Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities

Antares I: a Modular Photobioreactor Suitable for Photosynthesis and Bioenergetics Research

Oxygenic photosynthesis is responsible for most of the fixation of atmospheric CO2. The microalgal community can transport atmospheric carbon into biological cycles in which no additional CO2 is created. This represents a resource to confront the actual climate change crisis. These organisms have evolved to adapt to several environments and different spectral distribution of light that may strongly influence their metabolism. Therefore, there is a need for development of photobioreactors specialized in addressing spectral optimization. Here, a multi-scale modular photobioreactor made from standard glass materials, ad hoc light circuits, and easily accessible, small commercial devices is described. The system is suitable to manage the principal culture variables of research in bioenergetics and photosynthesis. Its performance was tested by growing four evolutionary-distant microalgal species with different endosymbiotic scenarios: Chlamydomonas reinhardtii (Archaeplastida, green primary plastid), Polytomella parva (Archaeplastida, colorless plastid), Euglena gracilis (Discoba, green secondary plastid), and Phaeodactylum tricornutum (Stramenophiles, red secondary plastid). Our results show an improvement of biomass production, as compared to the traditional flask system. The modulation of the incident light spectra allowed us to observe a far-red adaptation in Euglena gracilis with a difference on paramylon production, and it also significantly increased the maximal cell density of the diatom species under green light. Together, these confirm that for photobioreactors with artificial light, manipulation of the light spectrum is a critical parameter for controlling the optimal performance, depending on the downstream goals.

Probing the Fucoxanthin-Chlorophyll a/c-Binding Proteins (FCPs) of the Marine Diatom Fragilariopsis sp. by Resonance Raman Spectroscopy

We report resonance Raman spectra of the light-harvesting fucoxanthin-chlorophyll a/c-binding proteins (FCPs) of marine diatom Fragilariopsis sp. The Raman shifts in the 15N-isotope-enriched diatom provide the first spectroscopic evidence for the characterization of the Ca-N marker bands and, thus, of the penta- and hexacoordinated states of chlorophylls a/c in the FCPs. Under 405 and 442 nm Raman excitations, all of the marker bands of Chl a/c are observed and the isotope-based assignments provide new information concerning the structure of Chls a/c in the FCPs and their interactions with the protein environment. Therefore, the Raman spectrum at 405 nm originates from the π-π* transitions of Chl a/c and not from a different, non π-π* electronic transition, as previously reported (BBA Bioenergetics, 2010, 1797, 1647-1656). Based on the 15N isotope shifts of the Ca-N and in conjunction with other marker bands, two distinct conformations of five- and six-coordinated Chl a and Chl c are observed. In addition, two keto carbonyls were observed at 1679 (strong H-bonded) and 1691 cm-1 (weak H-bonded) in both the 405 and 442 nm Raman spectra, respectively. Collectively, the results provide solid evidence of the nature of the vibrational modes of the active Chl a/c photosynthetic pigments in the FCPs.

Simultaneous Application of Red and Blue Light Regulate Carbon and Nitrogen Metabolism, Induces Antioxidant Defense System and Promote Growth in Rice Seedlings under Low Light Stress

The purpose of this study is to determine the effect of light quality on growth, carbon and nitrogen metabolism, and antioxidant defense system of rice seedlings. Six light conditions were employed, including white (W), red (R), blue (B), combined LED of R and B at 3:1 (R3B1), combined LED of R and B at 1:1 (R1B1), as well as combined LED of R and B at 1:3 (R1B3). Combined application of red light and blue light could promote the growth of rice seedling leaves and roots under low light stress to varying degrees, increase the photosynthetic area by increasing the leaf area, improve the root characteristics by increasing the root volume, and increase the dry matter accumulation of rice seedlings. In addition, the combination of red light and blue light could increase carbon and nitrogen metabolites in rice seedling leaves, regulate the expression of genes related to carbon and nitrogen metabolism and enzyme activity, and enhance the antioxidant enzyme activity of rice seedlings. These results indicate that red light and blue light directly have synergistic effects which can regulate the carbon and nitrogen metabolism of rice seedlings, promote the morphogenesis of rice seedlings under low light stress, and promote growth, which has never been reported in previous studies. This study is a new discovery in the application of light quality in crop production and provides new avenues to enhance crop stress resistance. However, further study is needed to explore the physio-biochemical and molecular mechanisms of light quality in crop production.

Conservation of triplet-triplet energy transfer photoprotective pathways in fucoxanthin chlorophyll-binding proteins across algal lineages

Detailed information on the photo-generated triplet states of diatom and haptophyte Fucoxanthin Chlorophyll-binding Proteins (FCPs and E-FCPs, respectively) have been obtained from a combined spectroscopic investigation involving Transient Absorption and Time-Resolved Electron Paramagnetic Resonance. Pennate diatom Phaeodactylum tricornutum FCP shows identical photoprotective Triplet-Triplet Energy Transfer (TTET) pathways to the previously investigated centric diatom Cyclotella meneghiniana FCP, with the same two chlorophyll a-fucoxanthin pairs that involve the fucoxanthins in sites Fx301 and Fx302 contributing to TTET in both diatom groups. In the case of the haptophyte Emilianina huxleyi E-FCP, only one of the two chlorophyll a-fucoxanthins pairs observed in diatoms, the one involving chlorophyll a409 and Fx301, has been shown to be active in TTET. Furthermore, despite the marked change in the pigment content of E-FCP with growth light intensity, the TTET pathway is not affected. Thus, our comparative investigation of FCPs revealed a photoprotective TTET pathway shared within these classes involving the fucoxanthin in site Fx301, a site exposed to the exterior of the antenna monomer that has no equivalent in Light-Harvesting Complexes from the green lineage.

