Chlorophyll- a degradation during cellular senescence and death

Abstract

The present study consisted of two parts. First, baseline pigment distributions and potential analytical alterations in 29 microalgal species using ion-paired RP-HPLC/PDA methodology were established. The conversion of chlorophyll- a to chlorophyllide- a easily divided the organisms studied into high and low chlorophyllase activity groups. The idea of species specificity, as to chlorophyllase activity, was reinforced as different species in the same genus behaved differently. A novel compound, tentatively identified as 13 1-oxydeoxo-chlorophyll- a, was isolated from Euglena gracilis, Thalassiosira sp., Gymnodinium sp. and Dictyota dichotoma. The second and major portion of the study consisted of the dark aging of 9 microalgal clones under anoxic or oxic, warm or cold conditions for periods of up to 31 months. Species with very high chlorophyllase activity produced pheophorbide- a and/or pyropheophorbide- a, usually with chlorophyllide- a being observed as an intermediate. Other species produced only the isomers of pheophytin- a. Four organisms ( Anabaena flos-aquae, Closterium sp., Vaucheria sessilis and Nitzschia) were found to produce an early (2–4 months) predominance of pheophorbide- a which, as aging lengthened (4, 6 and 10+ months), gave way to pheophytin- a dominance. As pheophorbide- a is obviously not being converted to pheophytin- a, we propose that pheophorbide- a is being destroyed, possibly enzymatically, to leave pheophytin- a as a more stable survivor in these species. Chlorophyll- b degradation tracked the “ a-series”. That is, pheophytin- b or pheophorbide- b dominated according to species patterns for chlorophyll- a. In the green alga Closterium sp., having been aged for 7–31 months, a pigment (3–9% of total a-series) which we have tentatively identified as purpurin-18-phytyl ester was produced and was accompanied by relatively large amounts of pheophytin- a-allomer. We suggest that this compound, formed via the oxidative cleavage of the isocyclic ring, serves as a potential precursor to several open- β position geo-etioporphyrins. The overall degradation of chlorophyll- a and the overlap of catabolic biochemistry with true organic diagenesis is discussed.