Phytoplankton adaptation to marine ecosystems: insights into photophysiological functional diversity

The aim of this study was to investigate the diversity of the photoprotective responses activated by phytoplankton at short temporal scales, i.e. the non-photochemical quenching of Chl a fluorescence (NPQ) associated with the xanthophyll cycle (XC) activity. In this work, we tested the hypothesis that the process of photoprotection, in terms of modulation, extent and efficiency, might be considered as a functional trait in phytoplankton. This ecophysiological work has been performed on four species belonging to the phylum of Stramenopiles. The ecological diversity has been taken into account by studying the photoresponses of species isolated either in surface or deeper water layers, as well as of species adapted to grow either in oceanic or coastal or upwelling ecosystems. Species distinct photoresponses have been also assessed in relation to cell size constraints, using one microplanktonic and three picoplanktonic species. The relation between phytoplankton ecophysiological functional diversity and ecological adaptation has been studied in three picoeukaryotes (< 3.0 μm) isolated from distinct ecosystems, through the analysis of the physiological response curves obtained by plotting a measured variable against a range of a factor (i.e. light) affecting that variable. The photophysiological flexibility as a functional trait has been discussed in relation to niche adaptation. Because of their peculiar set of biological and ecological features, related to their minute cell size, and because of their very narrow size range, picoeukaryotes are interesting model organisms to address (and answer) questions concerning the ecophysiology of algae. The integrative physiological processes (i.e. photoacclimation, photoregulation, photosynthesis, growth), which are in part activated or modified in relation to changing light conditions, have been deeply investigated in the microplanktonic coastal diatom, Pseudo-nitzschia multistriata (Bacillariophyceae). A great plasticity in functioning and regulation has been shown to characterize the photoprotective/photoacclimative processes (XC and NPQ) of this diatom, in agreement with its ecological properties

The Velocity of Light Intensity Increase Modulates the Photoprotective Response in Coastal Diatoms

International audienceIn aquatic ecosystems, the superimposition of mixing events to the light diel cycle exposes phytoplankton to changes in the velocity of light intensity increase, from diurnal variations to faster mixing-related ones. This is particularly true in coastal waters, where diatoms are dominant. This study aims to investigate if coastal diatoms differently activate the photoprotective responses, xanthophyll cycle (XC) and non-photochemical fluorescence quenching (NPQ), to cope with predictable light diel cycle and unpredictable mixing-related light variations. We compared the effect of two fast light intensity increases (simulating mixing events) with that of a slower increase (corresponding to the light diel cycle) on the modulation of XC and NPQ in the planktonic coastal diatom Pseudo-nitzschia multistriata. During each light treatment, the photon flux density (PFD) progressively increased from darkness to five peaks, ranging from 100 to 650 mmol photons m 22 s 21 . Our results show that the diel cycle-related PFD increase strongly activates XC through the enhancement of the carotenoid biosynthesis and induces a moderate and gradual NPQ formation over the light gradient. In contrast, during mixing-related PFD increases, XC is less activated, while higher NPQ rapidly develops at moderate PFD. We observe that together with the light intensity and its increase velocity, the saturation light for photosynthesis (Ek) is a key parameter in modulating photoprotection. We propose that the capacity to adequately regulate and actuate alternative photoprotective 'safety valves' in response to changing velocity of light intensity increase further enhances the photophysiological flexibility of diatoms. This might be an evolutionary outcome of diatom adaptation to turbulent marine ecosystems characterized by unpredictable mixing-related light changes over the light diel cycle

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

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

Biochemical and molecular properties of LHCX1, the essential regulator of dynamic photoprotection in diatoms

International audienceAbstract Light harvesting is regulated by a process triggered by the acidification of the thylakoid lumen, known as nonphotochemical “energy-dependent quenching” (qE). In diatoms, qE is controlled by the light-harvesting complex (LHC) protein LHCX1, while the LHC stress-related (LHCSR) and photosystem II subunit S proteins are essential for green algae and plants, respectively. Here, we report a biochemical and molecular characterization of LHCX1 to investigate its role in qE. We found that, when grown under intermittent light, Phaeodactylum tricornutum forms very large qE, due to LHCX1 constitutive upregulation. This “super qE” is abolished in LHCX1 knockout mutants. Biochemical and spectroscopic analyses of LHCX1 reveal that this protein might differ in the character of binding pigments relative to the major pool of light-harvesting antenna proteins. The possibility of transient pigment binding or not binding pigments at all is discussed. Targeted mutagenesis of putative protonatable residues (D95 and E205) in transgenic P. tricornutum lines does not alter qE capacity, showing that they are not involved in sensing lumen pH, differently from residues conserved in LHCSR3. Our results suggest functional divergence between LHCX1 and LHCSR3 in qE modulation. We propose that LHCX1 evolved independently to facilitate dynamic tracking of light fluctuations in turbulent waters. The evolution of LHCX(-like) proteins in organisms with secondary red plastids, such as diatoms, might have conferred a selective advantage in the control of dynamic photoprotection, ultimately resulting in their ecological success

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

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

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

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