6V��Ӱ��

Speaker: Woodward Fischer, California Institute of Technology

Abstract: The emergence of oxygenic photosynthesis fundamentally transformed our planet; however, the processes that led to the innovation of biological water splitting have remained largely unknown. From an evolutionary perspective, it is notable that modern biological water-splitting in photosynthesis begins with Mn oxidation in the water-oxidizing complex of photosystem II, suggesting several evolutionary scenarios wherein Mn(II) once played a key role as an electron donor for photosynthesis prior to the evolution of oxygenic photosynthesis. 

To test this hypothesis, we examined the behavior of the ancient Mn cycle using newly obtained scientific drill cores through an early Paleoproterozoic succession of sedimentary rocks (2.415 Ga) preserved in South Africa. These strata contain the oldest substantial Mn enrichments (up to ∼17 wt %), well before those associated with the rise of oxygen such as the ~2.2 Ga Kalahari Mn deposit.  Using both bulk and microscale X-ray spectroscopic techniques coupled to optical and electron microscopy and stable carbon isotope ratios, we determined that the Mn is hosted exclusively in carbonate mineral phases derived from reduction of Mn oxides during diagenesis of primary sediments.

Additional observations of independent proxies for molecular oxygen—multiple S isotopes (measured in bulk by isotope-ratio mass spectrometry and in situ by secondary ion mass spectrometry) and redox-sensitive detrital grains—reveal that the original Mn-oxide phases were not produced by reactions with oxygen and instead points to a different high-potential oxidant. These results show that the oxidative branch of the Mn cycle predates the rise of oxygen, and supports the hypothesis that the water-oxidizing complex of photosystem II evolved from a former transitional photosystem capable of single-electron oxidation reactions of Mn.