Bakterioklorofill fluoreszcencia, mint a fotoszintetikus baktériumok fiziológiai állapotának jelzőrendszere

Abstract

Photosynthesis is a biological process whereby the energy of the Sun is captured and stored by series of events that convert the free energy of light into different forms of free energy needed to feed cellular processes. (Blankenship 2014). The photosynthesis provides the foundation for essentially all life and has altered the Earth itself over geologic time profoundly. It provides all of our foods and most of our energy resources. Since essentially all energy used on Earth can be traced back to the photosynthetic transformation of solar energy into chemical energy, it is not surprising that the study of photosynthesis is at the center of scientific interest (Govindjee et al. 2005; Eaton-Rye et al. 2012; Niederman 2017). In photosynthetic bacteria, the energy conversion processes are considerably simpler than in green plants. While there are two photochemical reactions in green plants, there is only one in the bacteria. In contrast to the linear electron transport chain of green plants, the electron transport in bacteria is cyclic, in which the free energy of the charge pair produced in the reaction center (RC) is utilized by a cyclic pathway of electron building up a proton gradient across the photosynthetic membrane. The reaction center and the cytochrome bc1 complex (via the Q-cycle) constitute a proton-pump mechanism that translocates protons from the cytoplasmic side to the periplasmic side of the membrane. In the modern photosynthesis research, the non-sulfur type of purple bacteria plays a significant role, because the three-dimensional determination of the reaction center at atomic level (Deisenhofer et al. 1984) has made it possible to identify the structure and function of a photosynthetic energy conversion system. Although the details of the transformation of energy may vary in different species, there are structural and functional similarities. The bacterial reaction center has a very high photochemical quantum yield (~ 100%) since nearly all of the absorbed photons create charge pairs (Wraight and Clayton 1974). The highest free-energy loss relates to the reduction of the primary quinone (QA), which also means that physiological conditions make this process irreversible. The photosynthetic bacteria protect and operate their energy conversion system with remarkable efficiency and rate. An important part of this process is the light-dependent production and protection of tripled states of bacteriochlorophylls (BChl) essential for the survival of photosynthetic organisms. The energy of the BChl tripled state can be transmitted easily to triplet molecular oxygen (3O2) that generates harmful singlet excited oxygen (1O2*, strong oxidant). To avoid this reaction, several pathways are operating in all of which carotenoid (Car) pigments play prominent role. In addition to high light intensity, photosynthetic bacteria are exposed to numerous stress effects including heavy metal ions. The organisms can maintain their functions even under harmful conditions. How do they do it and what can be learned from these experiences? What makes the intact photosynthetic bacterium and its reaction center robust and yet flexible enough to function efficiently under different stress conditions? These are the fundamental questions I set in the frontline of the dissertation.