Chlorophyll degradation during senescence and death-III: 3–10 yr experiments, implications for ETIO series generation

This report extends our previous studies by investigating oxic and anoxic alteration of chlorophyll in 13 species of microalgae incubated between 3 and 10 years. The experiments were designed to investigate the alteration of chlorophyll as cells senescence, die and decompose. They mimic the initiation of diagenesis without the intervention of (non-microbial) grazing or photochemistry. (dark) incubation provided two new observations and substantiated numerous other chlorophyll-a (CHLa) alteration reactions: (i) vinyl reduction, giving meso-CHLa and several derivatives, occurred, adding to observations on the phorbide/deoxophylloerythroetioporphyrin (DPEP) routes; (ii) room temperature anoxic incubation of a diatom (Navicula sp.) for 5.5 years resulted in production of purpurporphyrin-18 free acid (PurPorph-18-FA), a functionalized porphyrin and direct precursor of a specific C-30 β-H etioporphyrin. Oxic conditions typically led to high amounts of the allomers of CHLa, pheophorbide-a, and pheophytin-a, as well as to the appearance of purpurin-18 (PUR18) and/or its phytyl ester (PE). Mg-PUR18-PE, chlorin-p6-PE (the hydrolysis product of PUR18-PE) and two forms of chlorin-e6 (PE and free acid) were also often present. xperiments have proved quite valuable for investigation of senescence/death induced alteration of the chlorophylls. In this manner, the processes can be better understood, vs. reactions occurring during grazing (heterotrophy) and early diagenesis in sediments. Various routes to specific ETIO series porphyrins from the chlorophyll-a nucleus are suggested on the basis of these data.