Chlorophyll degradation in heat-treated Chlorella pyrenoidosa. A flow cytometric study

Abstract

The process of chlorophyll degradation of Chlorella pyrenoidosa was studied by flow cytometry using intrinsic chlorophyll fluorescence as the gauge. The small vegetative cells of a synchronous culture were subjected to a heat pre-treatment (46.5ºC for 1 h in the dark) and cultured again under continuous illumination thereafter. The cellular chlorophylls vanished within about 24 h after the heat pre-treatment in two distinguishable stages, which corresponded to the two major stages of the present model for chlorophyll degradation. It was found that, although both stages were light-dependent, they differed in several aspects. The first stage of degradation, in which the whole cell population uniformly converted its chlorophylls into pheopigments, started immediately after light-on during recultivation, and continued even after light-off for a long period (~60 h), indicating that all cells responded to light without delay, and the process continued once triggered. The second stage of degradation, in which cells opened their chlorophyll macrocycles and lost their green color, did not start until after about 8 h of illumination, and each individual cell actually entered this process at a somewhat different time (8–24 h). In addition, the conversion of each cell was fast, but the process in the population as a whole stopped whenever illumination was off. This indicates that the second stage proceeds in a random mode, and that light must be present during the conversion of each cell. It was also found that the process of pigment breakdown could be turned on by illumination as low as 4 mol m ^–2 s ^–1 , but was abolished by a period of dark treatment, suggesting that light played a triggering role, and the ‘signal’ for chlorophyll degradation set up by the heat pre-treatment was automatically canceled after a certain time without light triggering.