| Abstract: | The biosynthesis and turnover of the pigments fucoxanthin, diadinoxanthin (DD), and diatoxanthin (DT) were studied in exponentially growing cultures of the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle to investigate the dependence of pigment turnover on algal growth rates and light intensity. 14C-bicarbonate was used as a tracer. The labeling kinetics of fucoxanthin and DT were described satisfactorily by a simple precursor-pigment model with two free parameters, the precursor and pigment turnover rate. At growth irradiances < 200 μE · m?2· s?1, labeling kinetics of DD indicated the presence of two kinetically distinct DD pools and at least one precursor pool. The average growth rate-normalized pigment turnover rate of fucoxanthin was 0. The growth rate-normalized turnover rate of DT, determined only at high light irradiances (> 200 μE·m?2·s?1), was 1.3. At high light irradiances, the growth rate-normalized turnover rate of DD was 1.8. At low light irradiances, the turnover rates of the two DD pools were 3.7 and 0, respectively. The corresponding pigment turnover times were on the order of days to weeks, depending on the growth rate of the cultures. A comparison of pigment pool sizes, pigment turnover rates, and precursor turnover rates suggests that fucoxanthin is synthesized from a pool of DD and that DD and DT are synthesized from a common precursor, possibly β-carotene. No evidence was seen for dynamic xanthophyll cycling. This suggests that the commonly known “xanthophyll cycle” is the simple unidirectional conversion of DD into DT, or of DT into DD, in response to rapid irradiance changes. |
