Prof. Hans-Peter Grossart with a phytoplankton culture. | Photo: David Ausserhofer
They determine whether water remains clear and drinkable or becomes cloudy and toxic, and whether a body of water binds carbon or releases greenhouse gases: we are talking about microalgae, scientifically known as phytoplankton. Around 1,000 of them live in one millilitre of seawater. However, the number can easily vary by 1 to 2 orders of magnitude. Because they are so important, they also serve as a benchmark that politicians and authorities use to assess water quality – as is the case in the Water Framework Directive.
“But this key parameter that the authorities rely on – the diversity of phytoplankton, the indicator that secures drinking water for more than 180 million Europeans – still depends on inverse microscope technology developed in 1958, the year the first integrated circuit was soldered“, said IGB researcher Prof. Hans-Peter Grossart, co-author of the article. The so-called Utermöhl colonisation protocol, which is enshrined in Annex V of the WFD, continues to form the backbone of the official assessment of water status.
Example: Golden algae in the Oder disaster of 2022: Appearance says nothing about toxicity
One example of the limitations of this old method is the so-called golden algae, which played a devastating role in the Oder disaster of 2022. "You cannot tell whether phytoplankton is toxic or not by looking at it under a microscope. The toxin-producing species Prymnesium parvum, which killed an estimated 1,000 tonnes of fish and many other organisms such as mussels and snails in the Oder, looks exactly the same as its harmless relatives," explained Hans-Peter Grossart.
According to the authors, the solution is obvious: genetic analysis can be used to analyse the microbiome from water samples. The method is called rRNA amplicon profiling.
Faster and cheaper method with a better early warning system
"The process is faster and more cost-effective than the Utermöhl method, as has already been demonstrated in pilot projects for monitoring lakes, e.g. in lakes in the Alpine foothills. In addition, the genetic information can be stored digitally. This makes the data set future-proof, as it can be queried at any time for functional markers of emerging stresses," said Hans-Peter Grossart.
These new approaches could provide a better early warning system because they reveal subtle shifts in microdiversity and the mechanistic link between physical-chemical disturbance and the community's response – long before a change in the composition of phytoplankton would be visible under the microscope.
Proposals for adapting the measurement method in the WFD
The authors therefore propose including a ‘Genomic Plankton Index’ in Annex V of the WFD. This would improve water quality monitoring in Europe and increase Member States' responsiveness to environmental changes. A key proposal is to promote a so-called ring test through Horizon Europe, in which identical samples are sent to several national laboratories. The DNA results of these tests would be compared with the previous Utermöhl counts to ensure that the new metric is both interoperable and can be anchored in existing data.
In addition, the authors recommend establishing a ‘Genomics Observatory’ for the WFD. This observatory could link Europe's plankton samples to a cloud platform that integrates high-throughput sequencing data, AI-assisted online optical cell counts and satellite-based algal bloom alerts. By streaming these complementary data layers directly into EU monitoring dashboards and the EU's ‘Digital Twin Earth’ infrastructure, stakeholders would obtain cross-validated indicators of algal toxicity, invasive organism encroachment and biodiversity change in near real time.
