The influence of microorganisms on chlorophyll a degradation in the marine environment

This study investigated the action of natural microorganisms on the degradation (diagenesis) of chlorophyll a (Chl a). Unialgal cultures of the diatom Chaetocerus gracilis and the eustigmatophyte Nannochloropsis occulata were incubated with or without the addition of natural marine microorganisms in oxic (air) or anoxic (nitrogen sparge) conditions. Microorganisms were obtained from seawater recovered just above the sediment -water interface of a coastal lagoon of southeastern Florida. Incubations were over periods of 2, 4, 8 and 24 weeks. Pigments were analysed by high-performance liquid chromatography. The most rapid destruction of Chl a and its derivatives occurred in oxic conditions with added microorganisms. The next fastest rates of destruction were observed in oxic conditions without such additions. N. occulata and C. gracilis incubated under oxic conditions without added microorganisms were found to contain almost 50% or 25% of total chloropigments, respectively, as Chl a allomers after 24 weeks. Allomer formation is taken as a prelude to purpurin/chlorin formation and potential macrocyclic destruction. Pyropheophytin a and steryl chlorin (phorbide) esters were observed to form significantly under anoxic conditions, but only to a minor extent under oxic conditions, in cultures incubated with added microorganisms. The formation of the steryl chlorin esters was observed to occur in concert (R-2 > 0.98) with the disappearance of pyropheophorbide a and pyropheophytin a. The direct intervention of microorganisms in the course of diagenetic processing of Chl a is concluded. Thus, cellular senescence, grazing-induced cell disruption, physicochemical environment (O-2 and so on), and microbial processing all need to be considered in order to fully address chlorophyll digenesis.