Biological defunctionalisation of chlorophyll in the aquatic environment II: action of endogenous algal enzymes and aerobic bacteria

In order to obtain further information about the origin of sedimentary tetrapyrroles we have studied the defunctionalisation of algal chlorophyll a in the laboratory, and monitored changes in pigment composition by LC-MS. Cells of the diatom Phaedactylum tricornutum were lysed and incubated in the dark. This resulted in the formation of high relative abundances of chlorophyllide a and smaller amounts of phaeophytin a, phaeophorbide a and pyrophaeophorbide a within 3 h. This provides further evidence that algal senescence is probably an important process for the formation of such defunctionalised products in environments where zooplankton herbivory does not occur. In addition, the transformation time approximated to zooplankton gut clearance rates, implying that the chlorophyll transformations which have been observed during zooplankton herbivory may result in part from the endogenous enzymes of the ingested alga. Degradation of the marine diatom Thallasiosira weissflogii by a mixed aerobic bacterial community was performed in a system designed to mimic sedimentation of algal cells in the marine environment in the absence of grazing. This led to a decrease in the amounts of chlorophyll a, phaeophytin a and phaeophorbide a, and the appearance of small amounts of pyrophaeophorbide a. The appearance of such defunctionalisation products, also found in senescence-type experiments, suggests that the bacterial community did not play a role in their formation through microbial transformation.