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.
Besides their role as light energy collection proteins, LHC proteins also have a second important role when solar irradiation exceeds the photosynthetic capacity. In this case, LHC proteins can help to dissipate light energy as heat or fluorescence. Under these conditions, photosynthetic energy conversion of solar energy into photosynthetic product (“conversion efficiency”) is decreased. Because of their dual role in photosynthesis (light harvesting under low light conditions / light energy dissipation under excess light conditions) and the constantly changing environmental parameters such as light availability, temperature, water and nutrient supply, LHC protein expression is highly flexible and the fate of the individual LHC protein is finely controlled, from mRNA transcription to protein degradation. The overall amount of LHC proteins is constantly adjusted to the actual demand. Captured light energy is used by the photosynthetic organism to drive photosynthetic reactions and can be used to reduce carbon dioxide, allowing the synthesis of downstream molecules. In contrast to multi-cellular higher plants, several green algae have the ability to reduce protons and release hydrogen gas under anaerobic conditions, enabling the organism to retain basic levels of ATP generation in the chloroplast, while oxygen dependent mitochondrial respiration is blocked. The intriguing possibility of biosolar hydrogen production by green algae has recently gained considerable research interest, as well as public interest, because hydrogen is considered to be one of the most promising alternative energy carriers for the future.
