Spinach, a Key Ingredient in a Series of Groundbreaking Experiments at Freie Universität Berlin
X-ray experiments carried out at BESSY prove that photosystems from spinach can form manganese oxide nanoparticles – a product of oxygenic photosynthesis
№ 238/2020 from Dec 04, 2020
An interdisciplinary research team at Freie Universität led by Professor Holger Dau and Professor Robert Burnap from Oklahoma State University have proved that protein complexes in modern photosynthesis can form manganese oxide when exposed to light. These experiments support the hypothesis that manganese oxide was a product of photosynthesis in the Earth’s early history. “Photosystem II (PSII)” was isolated from spinach leaves, and the four manganese atoms that split water in modern photosynthesis to form atmospheric oxygen were then removed. With the aid of these manganese-free photosystems, the researchers recreated what they believe photosynthesis looked like billions of years ago. The study has since been published in Nature Communications, a renowned scientific journal.
The photosynthetic process used by algae, plants, and cyanobacteria is highly complex. Its current form evolved about 3 billion years ago and had an immense impact on the history of Earth. Oxygenic photosynthesis, the conversion of light energy into chemical energy, produces oxygen as a waste product. This led to an accumulation of oxygen in the Earth’s previously anoxic atmosphere to about 21%. The development of oxygenic photosynthesis thus enabled the development of life on Earth as we know it.
The team exposed a “green soup” of manganese-free photosystems to light in the presence of small volumes of manganese ions dissolved in water. The researchers noted that the photosystem formed manganese oxide nanoparticles within just a few minutes, with 50 to 100 manganese atoms per photosystem. The nanoparticles produced in this way resemble a mineral rock called birnessite. The team managed to verify its atomic structure by means of X-ray spectroscopy at BESSY (Berlin Electron Storage Ring Society for Synchrotron Radiation), one of the large research facilities belonging to the Helmholtz-Zentrum Berlin (HZB) in Adlershof.
Manganese is a chemical element that can be found next to iron in the periodic table of elements. It forms dark-brown manganese oxide when it combines with oxygen from the Earth’s atmosphere, which looks similar to red-brown iron oxide – otherwise known as rust. The fact that manganese oxide was created in an environment that is virtually free from oxygen molecules required a very special explanation. In 2013, a team of geologists led by Professor Jena E. Johnson proposed that mineral manganese deposits were a direct result of an evolutionary ancient version of photosynthesis.
When biophysicist Professor Dau heard about Professor Johnson’s hypothesis, he came up with an idea to prove that photosystems could actually produce manganese oxide. Several kilograms of fresh spinach leaves were pureed for the experiments. Photosystems containing chlorophyll were isolated from the mixture and then slightly modified by removing three small chlorophyll-free protein components. By analyzing the color changes of an electron acceptor added to the set-up using X-ray absorption spectroscopy, they were able to prove that electrons were removed from the dissolved manganese ions when exposed to light.
Work on photosynthetic manganese oxidation was initially carried out by students undertaking final projects and by researchers from two Berlin-based research associations – the Collaborative Research Center “Protonation Dynamics in Protein Function” (SFB 1078) and the Excellence Cluster “Unifying Systems in Catalysis” (UniSysCat). Einstein Fellow Professor Burnap, an internationally acclaimed microbiologist and expert in the light-driven formation of manganese-oxidizing photosynthesis, joined SFB 1078 in 2018. With support from the Einstein Foundation Berlin, Professor Burnap, together with Professor Dau and Dr. Dennis Nürnberg, has led a small research team in the Department of Physics at Freie Universität Berlin that researches the role of manganese in the evolution of photosynthesis.
Petko Chernev, Sophie Fischer, Jutta Hoffmann, Nicholas Oliver, Ricardo Assunção, Boram Yu, Robert L. Burnap, Ivelina Zaharieva, Dennis J. Nürnberg, Michael Haumann, and Holger Dau.
Light-driven formation of manganese oxide by today's photosystem-II supports evolutionarily ancient manganese-oxidizing photosynthesis. Nature Communications 11, Article number: 6110 (2020).
https://doi.org/10.1038/s41467-020-19852-0 (Open Access)
Prof. Holger Dau, Department of Physics, Freie Universität Berlin, Email: email@example.com