Xanthophyll synthesis in diatoms: quantification of putative intermediates and comparison of pigment conversion kinetics with rate constants derived from a model

Abstract

Recently, we reported the presence of the violaxanthin-antheraxanthin-zeaxanthin cycle in diatoms, and showed that violaxanthin is the putative precursor of both diadinoxanthin and fucoxanthin in the diatom Phaeodactylum tricornutum Bohlin (M. Lohr and C. Wilhelm, 1999, Proc. Natl. Acad. Sci. USA 96: 8784-8789). In the present study, two possible intermediates in the synthesis of violaxanthin from beta-carotene were identified in P. tricornutum, namely beta-cryptoxanthin and beta-cryptoxanthin epoxide. In low light, the latter pigment prevails, but in high light beta-cryptoxanthin accumulates, probably as the result of an increased activity of the xantophyll-cycle de-epoxidase. The apparent kinetics of several xanthophyll conversion steps were determined for P. tricornutum and Cyclotella meneghiniana Kuitzing. The experimentally determined conversion rates were used to evaluate the hypothetical pathway of xanthophyll synthesis in diatoms. For this purpose a mathematical model was developed which allows the calculation of theoretical rates of pigment conversion for microalgae under steady-state growth conditions. A comparison between measured and calculated conversion rates agreed well with the proposal of a sequential synthesis of fucoxanthin via violaxanthin and diadinoxanthin. The postulation of zeaxanthin as an obligatory intermediate in the synthesis of violaxanthin, however, resulted in large discrepancies between the measured and calculated rates of its epoxidation. Instead of zeaxanthin, beta-cryptoxanthin epoxide may be involved in the biosynthesis of violaxanthin in diatoms.

Functional heterogeneity of the fucoxanthins and fucoxanthin-chlorophyll proteins in diatom cells revealed by their electrochromic response and fluorescence and linear dichroism spectra

“…Phaeodactylum tricornutum (1090-1a) and Cyclotella meneghiniana (1020-1a) cells, obtained from the Culture Collection of Algae, Göttingen (SAG, FRG), were grown in ASP-medium according to [18] using the modifications described in [19] Absorbance spectra were recorded with a Shimadzu UV-3000 spectrophotometer, in double-beam mode, in the wavelength range of 400 -750 nm. The optical pathlength was 1 cm and the bandwidth was set to 2 nm.…”

Responses ofEmiliania huxleyi(Prymnesiophyceae) to step changes in photon flux density

“…First, DD is hypothesised to be a precursor in the biosynthesis of FUCO (Goericke & Welschmeyer, 1992b;Lohr & Wilhelm, 2001). Second, DD can be de-epoxidised to DT in the xanthophyll cycle Olaizola & Yamamoto, 1994).…”

“…The biosynthetic pathway (or network) leading from -carotene to FUCO has been hypothesised to go through the intermediates zeaxanthin (ZX), antheraxanthin (AX), violaxanthin (VX) and DD in diatoms (Lohr & Wilhelm, 2001;Wilhelm et al, 2006). In diatoms, the pools of ZX, AX and ZX are typically small compared the pools of DD and DT (Lohr & Wilhelm, 2001).…”

“…In diatoms, the pools of ZX, AX and ZX are typically small compared the pools of DD and DT (Lohr & Wilhelm, 2001). This pathway has not been confirmed in prymnesiophytes.…”

“…FUCO has been hypothesised to be a precursor for hexanoyloxyfucoxanthin since the latter pigment is simply FUCO with a hexanoyl group added (Stolte et al, 2000;Lohr & Wilhelm, 2001). Alternative routes through this biosynthetic network (Fig.…”