Low temperature sequestration of photosynthetic pigments: Model studies and natural aquatic environments

A series of synthetic C-3^1 alkylthioether chlorophyll derivatives, analogous to those found to occur naturally in sediment from Pup Lagoon, Antarctica, have been formed in laboratory reactions that simulate conditions that occur in many natural anoxic aquatic environments (low temperature, moderate pH, presence of H2S and limitation of oxygen). This has enabled a mechanism to be proposed to explain the formation of these derivatives in nature and provided valuable insight into their environmental significance and the potential of chlorophylls to form sulfur-crosslinks with macromolecular organic matter. Reaction of the chlorophyll a derivative, pyrophaeophorbide a, with H2S under simulation conditions led to its partial conversion to mesopyrophaeophorbide a. The transformation, which involves reduction of the C-3 vinyl group to an ethyl substituent, is a key event in the diagenetic pathway linking chlorophylls to many sedimentary alkyl porphyrins and provides evidence for the importance of low temperature abiotic reactions, involving H2S, in the geochemical reduction of organic matter. The reaction of pyrophaeophorbide a with H2S, in the presence of molecular oxygen, led to its near quantitative conversion into pyrophaeophorbide d via oxidative cleavage of the C-3 vinyl group to afford a formyl group at C-3. Such a transformation could represent a key intermediate step in the formation of C-3 b-H and b-CH3 fossil porphyrins.

Sediment samples from an Antarctic marine core were submitted to extraction by acetone and acid methanolysis and extracts analysed by LC-MSn. Acid extraction liberated appreciable amounts of tetrapyrroles believed to have been bound via ester linkages to components of the sediment matrix. In some cases the amount of acid-extracted tetrapyrroles was over double that extracted by acetone. Significant distributional differences existed between the acetone and acid extracted components, the latter being enriched in deesterified and oxidised transformation products of chlorophyll. Acid methanolysis also revealed the presence of derivatives of chlorophylls b and c, which were lacking from acetone extracts, as well as protoporphyrin-IX, believed to originate from heme-type precursors. These findings have implications for the use of sedimentary tetrapyrroles for palaeoenvironmental reconstruction.