Fish deaths in the Oder: algae toxin detected in Oder water

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Scientists of the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) and the University of Vienna believe that natural causes for the mass development of algae are unlikely.

Prymnesium parvum from the river Oder near Hohenwutzen, 15.08.2022. C: Katrin Pr
Prymnesium parvum from the river Oder near Hohenwutzen, 15.08.2022. C: Katrin Preuß, IGB

Recent investigations substantiate the suspicion that mass development of a toxic brackish water alga has taken place in the Oder River. This could have played a role in the massive death of fish, mussels, snails and possibly other animal species. The researchers still do not assume that this is a purely natural phenomenon, because the algae species Prymnesium parvum does not occur en masse in the affected sections of the Oder under natural conditions. For mass occurrence, it depends on salinity levels in this area, which can only be caused by industrial discharges. The Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) had detected and microscopically identified the toxic brackish water alga in all samples taken from the middle Odra river in the last days, but the detection of the associated algal toxin was still pending. Expert Elisabeth Varga from the Institute of Food Chemistry and Toxicology at the University of Vienna now analyzed the IGB samples and was able to confirm the assumptions about the algal toxin.

"We were able to detect a subspecies of the algal toxin, called ’prymnesins,’ beyond doubt and actually in significant amounts in Odra samples from different sites," explains scientist Elisabeth Varga of the University of Vienna, who conducted the analyses there at the Mass Spectrometry Center. "From previous research on this algal toxin, we know that the toxin is strongly bound to the alga itself. When this specific type of algae is present in very large quantities, as is the case in the Odra samples, very high toxin concentrations must also be assumed. Since all the samples were taken at an advanced stage of the algal bloom, a direct link to the death of fish and molluscs can be assumed," explains Elisabeth Varga. However, research is still needed on the toxicity of prymnesins, and it has not yet been conclusively clarified which other groups of organisms beyond fish and mollusks could be affected. This also applies to potential effects on the human organism.

Accurate quantitation from biomass samples of the alga itself and water samples drawn is currently underway, but it is more technically and time consuming. "Prymnesins are very specific organic compounds, as far as known with up to 107 carbon atoms. In principle, they can be measured with modern mass spectrometers, but in practice they are only analyzed in a few laboratories in Europe, partly because they are not being looked for," explains Stephanie Spahr, head of the IGB Organic Pollutants Working Group. Since no analytical standards for these toxins are yet available worldwide, the unambiguous assignment was made by means of previously characterized strains cultivated at the University of Copenhagen (Per J. Hansen working group) and characterized at the Danish Technical University (Thomas O. Larsen working group) as part of international projects.

"The Odra River is currently experiencing extreme mass development of planktonic algae. Prymnesium parvum is very dominant in all samples, in the Odra river the alga represents at least half of the total algal biomass, even after dilution by the inflow of the Warta river it is currently still 36 percent. To my knowledge, such a mass development has never been observed in our waters. It was probably made possible by salt discharges, abundant nutrients, high water temperatures and long residence times in barrages and in the developed river," emphasizes IGB scientist Jan Köhler, head of the Photosynthesis and Growth of Algae and Macrophytes research group.

"Now the genetic characterization of the algal strain is still missing. More than 50 strains of Prymnesium parvum have been described, which differ greatly in their environmental requirements and toxin production. The toxins are currently grouped into three groups, with each group consisting of more than 10 different toxins, which poses a significant challenge for identification," explains Jan Köhler.

"We are now also immediately contacting the relevant authorities so that the research results can be integrated into new measurement campaigns and programs," explains IGB scientist Tobias Goldhammer, head of the Nutrient Cycles and Chemical Analysis working group. In this way, he says, further developments on the Oder River can be better monitored. "The elevated salinity levels we measured in the samples occur more often in the Odra river, they are promoted by industrial loads in the upper reaches. In this respect, if the salt concentrations do not decrease and we continue to experience too hot and dry summers, such toxic mass developments could also occur again in the future," Tobias Goldhammer emphasizes.