what are cyanobacteria? Are they good or bad?

The summer of 2022 was the hottest recorded in France since 1900. And if you have thought of cooling off by going for a swim in a lake, a river or a leisure centre, you may have one was prevented due to a closure due to the proliferation of cyanobacteria, which can be toxic to our health or that of our animals.

But who are these cyanobacteria? What problems do they pose? Are they our friends or our enemies?

Cyanobacteria, engineers of the biosphere

Maybe we should start with introductions. Cyanobacteria are, as their name suggests, microscopic bacteria, blue-green in color.

But they are not only micro-organisms that spoil our swimming! Far from being simple “pests”, they are above all the inventors of oxygenic photosynthesis (which uses CO and produces O, like that used by trees). They are therefore at the origin of all the photosynthesis and oxygenation of our planet… without which we would not exist. Just that !

Little throwback. The origin of cyanobacteria dates back to very ancient Archean times, between 2.7 and 3.5 billion years ago. At that time, other bacteria were already using molecular machinery called photosystems, capable of converting light energy into chemical energy, allowing them to grow and multiply. Thanks to these photosystems, these bacteria can already convert inorganic carbon (such as atmospheric carbon dioxide) into complex molecules necessary for life (such as sugars, lipids or nucleic acids).

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But for the first time, cyanobacteria will associate two distinct and complementary photosystems. This will allow them to carry out a then unprecedented form of photosynthesis, particularly productive, which produces dioxygen as “waste”.

Thanks to this productivity, the ecological success of cyanobacteria is very rapid, and they develop very widely… transforming in the process all the chemistry of the biosphere.

Until then, in fact, the oceans and atmosphere contained very little oxygen, and therefore harbored anaerobic micro-organisms. However, the development of these cyanobacteria practicing oxygenic photosynthesis will produce a lot, a lot of oxygen! This oxygen accumulates first in the oceans and then, over the past billion years, in the atmosphere.

But that’s not all. About a billion years ago, cyanobacteria entered into symbiosis with a line of single-celled organisms with a nucleus. They thus gave rise to chloroplasts, small green compartments responsible for photosynthesis present in the cells of micro- and macro-algae, and terrestrial plants.

Cyanobacteria are therefore at the origin of all the photosynthesis and oxygenation of our planet! Our very existence is a consequence of this, on the one hand since oxygen is essential to all animal life, and on the other hand since we largely depend on plants for our food.

The harmful effects of cyanobacteria

But back to the present, and our rivers or our lakes. When the temperature rises, photosynthesis accelerates. If we add to this eutrophication, that is to say the enrichment of water by nutrients such as phosphorus or nitrogen from our fertilizers, the result is not long in coming: cyanobacteria proliferate. The clear water of the lake becomes a green or red soup, depending on the species… A simple consequence of their formidable efficiency!

These phenomena have been known since Antiquity, or among the Aztecs and the Mayas, in certain bodies of fresh water as well as in the oceans. However, global warming and the increase in human activities (agriculture, discharge of insufficiently treated wastewater, etc.) over the past few decades have increased the frequency and intensity of these episodes.

However, these proliferations of cyanobacteria are harmful to fauna, flora and human health.

Thus, even if photosynthesis produces oxygen, the biomass of cyanobacteria produced during the proliferations is rapidly degraded by bacteria which will consume this oxygen. It leads to ultimately to water anoxia (ie a lack of oxygen) and to the asphyxiation of animals, in particular fish which can thus die suddenly.

On the other hand, certain species of cyanobacteria (known as toxigenic cyanobacteria) synthesize powerful toxins, called cyanotoxins. Of greatest concern to human health are microcystins, cylindrospermopsins, toxoids, saxitoxins and nodularins.

After ingestion, contact or inhalation, they act on different organs such as the liver (hepatotoxic effects), the nervous system (neurotoxic effects), the reproductive systems (reprotoxic effects) or the mucous membranes (dermatotoxic effects), with consequences that can go as far as to death. Cases of poisoning, including fatal cases of dogs, are thus reported each year during the summer period.

In France, the most common cyanotoxins are regulated and regularly measured in drinking and recreational waters. When the threshold values ​​for toxigenic cyanobacteria or cyanotoxins are exceeded, the authorities may have to limit activities or even close access to water bodies, or limit the use or consumption of water.

Cyanobacteria are also allies for health

Fortunately, not all cyanobacteria produce toxins, and they do not produce that toxins.

They are indeed considered to be outstanding chemists, producing a wide variety of bioactive molecules, some of which find applications in the field of health. In a recent synthesis, our team identified the production by cyanobacteria of 10 classes of chemical compounds, presenting at least 14 types of potentially beneficial activities, most of which remain to be explored.



Charlotte Duval/MNHN

Among the most emblematic examples, dolastatin 10 is at the origin of the marketing of an anticancer drug (brentuximab vedotin) used in the treatment of lymphoma in Hodgkin’s disease.

Another example is that of the cyanobacterium Limnospira (formerly named Arthrospira) used as a dietary supplement for centuries and marketed under the generic name “Spirulina”. This cyanobacterium is rich in proteins, minerals, vitamins and unsaturated fatty acids.

With over 1,700 species known and many more yet to be described, cyanobacteria therefore represent an important resource for bio-inspired innovation.

Cyanobacteria, sentinels above all

It is likely that the proliferations of cyanobacteria will still be frequent next summer, and that they will sometimes deprive us of nautical activities, fishing, or swimming. More seriously, in many regions of the world, they directly affect the drinking water resources on which populations depend.

But beyond their impact on our activities, these episodes above all reveal the imbalances and poor health of aquatic ecosystems.

Strongly linked to human activities that contribute to the eutrophication of waters, the increase in cyanobacteria proliferations all over the world should challenge us to the threats that weigh on water quality and biodiversity, and underline the extent to which the human well-being is closely linked to that of aquatic ecosystems.

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