes of cyanotoxins are microcystin, cylindrospermopsin, and anatoxin (Figure 2). Microcystin is the most common cyanotoxin, with between 25–1000 variants. Upon ingestion of toxic water, microcystin primarily damages the liver but can also harm kidneys and reproductive systems. Cylindrospermopsin similarly affects the liver and the kidneys but does so by inhibiting protein synthesis. Anatoxin, on the other hand, is a neurotoxin which leads to paralysis and rapid death—one variant of anatoxin even is colloquially known as the ‘Very Fast Death Factor.’  Whereas microcystin and cylindrospermopsin are very stable in aquatic environments, anatoxin is fortunately very volatile, degrading rapidly with light exposure and as a result is uncommon in toxic blooms. 

Figure 2: Cyanotoxin anatomical targets. Whereas microcystin and cylindrospermopsin accumulate in the liver, anatoxin acts quickly in the central nervous system.

Just because cyanobacteria are capable of producing toxins, it doesn’t always mean they do. As a result, the presence of cyanobacteria in a lake or river does not necessarily indicate cyanotoxins, making it hard to predict if a lake or river is at risk of a toxic bloom. Since cyanotoxins have no apparent odor or color, it can be difficult to determine when water becomes contaminated.

Causes and cures of toxic blooms

Many toxic blooms are ignited by nutrient run-off into lakes and reservoirs. In particular, phosphorus, a nutrient that stimulates growth among photosynthetic life forms, is often used in fertilizers and ends up in these bodies of water. After a rainstorm, phosphorous from industrial fertilizers can be swept into lakes, leading to massive blooms (Figure 3). Careful management practices, such as diversion of runoff away from water sources and decreased use of industrial fertilizer, can reduce the occurrence of toxic blooms. 

Once a lake is ‘blooming’ there are few remedies besides waiting for the bloom to run its course and minimizing public exposure by shutting down access to the water. When possible, flushing the lake with more freshwater can help dilute toxic  cyanobacteria and send them down river at lower concentrations such that the cyanotoxin concentration will be less harmful. However, cyanotoxins can persist long after the bloom by soaking into the clay at the bottom of a lake bed, so careful steps must be taken before the water is deemed safe again. 

Figure 3: Frequent causes of toxic blooms in rivers and lakes. A) Cyanobacteria exist in all bodies of water, but only with increased nutrients and warmer weather will they quickly multiply and result in a toxic bloom. B) One cause of cyanobacterial blooms in rivers is the detachment of mats from the river floor, which float to the shallows, reproduce and release cyanotoxins into the water and onto the shoreline.

Cyanobacteria in river systems offer a different challenge, as different species are adapted to deal with flowing water. To escape the current in rivers, river-based cyanobacteria form mats with algae at the bottom of river beds. When the riverbed is disrupted by an increase in flow,  the cyanobacterial-algae mats detach and wash to the shoreline. Once in the warm, light-filled shallows of the river shoreline, the cyanobacteria can grow quickly in relatively stagnant water (Figure 3). Cyanotoxins are released into the water near the shore by both actively toxin-secretion and dead or ruptured cyanobacteria. 

When cyanotoxins are found in rivers, lake-based methods of bloom management can exacerbate the duration of toxic blooms. The sudden increase in water current from flushing a river can detach other river-bed cyanobacteria-algae mats, leading to a repeat of the cycle (Figure 3). Instead, the best method to both prevent and remedy river-based toxic blooms is to increase base flow of river water through the warmer months when cyanobacteria growth is favorable to prevent the formation of mats.

However, managing a steady river flow is difficult in some western states, which are increasingly prone to drought with the onset of global climate change. In drought conditions, reservoirs are dammed in the summer months leading to downstream rivers experiencing decreased amounts of water. However, to ensure rivers never dry out completely during the summer months, reservoir water is released, rapidly increasing the flow of water in downstream rivers, and thereby raising the risk of dislodging cyanobacterial mats. Analysis of cyanobacteria in rivers is still new, and more work is needed to find the best management practices to balance drought mitigation and the potential for toxic blooms.

Learning to work with cyanobacteria

After surviving all five of the last mass extinction events, it is safe to say that cyanobacteria are here to stay. And we have a lot to thank these microbes for—without them and their game-changing photosynthesis, we wouldn’t be here. However, with our freshwater sources in peril from climate change, we must find strategies to decrease risk of water contamination. The number of global toxic blooms have been rising over the last decade and the number is predicted to only increase in frequency with climate change, as warmer waters foster cyanobacteria growth. Awareness and understanding of cyanobacteria biology is an important step in learning to predict and mitigate toxic blooms, keeping our waterways safe and fun-filled. And who knows, maybe with our new microbial friends we can live on Mars.


Olivia Foster Rhoades is a PhD Candidate in the Biological and Biomedical Sciences Program at Harvard University.

Special thanks to Keenan Foster, Principal Environmental Specialist of the Sonoma County Water Agency for his time and expertise in informing this article.

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This article is part of our special edition on water. To read more, check out our special edition homepage!

One thought on “Blue-Green Planet: It’s a cyanobacterial world, and we just live on it.

  1. We’ve been receiving concerns regarding the impact of these cyanobacteria in our customer’s drinking water for about 7 years now. This was a new phenomenon for us to understand, but we learned that there are many people who’s drinking water gets directly and indirectly impacted. We appreciate you covering this topic Olivia.

    https://www.bigberkeywaterfilters.com/blog/microcystin/drinking-water-increasingly-threatened-by-toxic-blue-green-algae/

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