Spectral radiation dependent photoprotective mechanism in the diatom Pseudo-nitzschia multistriata

Abstract

Phytoplankton, such as diatoms, experience great variations of photon flux density (PFD) and light spectrum along the marine water column. Diatoms have developed some rapidly-regulated photoprotective mechanisms, such as the xanthophyll cycle activation (XC) and the non-photochemical chlorophyll fluorescence quenching (NPQ), to protect themselves from photooxidative damages caused by excess PFD. In this study, we investigate the role of blue fluence rate in combination with red radiation in shaping photoacclimative and protective responses in the coastal diatom Pseudo-nitzschia multistriata. This diatom was acclimated to four spectral light conditions (blue, red, blue-red, blue-red-green), each of them provided with low and high PFD. Our results reveal that the increase in the XC pool size and the amplitude of NPQ is determined by the blue fluence rate experienced by cells, while cells require sensing red radiation to allow the development of these processes. Variations in the light spectrum and in the blue versus red radiation modulate either the photoprotective capacity, such as the activation of the diadinoxanthin-diatoxanthin xanthophyll cycle, the diadinoxanthin de-epoxidation rate and the capacity of non-photochemical quenching, or the pigment composition of this diatom. We propose that spectral composition of light has a key role on the ability of diatoms to finely balance light harvesting and photoprotective capacity.

Figure 1. In situ light profile vs depth.

(A) PAR (µmol photons m^–2 s^–1) distribution along the water column, (B) blue, red and green radiation distribution along the water column (µW cm^–2; blue –422 to 496 nm, green –479 to 579 nm and red –589 to 656 nm), and (C) distribution of red : blue ratio along the water column. Data are mean of profiles done on 5 stations in the Mediterranean Sea in June–July 2008 (C. Brunet, unpublished data).

Figure 2. Spectral properties of the LEDs.

Blue (422−496 nm), green (480−580 nm) and red light (590−656 nm).

Figure 3. Growth curve of Pseudo-nitzschia multistriata.

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 (n = 3).

Figure 4. Variations of xanthophyll cycle pigment content.

(A) sum of diadinoxanthin and diatoxanthin ([Dd+Dt]; pg cell^–1); diadinoxanthin time distribution (Dd; pg cell^–1) in high (B) and low light (D); diatoxanthin time distribution (Dt; pg cell^–1) in high (C) and low light (E). 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. Dt content was below detectable level in R-L. Time is in hours after the start of the experiment. Data represent (A) mean ± SD (n = 21) and (B–E) mean ± SD (n = 3).

Figure 5. De-epoxidation state (DES = Dt/(Dd+Dt)) and non-photochemical quenching (NPQ).

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 (n = 3).

Figure 6. Variations of photosynthetic pigments content.

(A) Chlorophyll a (Chl a; pg cell^–1), (B) Fucoxanthin (Fuco; pg cell^–1), (C) β-carotene : Chl a ratio, (D) Chlorophyll c ₂ : Chl a ratio and (E) Chlorophyll c ₃ : Chl a 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 (n = 21).