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The photodegradation rates of chlorophyll-a and phytol in both its esterified and free forms were followed in killed cells of Phaeodactylum tricornutum Bohlin, a diatom possessing a strong chlorophyllase activity. The Chlorophyll Phytyl side chain Photodegradation Index (CPPI), defined as the molar ratio of phytyldiol (a specific photoproduct of the phytyl chain) versus phytol, was used to estimate the chlorophyll-a photodegradation rate. Despite the strong chlorophyllase activity of this alga and the high photoreactivity of free phytol, the CPPI gave a good estimation of the chlorophyll-a photodegradation state. These estimates were good for the axenic and non-axenic incubations, since aerobic bacterial degradation processes act upon free phytyldiol and phytol without high selectivity.

3,7,11,15-Tetramethyl-2-hexadecen-1-yl propionate (a model for the intact phytyl side chain of chlorophyll), 3-hydroxy-3,7,11,15-tetramethyl-1-hexadecen-1-yl propionate and 2-hydroxy-3-methylidene-7,11,15-trimethylhexadecyl propionate (models for a photo-oxidized chlorophyll phytyl chain) were heated in the presence of montmorillonite in order to simulate their abiotic degradation under geological conditions. Thermo-catalytic breakdown of phytyl propionate yielded most of the degradation products previously generated from phytol by heating in the presence of clays. Some interesting mechanisms involving protonation of the phytol double bond are proposed to explain the formation of phytanal and prist-1-ene observed during these manipulations. The abiotic degradation of 3-hydroxy-3,7,11,15-tetramethyl-1-hexadecen-1-yl propionate led to the production of 6,10,14-trimethylpentadecan-2-one, phytenals and phytenic acids as major products, whereas 2-hydroxy-3-methylidene-7,11,15-trimethylhexadecyl propionate afforded 6,10,14-trimethylpentadecan-2-one, 3,7,11,15-tetramethyl-2-oxohexadecanol-1 and 3,7,11,15-tetramethylhexadecan-1,2,3-triol. A kinetic study allowed comparison of the abiotic stability of intact and photooxidized chlorophyll phytyl chains in sediments and suggested potential markers for monitoring chlorophyll photodegradation.

Photodegradation of chlorophyll-a in seawater leads to the formation of some relatively stable photoproducts, acidic or alkaline hydrolysis of which affords 6,10,14-trimethylpentadecan-2-one. Owing to their high polarity, these compounds are not extracted during Soxhlet extraction of sediments with chloroform, but can be recovered in part in the form of 6,10,14-trimethylpentadecan-2-one after acidic and alkaline hydrolysis of the Soxhlet extraction residues. Hence, we suggest the use of this “bound” ketone as a marker for the photodegradation of chlorophylls with a phytol ester group in the marine environment.