PARC seeks to understand the structure and function of the wide variety of natural photosynthetic antenna systems in molecular terms, including their efficiency, mechanism of action, regulation, assembly and repair. Particular emphasis will be placed on developing new and modified antenna systems using techniques of molecular and synthetic biology that increase the efficiency of living photosynthetic organisms by reducing light saturation effects and expanding the spectral ranges of radiation that can be used for energy storage. The specific goals are as follows:
- To determine the molecular structures of native and modified antenna systems and correlate with function.
- To understand the subcellular organization of antenna systems in different photosynthetic organisms.
- To examine physiological consequences of modified/alternate antenna systems in diverse photosynthetic organisms.
The overall aim is to determine and manipulate the antenna size and composition to maximize photosynthetic efficiency in any such organism.
All natural photosynthetic organisms contain a light-gathering antenna system, which serves to increase the cross section for light absorption . The size of the antenna systems range from a few dozen pigments per reaction center (RC) in some species of proteobacteria and the heliobacteria to many thousands of pigments per RC in the green sulfur bacteria. The large size of the antenna system in most oxygenic photosynthetic systems, usually several hundred pigments per RC, causes the rate of photochemistry to be saturated at light intensities significantly less than full sunlight. Any excess light beyond what can be processed by the photosynthetic apparatus is lost in terms of photosynthetic efficiency and also must be dissipated safely to avoid photodamage. Oxygenic photosynthetic organisms contain an extensive suite of regulatory, protective and repair processes that prevent and repair photodamage . The onset of light saturation depends on both the rate of delivery of excitations from the antenna as well as the rate of photochemistry and utilization of photoproducts by the RCs and dark biochemical systems.
There has been considerable debate as to whether the efficiency of photosynthesis-based bioenergy conversion systems will be increased if the size of the light- gathering antenna is reduced by creating small antenna mutants, in which photochemistry is not light-saturated even at high light intensities . While there is some experimental evidence to support this view, the subject has not been tested in a comprehensive manner. Experiments and modeling are proposed under Theme 1 address this issue specifically.
Cyanobacteria.Genetic manipulations that lead to a decrease of the effective antenna size in photosynthetic microbes provide a useful approach in extending the understanding of light capture, and of distribution, regulation and use of excitation energy in these organisms.
Green Algae. In the Chlorophyta (green algae), light is primarily harvested by the chlorophyll a and chlorophyll b molecules within light-harvesting complex LHC-II . One goal of the proposed work is to modulate LHC abundance by controlling chlorophyll b accumulation.
Structure of the RC/LH1 Core Complex from Purple Photosynthetic Bacteria. Theme 1 will work to refine both the purification protocols and the crystallization conditions to grow crystals that diffract X-rays to higher resolution. These studies will be extended to include core complexes from other species. Additional work will include crystallization and structure determination of LH2 and related complexes with unusual spectral properties from purple bacteria and green non-sulfur bacteria.
Energetic and Spatial Relationships of Green Bacterial Photosystems. The photosystems of the green photosynthetic bacteria contain remarkable light-collecting complexes known as chlorosomes, which contain self-assembled pigment oligomers with essentially no protein content. This makes these complexes of considerable interest as models for artificial antenna systems.
Cyanobacterial Antenna Complexes: Phycobilisome Diversity and Organization. Specific analyses of altered antennas, resulting membrane attachment and photosystem association have largely not been performed. Antenna mutants will be examined using electron microscopy to determine the 3-D arrangement and therefore understand the affect of antenna alterations on phycobilisome architecture, energy capture, and membrane association.
Improving Solar Energy Efficiency by Expanding the Absorbance Characteristics of Phototrophs. How can chlorophyll d improve photosynthetic productivity? Expanding the solar harvesting efficiency of oxygenic photosynthesis has the potential to improve crop growth rate, yield, and season length and is applicable to both food crops and biofuels crops.
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