Cyanobacteria grow mainly in summer in waters such as lakes, ponds and some streams and rivers, and lead to a change in the colour of the water. Although they occur naturally in aquatic ecosystems, their proliferation, which results from excessive nutrient inputs to lakes and rivers, is becoming a growing international concern because of the ecological, health and economic consequences. This article provides some insights on this family of bacteria, which is gaining increasing attention.

1. What are cyanobacteria ?

Cyanobacteria are micro-organisms that thrive in terrestrial and aquatic environments, in both fresh and salt water. In favourable environmental conditions (i.e. with regard to temperature and nutrients), they can proliferate rapidly on a massive scale, sometimes in just a few days. This is known as an algal bloom. In some cases, these blooms lead to a change in the colour of the water (red, green, etc.), a foul odour and/or the accumulation of cyanobacteria on the water surface.

Some species of cyanobacteria produce toxins called cyanotoxins. The best known are microcystins, nodularins, cylindrospermopsins, anatoxins, saxitoxins and their derivatives.

In the aquatic environment, cyanobacteria can be divided into two groups according to how they live:

  • planktonic cyanobacteria remain suspended in the water column due to intracellular gas vesicles that give them buoyancy;

  • benthic cyanobacteria grow on the bottom of water courses, on mineral substrates (boulders, pebbles, sand, sediment, etc.) and even on the surface of aquatic plants.

Did you know ?

Although commonly called "blue-green algae", cyanobacteria are not algae but bacteria. To grow, they need access to the same nutrients as plants: phosphorus and nitrogen.

2. Where do cyanobacteria come from ?

Cyanobacteria are micro-organisms that have been present on Earth for two to three billion years. Found throughout the world, in plants, water and also in sand, they help to shape our planet.

Did you know ?

Cyanobacteria were the first organisms to make oxygen! They have enabled the existence of several life forms, including humans.

3. In which places and regions do they grow ?

Cyanobacteria need light, heat and nutrients to thrive.  In temperate zones, cyanobacterial blooms occur more often in the summer and early autumn, but can sometimes be observed as early as the spring. In France, cyanobacteria proliferate between May and October in calm, nutrient-rich waters such as lakes, ponds and some streams and rivers. In some rare cases, longer lasting blooms are seen throughout the year, and even specifically in winter.

In tropical and subtropical climates, such as in the French overseas départements, blooms can be observed throughout the year given the right conditions for their growth.

In general, episodes of cyanobacterial blooms are being seen more and more frequently on all continents.

4. What are the consequences of their proliferation ?

Cyanobacterial blooms are becoming a growing international concern because of the associated ecological, health and economic consequences. Indeed, proliferation of cyanobacteria on a massive scale can:

  • affect the health of ecosystems. High densities of cyanobacteria can alter the functioning of ecosystems by removing oxygen from the water, resulting in mass mortality of fish and invertebrates;

  • through the production of cyanotoxins, they can pose a risk to the health of humans and animals that consume contaminated water, or are in direct contact (through swimming or water sports, for example) or indirect contact (through the consumption of foodstuffs of animal or plant origin that are themselves contaminated) with contaminated water. Cases of animal mortality, mainly concerning dogs but sometimes also livestock or wildlife, have been recorded in recent years following exposure to blooms of toxin-producing cyanobacteria;

  • lead to limitations on aquatic uses such as bathing, boating or fishing due to the repulsive appearance of the water bodies (change in the colour of the water, unpleasant smells, etc.). Cyanobacterial blooms can therefore have direct negative effects on tourism along the shores of water bodies, and these may then be exacerbated by health restrictions on recreational uses.

5. Can accidental inhalation or ingestion of cyanobacteria be fatal to humans ?

In some cases, extremely rare so far, accidental inhalation or ingestion of cyanobacteria can be fatal. In Brazil, in 1996, 60 patients with kidney failure died following haemodialysis because the water used for this treatment was contaminated with microcystins. These are the most severe reported human health effects caused by the toxicity of certain cyanobacteria.

In France, to date, no lethal human poisoning associated with cyanotoxins has been recorded, mainly because of health checks carried out in drinking water and at bathing sites. On the other hand, episodes of dog deaths have been regularly attributed to cyanotoxins since 2005.

Did you know ?

The oldest documented case of human poisoning mentions the death of Chinese soldiers who drank green water from a river about 1000 years ago !

6. What are the routes of exposure to cyanobacteria and the associated risks ?

Cases of human poisoning are rarer than animal poisonings, mainly because the risk of exposure is managed (through quality monitoring of drinking water and of bathing sites). However, human exposure to cyanotoxins is possible through a variety of routes, the main ones being:

  • ingestion of poorly treated drinking water or untreated water (especially in developing countries);

  • inhalation, accidental ingestion of water, or skin contact with cyanobacteria and cyanotoxins during recreational activities (rowing, water-skiing, canoeing, etc.);

  • consumption of foodstuffs of plant origin contaminated by irrigation water;

  • consumption of foodstuffs of animal origin (fish) from contaminated waters;

  • oral intake of contaminated food supplements;

  • the intravenous route (haemodialysis).

