© Martina Bauchrowitz/IGB
"If you can adapt your diet to the respective environmental conditions like the mixotrophs, you may have great advantages over other organisms," says Ulrike Berninger. As an aquatic ecologist, she has already investigated mixotrophic plankton in many lakes and marine areas at home and abroad and in countless laboratory experiments. However, it has never before been studied in a large-scale field experiment within the natural food web. This special opportunity now arises at Lake Stechlin. Together with her bachelor student, Elisabeth Schlager, she is observing in this year's LakeLab experiment how the plankton organisms react to the changed light regime in the enclosures and whether the proportion, species diversity and significance of the mixotrophs change.
Mixotrophs can cover their carbon and energy requirements in two different ways. Like phytoplankton, they either carry out photosynthesis and build up the necessary biomolecules exclusively from inorganic substances with the aid of light energy – a way of life also known as photoautotrophic. Or they take up organic particles by phagocytosis and use the biomolecules present in them, like the heterotrophic zooplankton does. However, it is still unclear whether mixotrophs only use the different nutritional strategies to gain carbon, or whether this also involves other nutrients and/or trace elements.
Elisabeth Schlager is also fascinated by these special organisms, and when Ulrike Berninger told her about the project at the LakeLab earlier this year, she was immediately thrilled. Only two years ago, she decided to make her childhood dream come true and study biology after having graduaded and worked in mechanical engineering. "Things are different when you're in your mid-30s and you go back to university," says Elisabeth Schlager, "you're much more determined, you know more quickly which direction you want to go." In her case, it is ecology and the offer to investigate the mixotrophs in this year's LakeLab experiment as part of her bachelor thesis came just at the right time. In early summer, she carried out preparatory work in Ulrike Berninger's laboratory at the University of Salzburg for the conception of the study. Now she will stay at IGB Stechlin for the entire duration of the experiment in order to carry out feeding experiments and process the samples. "Participating in such an extensive field experiment as part of a bachelor thesis is a great opportunity," Elisabeth Schlager is enthused.
"Specifically, we are observing what is known as nanoplankton. These are organisms with a size between 2 and 20 micrometers. In a feeding experiment, we offer them fluorescent plastic particles the size of bacteria as food," explains Ulrike Berninger. After several hours of incubation in the differently manipulated LakeLab enclosures, the plankton organisms are killed, placed on filter membranes and finally counted under the epifluorescence microscope. When irradiated with a green filter, autotrophic plankton can be identified by its red autofluorescence and heterotrophic plankton by the yellowish fluorescing plastic particles they have ingested. As a consequence, mixotrophs can then be recognized by both the red autofluorescence and the fluorescence of the ingested particles. Thus, under the microscope, it is possible to determine the proportion of mixotrophs in the plankton community. Back in Salzburg in autumn, Elisabeth Schlager will devote herself entirely to counting the samples taken. It will then be possible to see to what extent the proportion of mixotrophs in the lake water has changed as a result of the different experimental conditions compared with the controls.
Ulrike Berninger's stay at Lake Stechlin is financed by the EU project AQUACOSM (MixoSky subproject). Elisabeth Schlager is financially supported by a scholarship of the "Stiftungs- und Förderverein" of the Paris-Lodron-University Salzburg as well as by IGB Stechlin.
