Without microorganisms, our ecosystems would not function properly: fungi “predigest” food, parasites keep blue-green algae in check, and water fleas play an important role in aquatic food web dynamics and structure. At IGB, many researchers are working on different microorganisms, studying both the ecology of these organisms and the extent to which they are threatened by climate change and other human-induced changes. While the findings are fascinating, they are also a stark reminder that the diversity of life in our waters is under threat - even the microbes.
When you meet Hans-Peter Grossart for a video call, you find yourself in an icy environment. “That behind me is Antarctica,” explained the limnologist and research group leader. But he is not wearing a hat or scarf, because the ice landscape is a digitised photograph. Hans-Peter Grossart is all too familiar with the real-world extremes of our planet’s South Pole – it’s where he and his team study the interaction between parasitic fungi and benthic diatoms. “We want to understand how climate change is affecting food webs and biodiversity in Antarctica and the Arctic,” the researcher said. He observes changes in various environmental parameters, such as rising temperatures and the increased input of “sweet” glacial meltwater: Does this increase or decrease parasitism, and how does this affect the food chain and biodiversity in general? His team has taken numerous samples, which are still being analysed.
Grossart and his team know from numerous recent research projects that fungi play a special role. For example, they are much more widespread than previously thought – occurring even in the deep sea, another extreme habitat. In the depths of the ocean, a number of cycles are driven by fungi, some of which also break down plastic. There are only rough estimates of fungi as a percentage of microorganisms in the different types of water. In freshwaters, they are thought to account for up to half of all single-celled microorganisms. Interactions between parasitic fungi and algae in lakes are important: parasitic fungi break down particular types of algae and bacteria, e.g. filamentous and toxic cyanobacteria, which then become food for zooplankton. “This means that the parasitic infestation makes part of the algal biomass available in the first place, which is a very important effect in the food web, as demonstrated by the team of my colleague at IGB, Justyna Wolinska,” reported Hans-Peter Grossart.
Fungi are important for a variety of nutrient cycles
Certain types of fungi can apply mechanical pressure to larger organic matter, such as leaves, to gain access to the cells. Cells that have been opened by the fungus can then be invaded by bacteria, causing the leaf to decompose more rapidly. “This is a fundamental mechanism that also occurs in soils,” remarked Hans-Peter Grossart. In the carbon cycle of freshwaters, microorganisms such as filamentous fungi contribute to the accumulation of organic matter. Larger aggregates sink faster to the bottom. But there are also fungi that work in the opposite direction, preventing aggregation and rapid sinking. Hans-Peter Grossart and his team are investigating parameters such as temperature, industrial pollution and nutrient availability to see how they affect these material flows.
Many fungi have special enzymes that allow them to interact with other organisms to transform persistent materials such as microplastics. “We have already tested a large number of fungal isolates and found some that break down plastics,” reported Hans-Peter Grossart. However, these results are obtained in a bioreactor, i.e. under laboratory conditions. Whether plastic can be transformed by fungi in a natural system depends on many factors, including temperature, nutrient conditions, microbial community composition and whether the fungus is able to grow on the plastic.
However, fungi are at risk. Fungicides and many other pollutants such as pharmaceuticals, metals, microplastics and nutrients may be affecting fungi and their delicate networks. How this happens is the subject of an ongoing research project led by Justyna Wolinska. Moreover, fungi are probably affected by the same factors as other aquatic organisms, which include habitat degradation, invasive species and climate change. Such threats can lead not only to species extinctions in aquatic fungal communities, but also to population declines and even to a total loss of their key functions in the ecosystem, which can ultimately produce cascading effects in aquatic food webs. For this reason, Hans-Peter Grossart and an international research team are calling for the protection of aquatic fungi to be recognised as a priority for the management of water bodies.
Using water fleas to study the effects of climate change
Dagmar Frisch from the “Evolutionary and Integrative Ecology” department also conducts research on microorganisms in the Arctic. She focuses on a special phenomenon in the animal kingdom: the dormant eggs of a Daphnia population in this extreme weather region. “Daphnia, or water fleas, are of great interest because they belong to the keystone species in freshwater ecosystems: they play a key role in food webs at an intermediate position, feeding on algae and serving as food to larger organisms including fishes,” reported Dagmar Frisch.
She works with Daphnia pulicaria, a species found mainly in lakes. The Greenland population she is studying differs from many other populations of this species in that the animals reproduce completely asexually: there are males, but they have no known function – the females clone themselves. Water fleas hatch from the eggs they produce, even without fertilisation, each with a triple set of chromosomes completely identical to their mother’s. Dagmar Frisch wants to know how climate change is affecting these populations and, in turn, the local ecosystem as a whole. To do this, she takes advantage of a special feature of this species: Daphnia eggs can survive in the sediment for several hundred years before being “awakened” to hatch. Daphnia eggs are contained in a chitinous shell that forms on the back of female daphnia and detaches when the animals shed their skin. The shell containing the eggs – the ephippium – slowly sinks to the bottom and rests in the sediment. Over the decades, layers of ephippia form in the sediment. Dagmar Frisch and her team take sediment cores and use radiocarbon dating to date the sediment layers from which the eggs were taken.
How well the eggs survive depends on the conditions in the sediment – they cannot always be resurrected, usually “only” to an age of just over 100 years. The purpose of the resting eggs: If a generation dies due to poor conditions, the population from the sediment can regenerate – usually the following year.
