• Prof. Dr. Dr. Christian Ulrichs in front of the FishCube door, Brain City Berlin

    © Ernestine von der Osten-Sacken

    23.07.2025

    Tomatoes and fish in a zero-waste cycle

It's all about future food production: in the CUBES Circle project researchers at the Thaer-Institute of Agricultural and Horticultural Sciences of Humboldt-Universität zu Berlin are exploring how established agricultural production systems can be interconnected in an smart, efficient, and sustainable way.The principle is inspired by nature, where waste is an essential part of the food cycle. Brain City Ambassador Prof. Dr. Dr. Christian Ulrichs coordinates the joint project, which includes several German universities and research institutions.

The tomatoes are bright red, round, and juicy. Their skin is nearly flawless – and the aroma is intense.
“Maybe I’m exaggerating a bit, but our tomatoes are probably the best in Berlin,” says Prof. Dr. Dr. Christian Ulrichs. “That’s because they’re grown sustainably and under controlled conditions. For example, they receive UV light through translucent films. Tomato plants respond to sunlight by producing flavor-enhancing pigments like lycopene.”

We’re standing on the upper level of the “CUBE” – the three-story lab of the “CUBES Circle” (Closed Urban Modular Energy- and Resource-Efficient Agricultural Systems) project. The building was opened in 2024 on the grounds of the Thaer-Institute in Berlin-Dahlem. Brain City Ambassador Prof. Dr. Dr. Christian Ulrichs, head of the Urban Plant Ecophysiology department at HU Berlin, coordinates the project, which he initiated together with biosystems engineer Prof. Dr. Uwe Schmidt.

Three established agricultural Systems – intelligently connected

CUBES Circle is one of eight nationwide projects funded by the Federal Ministry of Education and Research (now the Federal Ministry for Research, Technology and Space, BMFTR) as part of the “Agricultural Systems of the Future” initiative. The world population is growing rapidly – and arable land is becoming increasingly scarce. By the 2080s, the UN predicts a global population of around 10.3 billion (currently approx. 8.1 billion). Urban areas will become even more densely populated. Already, more than half of the world’s population lives in cities; by 2050, it will be nearly two-thirds. Climate change further exacerbates global resource scarcity.

The project integrates three production systems into a nearly closed energy and material cycle: plant cultivation, fish farming, and insect production. Each “production unit” is housed in a mobile, stackable module. These “CUBES” are smartly networked and communicate with each other. The cycle mimics nature: plant waste is fed to insects. Currently, desert locusts in the InsectCUBE on the ground floor nibble on leaves and stems – the previously used black soldier flies didn’t like tomatoes. The insects’ waste is used to feed fish, which swim in circles in blue aquaponic tanks in the FishCUBE. The nutrient-rich fish water then nourishes the plants. Even the CO₂ produced by the insects is used to fertilize the plants. Recently, a fourth production stage was added: mushrooms now help decompose organic matter.

CUBES Circle is unique in its complexity. “The system follows a zero-waste principle, creating a sustainable and highly efficient value chain,” says Ulrichs. “Thanks to its mobility, adaptability to rapidly changing urban environments, and inherent scalability, the CUBES can be used in rural, urban, and even desert-like areas.” The long-term goal of the project: by linking aquaculture, insect farming, and horticulture, the scientists aims to overcome the limitations of conventional agricultural systems and create a flexible, largely self-sufficient food system. Currently, the system is not yet fully autonomous but “embedded,” as Ulrichs explains. “That means we’ve defined interfaces with the surrounding environment – for example, we use waste heat and industrial residues and can serve as energy storage. What we take out, we must give back – in the form of young fish and plants. We also purchase young larvae. From the beginning, we saw this not as a revolution, but as an evolution.”

@ CUBES Circle

A nationwide synergetic collaboration

As an inter- and intradisciplinary project, CUBES Circle brings together partners with specific expertise. The Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) oversees aquaponics, the Technical University of Chemnitz handles data processing and networking, and the Technical University of Braunschweig is responsible for sustainability calculations. At the Weihenstephan-Triesdorf University of Applied Sciences, a converted shipping container serves as a CUBE and provides comparative data – since the modules in Berlin-Dahlem were built cost-effectively from wood. A psychologist is also involved: Prof. Dr. Linda Onnasch from the Institute of Psychology and Ergonomics at TU Berlin. “Public acceptance is crucial for the project,” explains Ulrichs. “The question is: can something like this be built in a neighborhood?” Like many forward-looking solutions, CUBES Circle may face skepticism. Surveys and interviews with potential users are therefore an essential part of the project – as is the targeted training of professionals, such as those in horticulture. These could be trained using augmented reality, for example.

Another key topic is animal welfare. “Fish welfare is well-researched. We know they like to swim in circles and don’t need cozy corners. But insect welfare has been handled rather casually so far,” says Ulrichs. “Insects have a different nervous system than we do. Many biologists therefore are critical of handling them in lab-like situations.” One of them is Dr. Wael Yakti, Brain City Ambassador and the team’s chief entomologist. He studies the conditions under which larvae and locusts thrive best in the InsectCUBE.

Engaging with society

CUBES Circle is funded by the BMFTR until the end of 2028. By then, Ulrichs hopes to make the system market-ready. “We’ve already conducted a site analysis for the upcoming transformation process and identified industries we could connect with – such as biogas plants or poultry farms. Suitable locations have been found in Brandenburg, the surrounding area, and the Ruhr region.” The next step: engaging with potential investors, policymakers, and especially the public. A digital twin of the system serves as a demonstration model for external stakeholders, a planning tool for the team, and a means of fostering public acceptance.

Another current cross-cutting topic is learning from the system and integrating new organisms and plants into the cycle – such as strawberries, bell peppers, or the spotted spiral ginger (Costus pictus), known for its antidiabetic properties. “Each CUBE module is divided into a production section and ongoing experiments.” The selection of plants and research approaches is also informed by acceptance studies. For Ulrichs, this is given: “Unlike the food industry, we don’t present consumers with finished products and try to convince them. As scientists, we engage with society from the outset. That’s clearly the path we must take as a university.”

The juicy red tomatoes from the sustainable CUBE production confirm Christian Ulrichs’ theory. A taste test revealed: they really are incredibly delicious!

cubescircle.de

Author: Ernestine von der Osten-Sacken

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