Ecological Rhine Station of the University of Cologne

User Story

A Life for Protozoa: A Biologist and His Fascination for the Microcosm

byZEISS Microscopy 27 May 2025 5 min read

Introduction

A Life for Protozoa: A Biologist and His Fascination for the Microcosm

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.

The Love of Single-Celled Organisms

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.

You just can’t let go of what once fascinated you.

Hartmut Arndt Professor for animal ecology at the University of Cologne, Institute of Zoology

The Rhine as a Research Paradise

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 River That Flows into the Laboratory

In the past, studying rivers meant a considerable logistical effort. Scientists had to collect samples on the banks in rubber boots or even explore the river bed with dredgers and diving bells. Today it's easier: the Rhine literally flows through the research laboratory on board. Using large steel pipes, the water is pumped directly from the river into the laboratory, filtered and fed into test facilities. Two laboratories have waterproof floors. Other rooms are used for microscopy, field sample handling and desk work. This allows researchers to observe the Rhine's microorganisms in real time – a revolutionary development in ecological research.

The Ecological Rhine Station of the University of Cologne is located on a ship that was constructed by joining two former Rhine ships. Now 60 m long and 11 m wide, it is anchored upstream from the city center and can be easily accessed during high water.

Water from the river is continuously pumped through various flumes and aquaria on board to conduct experiments with typical river organisms under defined conditions.

Due to the short distance from the submerged pumps, the water reaches the experimental vessels almost unchanged within a few seconds, allowing for microscopic investigation under flow-through conditions.

What the Microscope Reveals

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.

Today, everything is more molecular, more chemical. But classic microscopy remains irreplaceable.

Hartmut Arndt Professor for animal ecology at the University of Cologne, Institute of Zoology

Passing on Knowledge: Teaching and Expeditions

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.

And after Retirement?

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!”

A researcher with insatiable curiosity and an infectious enthusiasm – Prof. Dr. Hartmut Arndt shows us that true passion never retires.

Protozoa play a crucial role in ecosystems as they contribute to nutrient cycling, decomposition, and the overall health of aquatic environments. Prof. Arndt's research highlights that climate change can alter the habitats and survival strategies of these organisms, while the introduction of invasive species (neozoans) can disrupt local ecosystems, leading to a decline in native species. For example, he notes that invasive species like the Asian basket mussel have established themselves in the Rhine, impacting the native biodiversity and the ecological balance, which underscores the importance of studying protozoa in the context of these environmental changes.
Microscopy has evolved significantly, with advancements leading to high-resolution microscope cameras that provide sharp images of microorganisms, including Protozoa. Prof. Arndt emphasizes that while modern techniques have become more molecular and chemical, classic microscopy remains irreplaceable for observing the intricate details of these organisms while they are alive. This evolution in microscopy is crucial for researchers as it allows for a deeper understanding of the biodiversity and ecological roles of protozoa in their environments.
The Ecological Rhine Station utilizes a unique method where water from the Rhine is pumped directly into the laboratory through large steel pipes. This allows researchers to filter and observe microorganisms in real time, significantly enhancing the study of river ecosystems compared to traditional methods that required extensive logistical efforts.