Daphnia – one of the most important zooplankton species in aquatic ecosystems. | Photo: IGB/David Ausserhofer
Ms. Wolinska, Mr. Agha, your work involves exploring parasites, a very particular group of organisms. What do you find fascinating about them?
Ramsy Agha: We biologists usually neglect parasites because we consider them to be an exceptional case in nature. We now know, however, that they occur frequently and in large numbers. That’s why I consider it important to pay greater attention to this previously overlooked group of organisms: What role do they play in ecosystems? We want to bring parasites out of the shadows into the spotlight.
Justyna Wolinska: One reason parasites have been overlooked until now is that they are difficult to handle. Parasites are usually very small, and even when this is not the case, we cannot see them because they usually live within another organism. So you have to look inside to uncover the prevalence of parasites. A key reason why I became interested in parasites was the negative attitude we have towards them: since parasites cause disease, we see them as something bad.
But parasites are not always bad, right?
JW: Exactly! Our group discovered that, quite the contrary, parasites may well play a positive role in the ecosystem. That was a completely unexpected finding, it was really fascinating!
RA: It’s a kind of paradigm shift. Disease is something negative, but in an ecological context it can also be something positive.
What did you investigate?
RA: We looked at the relationship between parasites and their host, which, in our case, were fungal parasites and cyanobacteria. In the process, we discovered that this parasite also has an effect on a third organism, namely Daphnia – one of the most important zooplankton species in aquatic ecosystems. We observed that Daphnia benefit from infected cultures of the cyanobacterium. This is interesting because this type of prey is usually nutritionally inadequate for this organism. The zooplankton uses the parasite as prey, and, because it contains important fatty acids, Daphnia get a dietary upgrade. This can be seen from the fact that Daphnia populations grow strongly.
JW: The fungal infestation also enables Daphnia to consume cyanobacteria more efficiently. Cyanobacteria consist of long filaments, making them a less palatable food source: the filaments block the filtering apparatus of Daphnia, which they use to ingest food in the water. The parasitic infection causes the fragmentation of filaments into shorter lengths, as we observed, making them easier to ingest by Daphnia.
RA: Let me illustrate this with an example: In summer, Berlin’s lakes are often green, rather than blue, because masses of phytoplankton, often cyanobacteria, float on the surface. Why is this the case? One important reason for this is that cyanobacteria are not consumed efficiently, that is, they are not very palatable for their predators. Thanks to the parasite, this source of food improves in quality, keeping cyanobacteria better at bay.
Does this mean that our lakes would be even more contaminated in summer in the absence of these parasites?
JW: Yes, this can very much be the case. These parasites are a group of very primitive fungi that exist almost anywhere and can be very virulent, i.e. they would be able to eliminate cyanobacteria populations in no time. This gave us the idea that these parasites have a major impact on carbon transfer in their respective ecosystem. Since these parasites can infect any phytoplankton group, and since phytoplankton accounts for around half of the world’s carbon fixation, such infections are likely to have an impact on the global carbon cycle and climate regulation.
What finding surprised you the most?
RA: That parasites also act as prey! And that besides providing nutrition, the infection also makes it easier for another prey to be ingested. Parasites therefore have very complex effects: they act as an additional link in the food web, but also modulate existing ones.
JW: Until now, aquatic food webs were considered as a simple cascade: phytoplankton, zooplankton and fish are linked by predator-prey interaction. But then along comes this widespread parasite, infecting a very common phytoplankton species – cyanobacteria – and it transpires that the food web is much more complex than we thought.
Do you have any idea what impact your findings could have in times of global warming?
RA: We can’t say much about that yet, but it’s an issue that we are keen to address. We know that cyanobacteria will become more common as a result of climate change. In the laboratory, we investigated what happens at different temperatures, and discovered that infections also increase with a rise in temperature. But that’s not what happens in nature, where temperatures rise slowly, over longer periods of time, giving host and parasite organisms time to adapt. We want to simulate such conditions in the laboratory, and investigate them using approaches from experimental evolution, that is, we allow organisms to adapt to new conditions over a longer period of time, and then compare the resulting disease dynamics. These experiments are particularly exciting because they enable us to observe evolution in real time! We plan for that a year-long study.
What issue will you address in the process?
JW: Our goal is to gain a better understanding of the fundamental processes the parasites are involved in; we will additionally investigate other parasites that have an impact of zooplankton. We are also planning more complex experiments, which may enable us to look into the future. A number of lakes in Poland have experienced discharge of cooling water from coal-fired power plants for 60 years, causing a total increase in water temperature of 4 degrees Celsius. This temperature increase makes them perfect models for what to expect in future decades. As part of a large-scale project, we collect samples of phytoplankton and zooplankton from these lakes and compare them with control lakes, i.e. other nearby lakes that have not been fed with warm. In particular, we compare how parasitic epidemics spread in heated lakes compared to the control lakes.
Ms. Wolinska, in addition to your research, you are also involved in IGB’s Inclusion and Diversity Group, which was established in 2019. Why is this issue important to you?
In my opinion, institutes that pay more attention to inclusion and diversity provide a friendly and safe environment. Diversity means differences that constitute our respective identity, such as geographical or ethnic origin, gender, age, skills or religion, and it is important to recognise these. All individuals contribute unique perspectives, and this mix of perspectives paves the way for smarter decisions, better ideas and outcomes, greater innovation; you maximise a group’s potential. So if we pay more attention to diversity and inclusion, we can benefit enormously, enhancing not only our culture at the institute, but also our scientific work.
What have you achieved so far?
Research shows that a long list of requirements in job advertisements makes women less likely to apply for the job. We spoke to the institute management, and suggested advertising IGB vacancies differently in the future, and reducing the number of requirements to a reasonable amount. This suggestion has already been implemented. We also organised a colloquium that addressed the issue of subconscious prejudices, and how to overcome them. After all, even if we have good intentions, we are often guided by attitudes that influence our judgement, such as in application procedures. Incidentally, the same applies to scientific work, as I discovered while teaching a class for 200 students a long time back...
Tell me more.
We had prepared an experiment to measure the reactivity of two different groups of Daphnia: one group had experience with predators, whereas the other had none. The students were asked to determine how strongly Daphnia react when threatened with a needle inserted into the Petri dish, i.e. to measure the distance they shrank back.
And?
In the experiment, 80 per cent of the students proved that Daphnia with “predator experience” responded more strongly. But the point was that there were no differences between the two groups – all Daphnia had been reared under the same conditions. A clear case of observer bias that shocked me at the time: for a while, I even toyed with the idea of abandoning science. Today, I see that experiment as a convincing example of the need to maintain sample blinding at all times, and I mean at all times. Greater impartiality would be important in other areas of scientific life, too.
The interview was conducted by Wiebke Peters.
