2024: Best Master Thesis

Tilman Linke's Master thesis was awarded with the Prize for the Best Plant Science Master Thesis, which was carried out at Universität Leipzig in the year 2024 with the title:

Development of Photo-Calorespirometry, as a Novel Method to Directly Access Photosynthetic Rates and Efficiencies of cyanobacteria in real-time

The novel Photo-calorespirometry method was established and applied for the first time to cyanobacteria, using Synechocystis sp. PCC 6803 as a model, to directly quantify photosynthetic rates and efficiencies on an energy level in real-time

Photosynthesis represents a natural phenomenon performed by phototrophs, such as plants, microalgae, or cyanobacteria. Triggered by the climate crisis, an increasing focus on the generation of renewable energy carriers (e.g. hydrogen) employing phototrophs in photo-biotechnologies is emerging since they convert light energy into chemical energy. The conversion efficiency of light into chemical energy (PE) is highly dynamic and relies on a variety of alternating environmental conditions (e.g. light and substrate availability, temperature etc.), as well as internal metabolic adaptation mechanisms to balance physiological requirements regarding the fluctuating availability of light. However, the currently available techniques measure PE only indirectly, with a poor time resolution, or fail to consider the whole photosynthetic process. For that purpose, photo-calorespirometry (Photo-CR) was developed to directly measure the photosynthetically fixed energy (P_PS) as heat. Photo-CR combines calorimetry with the simultaneous monitoring of photosynthetic oxygen evolution rates (r_O2) in real-time. In this work, Photo-CR was established and validated the first time for the model cyanobacterium Synechocystis sp. PCC 6803. The experimental setup of Photo-CR has been further optimized to enhance the accuracy of the thermokinetic data, facilitate the implementation of the measurements, and minimize sources of error. A maximum fixed light energy yield over the cell dry weight of 26.4 kJ/g^_CDW, and  P^max_PS = 5.2 W/g_CDW was achieved after these adjustments. PE was analyzed at changing light intensities, resulting in a maximum of around 2.3 % and a half-maximum PE at 360 µmol_Photons/m²*s. Additionally, the outcomes indicated photo-protective mechanisms as a response to high light energy inputs. A strong correlation between calorimetry and respirometry was observed with an oxycaloric equivalent of 488.5 ± 0.3 kJ/mol_O2, providing further insights into potential side-reactions with additional energy expenditures under the consumption of oxygen (e.g. Mehler reaction, photo-respiration). Besides, the influence of distinct nitrogen sources, with different degrees of reduction, on the photosynthetic performance of Synechocystis under high and low light conditions was evaluated. Photo-CR unraveled beneficial impacts of nitrate as an additional electron sink under high light conditions, suggesting that surplus light energy can inhibit photosynthesis and that electron sinks can prevent this by receiving photosynthetically derived electrons. In conclusion, this master thesis emphasized Photo-CR as a non-invasive, holistic, quick and versatile technique to screen for the most efficient aquatic phototrophs, to elucidate optimized parameters to improve photosynthetic performances, and to expand our understanding of the underlying metabolic regulations during photosynthesis.

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Tilman Linke conducted this work at the Helmholtz Center for Environmental Research (UFZ) in the working groups of Prof. Dr. Thomas Maskow and Prof. Dr. Andreas Schmid.