Comparison of the photodegradation rates of chlorophyll chlorin ring and phytol side chain in phytodetritus: is the phytyldiol versus phytol ratio (CPPI) a new biogeochemical index?

Rates of photodegradation of chlorophylls in their entirety and of chloroph yll phytyl chains, and time evolution of the Chlorophyll Phytyl side-Chain Photodegradation Index (CPPI) under visible light exposure were determined and compared in laboratory experiments. Killed phytoplanktonic cells of two widespread Bacillariophyceae (Thalassiosira rotula Meun. and Skeletonema c ostatum (Grev.) Cleve), one widespread Prymnesiophyceae (Emiliania huxleyi (Lohm.) May and Mohler) and one Chlorophyceae (Dunaliella tertiolecta Butch ) were used as source of pigments (chlorophyll a, chlorophyll b and phaeoph ytin a). Chlorophyll and chlorophyll phytyl chain photodegradation and CPPI evolution showed apparent first-order kinetics with respect to light expos ure. The in vitro determined degradation rates were approximately 3.5 times higher for the chlorophyll tetrapyrrolic structure than for the phytyl cha in. Few variations of the relative photodegradation constant rates were obs erved between strains: less than 34% for chlorophyll a and 25% for chloroph yll phytyl chain at 20 degrees C. Nonetheless, the photo-oxidation process of the phytyl chain (Q(10) = 2.0) seems more affected by temperature than d oes chlorophyll a (Q(10) = 1.4). Phytol loss in the dark controls have been attributed to bacterial degradation, and, in the case of S. costatum, chlo rophyllase dephytylation seems to have enhanced the potential for bacterial biodegradation. From these results, it clearly appears that in small detri tal particles not only chlorophyll a, chlorophyll b and phaeophytin but als o their phytyl chains may be rapidly photodegraded in the upper portion of the euphotic zone. The use of the CPPI for monitoring the chlorophyll photo -oxidation process in the aquatic environment is discussed. (C) 1999 Elsevi er Science B.V. All rights reserved.