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Project Progress

The innovation of oxygenic photosynthesis is argued to have transformed the Earth’s atmosphere and been the driving force that led to the evolution of O₂-based respiratory metabolisms. At the heart of this biological innovation is the photosystem II (PSII) enzyme complex with ability to split water into O₂. Previously, it was postulated that the first oxygenic PSII may have originated from an anoxygenic phototroph that lived on ferrous metal complexes for an electron source and later bound and photo-oxidized Mn²⁺ in the form of bicarbonate complexes. To test this hypothesis, both electrochemistry and electron paramagnetic resonance (EPR) spectroscopies were used to characterize Mn²⁺ bicarbonate complexes that form in solution and their efficacy as electron donors to Photosystem II. The charge and structure of these complexes together with the greatly reduced oxidation potential to Mn³⁺ explains why bicarbonate stimulates the rate of photo-assembly of the Mn₄CaO_x-cluster during biogenesis of Photosystem II in whole cells (Dasgupta et al.., submitted to J. Phys.Chem B.). This work supports the hypothesis that carbonate is essential in some Photosystem-II organisms and could have played a key role in the evolution of oxygenic photosynthesis in the CO₂-rich Archean era.

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Highlights

• A pulse-laser flat-repetition-rate fluorometer (FRRF) was successfully used to characterize photosynthetic function for in situ biofilms and planktonic cells in hot spring settings.
• An astrobiology course was taught at Princeton University, including a field trip to LaDuke hot spring near Yellowstone National Park in Fall 2005.
• A modified design was completed for a fluorometer that measures quenching of the variable portion of chlorophyll fluorescence emission from the Photosystem II reaction center by the O₂-evolving enzyme.