Decision support tool

Background information

Toxic cyanobacteria

Cyanobacteria [1] are a natural part of the biological communities in water bodies. Some cyanobacteria float freely in the water column (planktic), while others colonize surfaces like stones or the bottom of the water body (benthic). At high nutrient concentrations (esp. phosphorus), populations of cyanobacteria may develop massively.

Beside chlorophyll, cyanobacteria contain accessory photopigments, principally the blue phycocyanin and the red phycoerythrin. These pigment combinations cause the different colourings of cyanobacteria which lead to discoloration of the water when cyanobacteria proliferate massively. However, only decaying cyanobacterial blooms appear “truly blue” due to the release of the water-soluble blue phycocyanin from the damaged cells.

Surface scum of Microcystis aeruginosaSurface bloom of Planktothrix rubescens (© ARPA, Sicilia)Cyanobacteria appear truly bluish primarily when cells are lysing

Many species of cyanobacteria may produce toxins which can lead to intoxications in animals and humans. Especially abundant freshwater cyanobacteria like Microcystis, Planktothrix, Aphanizomenon and Anabaena may be toxic and, moreover, develop massively (“blooms”) under suitable conditions, eventually outcompeting all other phytoplankton. To assess whether cyanobacteria pose a threat to a drinking water supply or bathing water, it is crucial to identify potentially toxic cyanobacteria quickly and reliably.

A detailed identification of cyanobacteria at species level is not only difficult, but often not necessary; mostly, the identification to genus level is sufficient and can be performed by staff after some training.

In the following you will find an overview of the most frequently encountered cyanobacteria which can produce toxins. More information about cyanobacteria can be found in the literature, as well as photos or taxonomic information and references in the internet.

With these sources – and eventually also with additional limnological expertise – you will be able to identify the most important cyanobacteria in your water body and establish a list for their identification in the future.

Abundant toxic cyanobacteria

The following cyanobacteria often develop massively and may produce toxins.


Microcystis aeruginosaMicrocystis viridisMicrocystis ichthyoblabe


Planktothrix agardhii


Aphanizomenon flos-aquae (© A. Ballot) Aphanizomenon gracile (© A. Ballot) Cuspidothrix issatschenkoi (© A. Ballot), vormals Aphanizomenon issatschenkoi

Anabaena (syn. Dolichospermum)

Anabaena lemmermannii (© A. Ballot)Anabaena flos-aquae (© A. Ballot) Anabaena planctonica (© A. Ballot)


Nodularia spumigena - coiled filaments (© B. Schubert)


Cylindrospermopsis raciborskii (© T. Endrulat)

Other toxin-producing cyanobacteria

The following cyanobacteria may also produce toxins. However, the health hazards to drinking water production and bathing are globally regarded to be less relevant as mass developments of these genera appear to be less common. An exception is the genus Lyngbya occurring often massively in (sub)tropical environments where it can cause severe dermatitis (“seaweed dermatitis” or “swimmers’ itch”).










[1] Cyanobacteria

The ability of cyanobacteria to perform oxygenic photosynthesis has formerly lead to classify them as algae and colloquially they are still called ”blue-green algae”. However, cyanobacteria are Gram-negative bacteria.

[2] Filament

Chain-like series of cells

[3] Heterocysts

Specialized nitrogen-fixing cells formed by some cyanobacteria.

[4] Akinetes

Resting cells formed by some cyanobacteria

[5] Stratification - Epilimnion, metalimnion and hypolimnion

Thermal stratification of lakes is the change in temperature with depth caused by the change in water’s density with temperature. One differs between the epilimnion (the warmer top layer), metalimnion (or thermocline, which is the small layer with a strong decrease in temperature ) and hypolimnion (the colder bottom layer).