Xanthophyll cycle modulation.

<p>Evolution of diatoxanthin (Dt)/chlorophyll (Chl) <i>a</i> (in mol Dt/100 mol Chl <i>a</i>) over the light gradient, in <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h (A), 3 h (C) and 2 h kinetics of light increase (E). Values are means ± SD (<i>n</i> = 3). Relationship between Dt and diadinoxanthin (Dd)/Chl <i>a</i> (in mol pigment/100 mol Chl <i>a</i>), during the 5 h (B), 3 h (D) and 2 h kinetics of light increase (F). In (B) and (F) data measured at PFD ≤350 µmol photons m⁻² s⁻¹ (black dots, <i>n</i> = 39) and ≥500 µmol photons m⁻² s⁻¹ (white dots, <i>n</i> = 6) are discerned. In (D) data measured at PFD ≤250 µmol photons m⁻² s⁻¹ (black dots, <i>n</i> = 33), at 280 and 350 µmol photons m⁻² s⁻¹ (grey dots, <i>n</i> = 6), and at PFD ≥500 µmol photons m⁻² s⁻¹ (white dots, <i>n</i> = 6) are discerned.</p

Non-photochemical fluorescence quenching (NPQ), and relationship between NPQ formation and diatoxanthin (Dt) synthesis.

<p>Induction of NPQ over the light gradient in <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h (A), 3 h (C) and 2 h kinetics of light increase (E). Values are means ± SD (<i>n</i> = 3). Relationship (<i>n</i> = 45) between NPQ and Dt Chl <i>a</i>⁻¹ (in mol Dt/100 mol Chl <i>a</i>) in <i>P. multistriata</i> cells during the 5 h (B), 3 h (D) and 2 h kinetics of light increase (F). Black and white dots are data measured at PFD ≤280 and ≥350 µmol photons m⁻² s⁻¹, respectively.</p

Photosynthetic and physiological properties, and photosynthetic pigment content of <i>Pseudo-nitzschia multistriata</i>.

<p>The measurement of photosynthetic and physiological properties was performed on cells in the exponential growth phase, during preacclimation, the day before the experiments started. The growth rate did not change during experiments. _relETR_max, maximal relative electron transport rate (in mol e⁻ g Chl <i>a</i>⁻¹ h⁻¹); Ek, saturation light for photosynthesis (in µmol photons m⁻² s⁻¹); µ, growth rate (in d⁻¹); F_v/F_m, photosystem II maximal photochemical efficiency. Values are means ± SD (<i>n</i> = 9). Chlorophyll <i>a</i> cellular content (Chl <i>a</i>, in 10⁻¹⁶ mol Chl <i>a</i> cell⁻¹) and photosynthetic accessory pigments Chl <i>a</i>⁻¹ content (in mol pigment/100 mol Chl <i>a</i>) measurements were performed during experiments. Fuco, fucoxanthin: Chl <i>c</i>, chlorophyll <i>c</i>₁,₂,₃. Pigment data are means ± SD of the all data set (<i>n</i> = 135).</p

Influence of the kinetics of light increase on the photoprotection modulation.

<p>(A) Evolution of the number of absorbed photons per Chl <i>a</i> integrated over time (integrated absorbed light, Int Abs Light; expressed in mol photons mg Chl <i>a</i>⁻¹) over the light gradient, at the PFD peaks of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h (white dots), 3 h (black squares) and 2 h kinetics of light increase (black triangles). Induction of the sustained light-acclimated NPQ (NPQ_sl; B) and evolution of the de-epoxidation state (DES = Dt/[Dd+Dt]; C) <i>versus</i> Int Abs Light during the 5 h (white dots), 3 h (black squares) and 2 h kinetics of light increase (black triangles). Values are means ± SD (<i>n</i> = 3).</p

Preacclimation and experimental light conditions.

<p>(A) <i>Pseudo-nitzschia multistriata</i> cells were grown under a sinusoidal light regime set to peak at the PFD of 100 µmol photons m⁻² s⁻¹ (preacclimation light, PL; dashed line). After two weeks of preacclimation, cells in the exponential growth phase were shifted to three experimental light treatments, the 5 h (diel cycle-related PFD increase; B), 3 h and 2 h kinetics of light increase (mixing-related PFD increases; C and D, respectively), each characterized by light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹. In each panel, experimental light increases (solid lines) are compared to PL (dashed line). Triplicate samples were taken at three sampling time points during light increase (dots, B−D). Firstly, cultures were sampled in darkness. Then, after 3 h (5 h kinetics), 2 h (3 h kinetics), and 1.5 h (2 h kinetics), samples were taken at the PFD of 42, 123, 150, 164 and 280 µmol photons m⁻² s⁻¹ for the light condition peaking at 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, respectively. Lastly, cultures were sampled at PFD peaks.</p

Sustained light-acclimated non-photochemical fluorescence quenching (NPQ_sl).

<p>Induction of NPQ_sl over the light gradient, in <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500, and 650 µmol photons m⁻² s⁻¹, during the 5 h (A), 3 h (B) and 2 h kinetics of light increase (C). Black dots are values estimated for the first and second sampling time point, white dots are values estimated for the last sampling time point. Values are means ± SD (<i>n</i> = 3).</p

Carotenoid content of <i>Pseudo-nitzschia multistriata</i> cells.

<p>β-carotene (β-Car), violaxanthin (Vx) and zeaxanthin (Zx)/chlorophyll (Chl) <i>a</i> (in mol pigment/100 mol Chl <i>a</i>) of <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h, 3 h and 2 h kinetics of light increase (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103782#pone-0103782-g001" target="_blank">Fig. 1B−D</a>). Pigment values are means ± SD.</p

De-epoxidation state (DES = Dt/(Dd+Dt)) and non-photochemical quenching (NPQ).

<p>Time distribution of DES in (A) low and (B) high light. Time distribution of NPQ in low (C) and high light (D). B-L, BR-L, BRG-L, R-L are blue, blue-red, blue-red-green, and red low light conditions, respectively; B-H, BR-H, and BRG-H are blue, blue-red, blue-red-green high light conditions, respectively. Time is in hours after the start of the experiment. Data represent mean ± SD (<i>n</i> = 3).</p

Variations of photosynthetic pigments content.

<p>(A) Chlorophyll <i>a</i> (Chl <i>a</i>; pg cell^–1), (B) Fucoxanthin (Fuco; pg cell^–1), (C) β-carotene : Chl <i>a</i> ratio, (D) Chlorophyll <i>c</i>₂ : Chl <i>a</i> ratio and (E) Chlorophyll <i>c</i>₃ : Chl <i>a</i> ratio. B-L, BR-L, BRG-L, R-L are blue, blue-red, blue-red-green, and red low light conditions, respectively; B-H, BR-H, and BRG-H are blue, blue-red, blue-red-green high light conditions, respectively. Data represent mean ± SD (<i>n</i> = 21).</p

Growth curve of <i>Pseudo-nitzschia multistriata</i>.

<p>Growth under (A) low and (B) high light. B-L, BR-L, BRG-L, R-L are blue, blue-red, blue-red-green, and red low light conditions, respectively; B-H, BR-H, and BRG-H are blue, blue-red, blue-red-green high light conditions, respectively. Red high light prevented cell growth. Experiments were performed during the exponential phase on days 3 to 5 (B-L), 1 to 3 (R-L, BR-L, BRG-L, B-H) and 2 to 4 (BR-H, BRG-H). Data represent mean ± SD (<i>n</i> = 3).</p