Photosynthetic organisms have developed the ability to use solar energy to produce energy rich chemical molecules which form the basis for all life on our planet. Most plants and photosynthetic micro-organisms are sessile or limited in mobility; therefore they have had to develop strategies to adjust to diurnal and seasonal changes in their environment. Sun light is the driving force of photosynthesis and therefore is of special importance for the survival of photosynthetic organisms. Consequently, the interplay between light perception and cellular response reactions is of fundamental interest to photosynthesis research. The availability of light energy changes dramatically in the daily rhythm and can range from rate limiting (night, sun rise/dawn) to excess abundance. Furthermore, rapid and unpredictable changes during the light period (caused e.g. by clouds) commonly occur. Photosynthetic organisms have developed an intriguing network of adaptation reactions to adjust to these changes. The first step of photosynthesis, and therefore direct interconnection between the physical world (electromagnetic irradiation from the sun) and the living cell, is commonly referred to as light harvesting. Light harvesting reactions are of crucial importance for all following photosynthetic reactions and are in the focus of this research project.
Simplified scheme of energy conversion in Chlamydomonas. Solar energy is captured by LHC proteins, electrons (e-) are released from H2O and transferred along the electron transport chain to reduce ferredoxin (Fd). Fd can then be used to reduce oxidised carbon (CO2) or to drive hydrogen production under anaerobic conditions.
Photosynthetic organisms are constantly challenged by a fluctuating supply of light or carbon dioxide. The light harvesting efficiency and hence the energy input at photosystem II has to be tightly adjusted to the prevailing external condition. This guarantees efficient light capture under light-limited conditions and avoids the formation of damaging reactive oxygen species when light is in excess. Translation control of photosystem II-associated light harvesting proteins (LHCBMs) mediated by the cytosolic translation repressor NAB1 is a key process, required for an adjustment of PSII light capture capacity in response to changes in external conditions. We are using NAB1-mediated translation control of LHCBM mRNAs as a model system to obtain novel insights into the crosstalk between chloroplast, nucleus and cytosol during light acclimation processes.
Metabolic processes in the two organelles chloroplast and mitochondrion are tightly linked to each other by an intense inter-organellar crosstalk involving the exchange of metabolites. For instance, efficient photosynthesis relies on mitochondrial respiration in the light, since excess reducing equivalents accumulating in the plastid under certain conditions can be exported and consumed by mitorespiration acting as a valve system. Hydrophobic core subunits of the respiratory chain are encoded by the mitochondrial genome and expressed by an organell-specific machinery, decoupling their expression from the nuclear control that regulates expression of the majority of mitochondrial proteins. Organellar gene expression (OGE) is among the candidate signal sources thought to initiate and modulate retrograde signaling events and the acclimation of photosynthetic eukaryotes to environmental changes critically depends on a tight coordination of nuclear, plastidic and mitochondrial gene expression. mTERF (mitochondrial transcription termination factor) proteins have been identified diverse eukaryotic systems. Among them is the C. reinhardtii mTERF designated MOC1 which acts as a mitochondrial transcription termination factor in vivo and is required to prevent read-through transcription at its binding site. An inactivation of the MOC1 gene in mutant stm6 de-regulates mitochondrial gene expression and respiration. Interestingly, this de-regulation also perturbs light-acclimation processes in the chloroplast and favours the enhanced production of biohydrogen. Future research will be focussed on the role of C. reinhardtii mTERFs within light acclimation processes and the interactions between chloroplast and mitochondria in photosynthesizing C. reinhardtii cells.
Phototrophic microbes have a great potential for the sustainable generation of bulk products such as food, feed, materials, chemicals and fuels. Compared to terrestrial plants, the efficiency of solar energy to biomass conversion is significantly higher in phototrophic microbes including unicellular microalgae, where in contrast to higher plants every cell is photosynthetic. Furthermore, algal biomass is rich in valuable components like neutral lipids as a feedstock for biodiesel production or compounds of high value for human nutrition including long-chain polyunsaturated fatty acids. Starch produced in a photoautotrophic regime might be converted into value-added products like bioethanol via fermentation and residual biomass (e.g. after biohydrogen production) can be integrated into bio-refinery concepts by its use as a substrate for biomethane production. Microalgae amenable to genetic engineering can be equipped with synthetic metabolic pathways and used as biocatalysts converting solar energy, water and carbon dioxide into various carbon-based compounds (e.g. drop-in transportation fuels). Photon conversion efficiencies observed in algal mass cultures are, however, far below the theoretical upper limit which calls for engineered algal strains converting solar energy more efficiently into biomass or carbon-based chemicals.