In Situ Measurement of Variable Fluorescence Transients

Abstract

Chlorophyll variable fluorescence provides considerable insight into the photosynthetic physiology of plants and algae, in particular the structure and function of Photosystem II (PSII). A longstanding method for measuring variable fluorescence relies on the addition of DCMU, an herbicide, which blocks electron flow through PSII and eliminates photochemistry as a quencher of fluorescence (Malkin and Kok 1966; Trebst 1980). When the photochemical pathway is blocked by DCMU a sample’s fluorescence yield F is greater than when it is not blocked, and this variable fluorescence difference between F measured before and after the addition of DCMU is a valuable indicator of photochemistry in photosynthetic organisms. Unfortunately the DCMU method is not well suited for use in the field. It is possible to measure variable fluorescence using DCMU on discrete or continuous samples of natural phytoplankton assemblages (e.g., Cullen and Renger 1979; Roy and Legendre 1979; Vincent 1981) but it is difficult to do so in situ under the ambient light and nutrient conditions that phytoplankton experience in the natural environment.