Background information

Microcystins (MC) can be detected in water bodies worldwide. MC-producing cyanobacteria including Microcystis, Planktothrix and Anabaena are apparently cosmopolitan and the eutrophication of many water bodies enables their (massive) proliferation. Studies from different countries have found MCs in 80-100% of samples containing Microcystis, Planktohrix and/or Anabena. Thus, the presence of microcystins should be assumed during occurrence of these cyanobacteria, unless proven otherwise.

With Microcystis the concentration of microcystins in the open water are mostly below 100 µg/l. However, Microcystis usually forms surface scums or accumulations at shorelines during calm weather conditions, which can lead to microcystin concentrations that are several orders of magnitude higher compared to those in the open water. Concentrations between 1000 – 40000 µg/l have been reported from blooms and shoreline accumulations with a high variation over time and space due to changing wind directions and speed and the morphology of the water body (Fig.1).

Planktothrix rubescens displays another pattern of concentration variation within a water body: during summer stratification of a water body this species grows in the metalimnion with microcystin concentrations usually between < 1-10 µg/l in this layer. During the autumn turnover the cells (and MC) are entrained into the whole water column and also surface scums with extremely high microcystin concentrations have been reported ( >> 10000 µg/l).

Also in dense populations of Planktohrix agardhii MC concentrations can amount 100 µg/l or even more, but as this species usually occurs in shallow and thus frequently mixed lakes surface scums or accumulations at shorelines do not occur.

MCs are largely cell-bound and enter the water phase only during dying-off of the cells. In healthy populations, the dissolved microcystins usually account for less than about 10% of the total microcystin concentration. Higher concentrations of dissolved microcystins are observed after the break-down of blooms, but are often (though not always) quickly diluted within the water column and degraded by bacteria within several days.

Relationship of microcystin to biovolume

A quick estimation of the risk due to microcystins when a cyanobacterial bloom occurs can be done roughly with toxin concentrations per biomass units derived from field samples.

For microcystins, a number of data sets are available which use either the biovolume (BV) or chlorophyll- a (Chl-a) concentration as biomass reference unit. Culture strains mostly produce 0.5-5 µg microcystin per mm³ BV. In the field, concentrations are often below 1 µg microcystin per mm³ BV or 0,3 per µg Chl-a (Fig.1) probably due to the mixture of microcystin-producing and non-producing genotypes. In some cases, however, higher (up to 6 –fold) concentrations per biomass unit are reported.

Uncertainty arises when using chlorophyll-a as a reference unit due to the variability of cellular Chl-a content due to environmental conditions as well as to the variable contribution of the cyanobacteria to the whole phytoplankton. Though biovolume is the more accurate reference unit, the determination of biovolume is less widely practised and requires more experience than chlorophyll-a analysis. For a first “worst case” estimation of the toxin concentration per Chl-a (0.3 µg MC/µg Chl.a) or biovolume (1 µg MC/mm³ BV) are therefore both acceptable.

Fig. 1: Concentrations of microcystins along the shorelines of the river Havel (Berlin, Germany) during a mass development of Microcystis sp. (MC: columns, MC/Chl-a relation: black dots).

Many populations of Nodularia spumigena in the temperate as well as subtropical climate zones have been found to contain nodularins. Reported concentrations of nodularins range from a few µg up to ~ 18000 µg/l in e.g. cell accumulations at the shorelines of the Baltic Sea.

The frequency of cylindrospermopsin detection differs between countries apparently reflecting the distribution of CYN-producing cyanobacteria. In Australia the frequent occurrence of CYN is largely attributed to C. raciborskii and Aph. ovalisporum, while in North-East Germany the abundance of Aphanizomenon and Anabaena species have led to a detection of CYN in 70% of investigated lakes and CYN occurrence was thus as frequent as that of microcystins. In other countries including the Czech Republic or different parts of the USA, CYN has been detected much less frequently.

The concentrations of CYN are often < 10 µg/l, but occasionally up to about 100 µg/l, and the highest concentration found to date is 1050 µg/l. Blooms with scums and accumulation of cells of the CYN-producers, and thus of CYN, have not yet been reported, but are likely regarding the ecological behaviour of some CYN-producers including Aphanizomenon and Anabaena spp..

For risk assessment it is important to note that CYN occurs often up to 90% outside of the cells (i.e. dissolved in the water phase), and that it has been found to be very persistent in some water bodies. Dissolved CYN can thus still be detected, even year-round (Fig. 1), when the CYN-producing cyanobacteria have already disappeared for some time.

In case of CYN in raw water, a drinking water treatment must also be able to remove dissolved toxins (a different scenario from that for the largely cell-bound microcystins), and the surveillance of the raw water cannot be based on cyanobacterial biomass alone.

Fig 1: Concentrations of dissolved (CYNdiss) and cell-bound (CYNpart) cylindrospermopsin in Stolpsee (Germany) 2008.

Worldwide ATX-a and PSP (saxitoxins) are detected at much lower frequency than microcystins and also the concentrations found are mostly in the lower µg/l range. This reflects the apparently lower abundance of potential neurotoxin-producers like Aphanizomenon and Anabaena. In blooms of neurotoxic cyanobacteria, however, also high concentrations of more than 1000 µg/l have been found.

Also ATX-a and PSP can sometimes occur in dissolved form in the water. Low concentrations are quickly diluted and/or degraded, however, PSPs have been found to be very persistent in an Australian water body, and some congeners have furthermore been transformed in more toxic forms.

Anatoxin-a (and homoanatoxin-a) from benthic cyanobacteria (Oscillatoria, Phormidium) have been reported as causative agents in animal poisonings. For homoanatoxin-a, as well as ATX-a(s), only very limited data, mostly qualitative, are available.