Low temperature abiotic formation of mesopyrophaeophorbide a from pyrophaeophorbide a under conditions simulating anoxic natural environments

Reduction of the C-3 vinyl group to an ethyl substituent is a key, yet poorly understood, event in the diagenetic pathway linking chlorophyll a to sedimentary alkyl porphyrins. Laboratory simulation experiments employing conditions that mimic those of natural anoxic environments (aqueous media, low temperature, presence of H2S and limitation of oxygen) lead to partial conversion of the chlorophyll a derivative methyl pyrophaeophorbide a to mesopyrophaeophorbide a via reduction of the C-3 vinyl substituent mediated by H2S. The mild conditions employed, and the apparent susceptibility of methyl pyrophaeophorbide a to reductive transformation, implies that reduction mediated by H2S could be a widespread process occurring in anoxic depositional environments. Our findings have implications for the geological preservation of chlorins and other biological markers and provide clear evidence that reactions involving H2S are important in the geochemical reduction of sedimenting organic matter and can occur at temperatures within the range for natural environments.