On the left bank of the Rhine, a little away from the hustle and bustle of Cologne’s city center, lies the Ecological Rhine Station of the University of Cologne. Here, on a converted research vessel, we meet Prof. Dr. Hartmut Arndt – a man who has dedicated his life to the smallest creatures in the world: Protozoa. He welcomes us with a warm smile. His enthusiasm is infectious, and after just a few minutes you can feel it: This is someone who has found his passion.

Hartmut Arndt’s journey into the world of biology began in Rostock, where he studied and immersed himself in experimental ecology. But one particular encounter was to change his path forever: the encounter with Protozoa. These tiny, single-celled creatures populate almost all humid habitats and play a crucial role in the ecosystem. They are masters of survival, feeding and moving in a wide variety of ways.
Arndt’s enthusiasm for these small organisms became the driving force of his life.

After working in Plön and on Hiddensee, Arndt came to Cologne in 1997 – a decisive step, as the Rhine offers a unique laboratory for his research. The waters of this mighty river are home to countless organisms: from fish and mussels to tiny plankton and the very protozoa that make Arndt’s heart beat faster.

The Rhine station, which offers scientists and students of the Institute of Zoology a unique working environment, has been a second home for Arndt for over 25 years. Among other things, he researches the effects of climate change on the ecosystem, invasive species (neozoans), and the influence of ciliates and flagellates on water quality.

A look through the microscope opens the door to a hidden world. Around 95 percent of the creatures discovered in the Rhine are neozoa – introduced species that arrive here through the currents or the ballast water of large merchant ships. Some of these new inhabitants, such as the Asian basket mussel, have already become firmly established in the ecosystem. But things look less rosy for native species such as the eel: “Only one-fifth of the migrating eels that have already reached the Rhine in Cologne on their way to reproduce in the Atlantic Ocean actually make it to the North Sea. Unfortunately, species extinction is real, and we see it up close here,” says Arndt with a serious expression.

However, he is not only fascinated by the disappearance of old species, but also by the emergence of new ones. For example, there are hybrids of sculpins that have evolved due to changing environmental conditions – an example of how nature always finds creative solutions.

Technology has developed rapidly in recent decades. In the past, researchers were happy if they could take any pictures at all with an external camera. Today, high-resolution microscope cameras deliver razor-sharp images of fascinating microorganisms.

In addition to research, Arndt is passionate about training the next generation of biologists. The Ecological Rhine Station is an important place of learning for students in bachelor’s and master’s degree courses in biology and environmental sciences. Equipped with state-of-the-art technology, young scientists can combine theory and practice directly on the river.

But Arndt’s thirst for knowledge does not end at the Rhine. Deep-sea expeditions, including one with a submarine around the Azores, have given him insights into the most hidden corners of the Earth.

Retreating to a cozy retirement life? No way! “My life’s work is only just beginning,” says Arndt with a laugh. He has set up his own microscopy laboratory at home, where he continues to work on the taxonomy of flagellates. Even there, he can’t stop researching, discovering, and classifying.

And when we ask him whether he has now become the infamous “bristle counter”, he laughs heartily: “Yes, exactly that!”