Regardless of the routes of exposure considered, the most commonly reported symptoms are gastrointestinal signs, fever and skin irritations. Ingestion or inhalation of cyanobacteria can also cause hepatic toxicity (liver problems) and neurotoxicity (tremor, tingling, paralysis, etc.).

The time to onset of symptoms ranges from minutes or hours for skin symptoms and neurological disorders to several hours for liver toxicity.

In France, 95 cases of human poisoning by cyanobacteria were recorded by poison control centres between 1 January 2006 and 31 December 2018This number is probably vastly underestimated due to a lack of awareness of this phenomenon by the general public and to non-specific symptoms, which in addition can disappear quickly and are not necessarily reported by people to doctors and health authorities.

7. What is the best way to prevent cyanobacterial blooms ?

Planktonic cyanobacterial blooms occur mainly in stagnant waters (very slow-moving water bodies and rivers) where there is an excessive supply of nutrients, leading to plant growth, oxygen depletion and ecosystem imbalance. Cyanobacteria require high concentrations of phosphorus and nitrogen to grow, which can be supplied from many sources: livestock manure, compost, sewage sludge, fertilisers applied to agricultural soil, insufficiently treated wastewater discharges, and leaching from soil during heavy rainfall. Reducing phosphorus and nitrogen inputs to surface waters is currently still the only sustainable way to restore and/or protect these ecosystems from planktonic cyanobacterial blooms.  

Benthic cyanobacterial blooms are most often found in shallow flowing waters (small rivers and some large rivers). Current knowledge of these blooms is far more limited than for planktonic cyanobacteria. However, it seems that the development of cyanobacterial plaques (or biofilms) is most likely when the water level is at its lowest, in areas less than one metre deep and with a slow current. The detachment of these biofilms, their transport and then their accumulation on the banks occur as a result of various processes that are still poorly understood.

Because the factors and processes regulating cyanobacterial blooms are particularly complex, these phenomena are often difficult to predict.

8. Does climate change have an impact on the proliferation of cyanobacteria ?

The impact of climate change on cyanobacterial blooms is currently being debated by the scientific community. The global increase in temperatures, along with changes in rainfall patterns (multiplication of periods of major drought, storms and heavy rainfall, etc.), cause changes in the functioning of water bodies. These changes appear to promote cyanobacterial blooms. However, the multiple interactions between all these factors and processes are still largely unknown. It is therefore very difficult to predict what their impact on cyanobacterial blooms will actually be.

9. What is ANSES doing about it ?

To limit user exposure to cyanobacteria, ANSES:

  • updates the list of toxin-producing cyanobacteria in fresh water that pose a threat to humans;

  • establishes toxicity reference values for several cyanotoxins in order to characterise the health risk to users and determine management thresholds;

  • offers guidance to improve how the risk of cyanotoxins is taken into account and managed for different water uses (drinking water, recreational water, water intended for fishing, etc.);

  • conducts various research:

    • in its Laboratory for Food Safety: investigations on food poisoning cases;

    • in the Toxicology of Contaminants Unit of the Fougères Laboratory: studies to assess the fate of toxins and their toxicity after ingestion;

    • in its Laboratory for Hydrology: work to standardise methods for sampling, detecting and quantifying cyanobacteria. The aim is to provide laboratories accredited by the Ministry of Health with universally-shared methods in order to ensure uniform health surveillance throughout France;

    • in general, studies to increase the level of knowledge of microcystins and enable a better assessment of the risks associated with these toxins.

10. What can you do to avoid cyanobacterial poisoning ?

In areas of cyanobacterial growth and accumulation, you are recommended to:

  • avoid water sports (swimming, water-skiing, rowing, canoeing, paddleboard, etc.);

  • supervise young children to prevent them from playing with clumps of cyanobacteria that have accumulated on the surface, banks, rocks or pebbles along bodies of water;

  • keep dogs on a lead to prevent them from gaining access to bodies of water;

  • if you develop suspicious clinical signs (such as gastroenteritis, itching, redness, conjunctivitis, dizziness, impaired senses) as a result of exposure to contaminated water while bathing or engaging in water sports, take a shower and consult your doctor.

In general, regarding the consumption of freshwater fish:

  • remove the head and guts of fish before consumption (or before freezing);

  • avoid consuming small freshwater fish whole (fried);

  • limit as far as possible the consumption of fish from environments regularly affected by cyanobacterial blooms.