Influence of the diadinoxanthin pool size on photoprotection in the marine planktonic diatom Phaeodactylum tricornutum

Abstract

The pool size of the xanthophyll cycle pigment diadinoxanthin (DD) in the diatom Phaeodactylum tricornutum depends on illumination conditions during culture. Intermittent light caused a doubling of the DD pool without significant change in other pigment contents and photosynthetic parameters, including the photosystem II (PSII) antenna size. On exposure to high-light intensity, extensive de-epoxidation of DD to diatoxanthin (DT) rapidly caused a very strong quenching of the maximum chlorophyll fluorescence yield (F(m), PSII reaction centers closed), which was fully reversed in the dark. The non-photochemical quenching of the minimum fluorescence yield (F(o), PSII centers open) decreased the quantum efficiency of PSII proportionally. For both F(m) and F(o), the non-photochemical quenching expressed as F/F' - 1 (with F' the quenched level) was proportional to the DT concentration. However, the quenching of F(o) relative to that of F(m) was much stronger than random quenching in a homogeneous antenna could explain, showing that the rate of photochemical excitation trapping was limited by energy transfer to the reaction center rather than by charge separation. The cells can increase not only the amount of DT they can produce, but also its efficiency in competing with the PSII reaction center for excitation. The combined effect allowed intermittent light grown cells to down-regulate PSII by 90% and virtually eliminated photoinhibition by saturating light. The unusually rapid and effective photoprotection by the xanthophyll cycle in diatoms may help to explain their dominance in turbulent waters.

Figure 1

Fluorescence yield recordings by the pulse-amplitude-modulated (PAM) fluorometer of CL cells (A) and IL cells (B). After a few minutes of darkness (with the modulated detecting beam on), cells were illuminated for 5 min at 450 μE m⁻² s⁻¹ (between up and down arrows). At regular intervals, strong light pulses were fired to probe the fluorescence yield without photochemical quenching.

Figure 2

NPQ as a function of illumination duration for two light intensities: 450 μE m⁻² s⁻¹ (A) and 2 mE m⁻² s⁻¹(B). Quenching of F_m (circles, SV_m) and of F_o (triangles, SV_o) in CL cells (white symbols) and IL cells (black symbols). The data were obtained from measurements as in Figure 1.

Figure 3

NPQ as a function of light intensity for a fixed illumination duration of 5 min. SV_m (circles) and SV_o (triangles) for CL cells (white symbols) and IL cells (black symbols).

Figure 4

Correlation of NPQ and DT concentration. SV_m (circles) and SV_o (triangles) for CL cells (white symbols, shown enlarged in the inset) and IL cells (black symbols). Linear regressions were: SV_m(CL) = 0.95 [DT], SV_m(IL) = 1.04 [DT], SV_o(CL) = 0.39 [DT], and SV_o(IL) = 0.72 [DT] with 0.98 < R² <0.99. [DT]/[Chl a] was determined by HPLC on cells sampled at the end of the light treatment. The duration of light was varied from 30 s to 60 min and the light intensity from 0.05 to 2 mE m⁻² s⁻¹.

Figure 5

Steady-state O₂ yield per flash (Y_SS) as a function of flash intensity. Dark-adapted (SV_o = 0) CL cells (white circles) and IL cells (white triangles), CL cells at SV_o = 0.5 (black circles) and IL cells at SV_o = 1 (black triangles). SV_o was measured simultaneously to Y_SS (after 7 min of darkness) with the PAM fluorometer. The intermediary curve correspond to CL cells with a maximal SV_o value of 1.5 after cessation of illumination (15 min at 2,000 μE m⁻² s⁻¹) lowered to 0.5 min when the flash series was fired 7 min later. The curve to the right corresponded to IL cells with a maximal SV_o value of 4 after cessation of illumination (5 min at 450 μE m⁻² s⁻¹) lowered to 1, when the flash sequence was fired. Inset, Relative reciprocal light intensity needed for one-half saturation of Y_SS (I_{1/2 [DT = 0]}/I_1/2) versus F_o′/F_o.

Figure 6

Energy dissipation in the antenna of open PSII reaction centers (1 − F_o′/F_o) as a function of illumination duration. CL cells (white symbols) and IL cells (black symbols) at 450 μE m⁻² s⁻¹ (dashed lines) and 2 mE m⁻² s⁻¹ (black lines). Inset, First 5 min enlarged.

Figure 7

Photoinhibition kinetics of PSII as a function of illumination duration. CL cells (white symbols) and IL cells (black symbols) at 450 μE m⁻² s⁻¹ (squares) and 2 mE m⁻² s⁻¹ (circles). After the light treatment, cells were allowed to settle on a rate electrode for 7 min in darkness. PSII activity was estimated by measurement of the steady-state O₂ yield per flash after a sequence of twenty single-turnover flashes.