In the laboratory, Frisch and her team resurrect dormant eggs of different ages and analyse how the animals that develop from them differ from one another. An individual and its copies in different sediment layers provide the researchers with a kind of time series that can be used to track the effects of different temperatures: for example, on the number of eggs per generation or the time it takes for the Daphnia to reach sexual maturity. “Usually you collect individuals from populations in different places and then compare their genomes, for example: Where are the genes that have changed through selection, where are the differences? In this case, you’re following the same population and its evolution, which makes it so special,” enthused Dagmar Frisch. The researchers have already discovered that their temperature tolerances vary. Present-day Daphnia from the habitat under study are more sensitive to heat, so their populations could be at risk from climate change.
Lynn Govaert seeks to discover underlying mechanisms
Changes over many generations also play a central role in Lynn Govaert’s research: she is investigating the rules that govern community dynamics using tools from evolutionary ecology. Environmental conditions affect individuals and also influence their genetic makeup over time; animals that have undergone physical and behavioural changes in turn affect their population dynamics but also their interaction with other species. “I am interested in the rules governing these complex dynamics and what we can learn from them for the future,” remarked Lynn Govaert. Before the Belgian mathematician came to IGB in spring 2021, she had already studied how single-celled organisms known as ciliates evolve under changing conditions.
Ciliates are interesting because they have very short generation times, ranging from approximately four hours to two days depending on the species. This means that evolutionary changes are already visible after a few weeks. They are also relatively easy to maintain in the laboratory, respond quickly to a variety of stressors and have different responses to environmental change. In the coming years, Lynn Govaert wants to study how different ciliate species interact when environmental conditions such as the salinity of their water habitat or water temperature are changed.
“Evolutionary ecology is a young field of research with much to discover,” noted the researcher. She is aided by modern technology. Lynn Govaert and her team use a software program that allows them to tag a large number of single ciliate individuals in a randomly sampled “sip” of the miniature ecosystems and follow them under the microscope. Lynn Govaert’s goal is to understand the evolution of ciliates in their complexity to the point where it is possible to discover mechanisms and make predictions for natural systems.
Heat affects Daphnia size and fecundity
Justyna Wolinska also studies evolutionary processes and what they can teach us about the coexistence of the smallest organisms. To this end, she and her team use a number of lakes in Poland that have experienced discharge of cooling water from coal-fired power plants for 60 years. As a result, the temperature of these water bodies is on average 3 to 4 degrees Celsius higher than the surrounding lakes. The researchers use these cooler water bodies as control lakes. Sixty years represents hundreds or even thousands of generations during which the species studied – Daphnia and the tiny parasites that live with them – have been able to evolve together and adapt to the higher water temperatures.
One of the results of Wolinska’s research is that the Daphnia grow larger and lay more eggs in the warmer lakes – although these conditions are not actually suitable for the species: when Daphnia from cooler lakes are exposed to the same temperatures, they produce fewer eggs. The common microparasite infecting Daphnia was found less frequently in the warmer lakes. “This may sound like good news, but it is not. After all, these parasite species have many functions in the ecosystem. If these key actors disappear, other trophic levels will be affected,” stated Justyna Wolinska. Microparasites and the epidemics they cause increase the evolutionary pressure on Daphnia to be genetically diverse and hence better able to adapt to stressful conditions. And given that Daphnia are at the centre of the food web, their potential decline might have consequences for other species.
Even small quantities of microplastics have a major impact on microorganisms
Wolinska and her team are currently investigating how microplastic and nanoplastic particles affect the host–parasite relationship in water. “There are numerous studies on the toxicity of microplastics to individual species, but we still don’t know how they affect interactions and contribute to the spread of diseases, for example,” stated the evolutionary biologist. The researchers tested low levels of nanoparticles and microplastics in water, expecting that such low levels would have no effect. But contrary to expectations, infection rates in Daphnia increased dramatically. The mechanism behind this could be that the plastic particles damage their immune system, leaving them unable to defend themselves against the parasites. The researchers made similar observations when cyanobacteria interacted with chytrid fungi. In summer, these fungi help to prevent the spread of algal blooms. “We observed that when the cyanobacteria were exposed to the smallest plastic particles, they were covered by them. As a result, the chytrid fungi were unable to attack the cyanobacteria,” stated Justyna Wolinska, describing the connection. Increased amounts of plastic in our freshwaters could therefore contribute to more cyanobacteria blooms and also put pressure on Daphnia populations.
The enormous impact that microorganisms can have on aquatic ecosystems was demonstrated in the Oder River disaster in the summer of 2022, when toxic algae triggered a massive fish kill. Jan Köhler is investigating the conditions that allowed the algae to multiply so suddenly and exponentially.
Meanwhile, Dirk Schulze-Makuch knows that microorganisms are also the key to extraterrestrial life: his work in the Atacama Desert has shown that certain species of bacteria can survive without that it ever rains; they only need sufficient moisture in the atmosphere. On Mars, such bacteria may already have been detected but inadvertently eradicated – with too much water. During Mars missions in the 1970s, for example, scientists treated the soil with water to “wake up” any life that might be present. As discussed by Schulze-Makuch in a report for the US-American online forum Big Think, this method may well have wiped out any bacteria that may have been on Mars. After all, microbes that could survive on Mars would be adapted to extremely arid conditions. The smallest organisms are not only essential to our survival, they are also adaptable. But if their basis for life is disrupted, it can have far-reaching consequences for the ecosystem around them.
Text: Wiebke Peters