Morphological, physiological, and transcriptional responses of the freshwater diatom Fragilaria crotonensis to elevated pH conditions

Harmful algal blooms (HABs) caused by the toxin-producing cyanobacteria Microcystis spp., can increase water column pH. While the effect(s) of these basified conditions on the bloom formers are a high research priority, how these pH shifts affect other biota remains understudied. Recently, it was shown these high pH levels decrease growth and Si deposition rates in the freshwater diatom Fragilaria crotonensis and natural Lake Erie (Canada-US) diatom populations. However, the physiological mechanisms and transcriptional responses of diatoms associated with these observations remain to be documented. Here, we examined F. crotonensis with a set of morphological, physiological, and transcriptomic tools to identify cellular responses to high pH. We suggest 2 potential mechanisms that may contribute to morphological and physiological pH effects observed in F. crotonensis. Moreover, we identified a significant upregulation of mobile genetic elements in the F. crotonensis genome which appear to be an extreme transcriptional response to this abiotic stress to enhance cellular evolution rates-a process we have termed "genomic roulette." We discuss the ecological and biogeochemical effects high pH conditions impose on fresh waters and suggest a means by which freshwater diatoms such as F. crotonensis may evade high pH stress to survive in a "basified" future.

Excitation relaxation dynamics of carotenoids constituting the diadinoxanthin cycle

Carotenoids (Cars) exhibit two functions in photosynthesis, light-harvesting and photoprotective functions, which are performed through the excited states of Cars. Therefore, increasing our knowledge on excitation relaxation dynamics of Cars is important for understanding of the functions of Cars. In light-harvesting complexes, there exist Cars functioning by converting the π-conjugation number in response to light conditions. It is well known that some microalgae have a mechanism controlling the conjugation number of Cars, called as the diadinoxanthin cycle; diadinoxanthin (10 conjugations) is accumulated under low light, whereas diatoxanthin (11 conjugations) appears under high light. However, the excitation relaxation dynamics of these two Cars have not been clarified. In the present study, we investigated excitation relaxation dynamics of diadinoxanthin and diatoxanthin in relation to their functions, by the ultrafast fluorescence spectroscopy. After an excitation to the S₂ state, the intramolecular vibrational redistribution occurs, followed by the internal conversion to the S₁ state. The S₂ lifetimes were analyzed to be 175 fs, 155 fs, and 140 fs in diethyl ether, ethanol, and acetone, respectively, for diadinoxanthin, and 155 fs, 135 fs, and 125 fs in diethyl ether, ethanol, and acetone, respectively for diatoxanthin. By converting diadinoxanthin to diatoxanthin, the absorption spectra shift to longer wavelengths by 5-7 nm, and lifetimes of S₂ and S₁ states decrease by 11-13% and 52%, respectively. Differences in levels and lifetimes of excited states between diadinoxanthin and diatoxanthin are small; therefore, it is suggested that changes in the energy level of chlorophyll a are necessary to efficiently control the functions of the diadinoxanthin cycle.

Flow cytometry and 5-ethynyl-2'-deoxyuridine (EdU) labeling to detect the cell cycle dynamics of Phaeodactylum tricornutum under light

Cell cycle studies in plants and algae are highly dependent on reliable methods for detecting cellular DNA replication. With its short growth cycle and ease of genetic transformation, Phaeodactylum tricornutum is an important model organism for the study of pennate diatoms. Here we explored two different methods to detect the cell cycle of P. tricornutum, one using SYBR-green I to via flow cytometry, and the other using EdU labeling to observe cell cycle changes under fluorescence microscopy. Both EdU labeling fluorescence microscopy and SYBR-green I staining flow cytometry accurately indicated that the cells of P. tricornutum enter the G2/M phase after 12 h of light exposure. The results indicate that SYBR Green I was an adequate detection method for nuclear DNA quantitation in cells of P. tricornutum using a flow cytometer and without RNase A treatment. In addition, EdU can be applied to P. tricornutum to reliably detect cell proliferation. Besides, Mg-ProtoIX was able to reverse the cell cycle division inhibition of P. tricornutum and allow the nuclear DNA replication to proceed normally. Taken together, the photoperiodic division time point was clearly identified, which sheds light on the regulation of cell division mechanism in P. tricornutum.

Evidence for reversible light-dependent transitions in the photosynthetic pigments of diatoms

Marine diatoms contribute to oxygenic photosynthesis and carbon fixation and handle large changes under variable light intensity on a regular basis. The unique light-harvesting apparatus of diatoms are the fucoxanthin–chlorophyll a/c-binding proteins (FCPs). Here, we show the enhancement of chlorophyll a/c (Chl a/c), fucoxanthin (Fx), and diadinoxanthin (Dd) marker bands in the Raman spectra of the centric diatom T. pseudonana, which allows distinction of the pigment content in the cells grown under low- (LL) and high-light (HL) intensity at room temperature. Reversible LL–HL dependent conformations of Chl c, characteristic of two conformations of the porphyrin macrocycle, and the presence of five- and six-coordinated Chl a/c with weak axial ligands are observed in the Raman data. Under HL the energy transfer from Chl c to Chl a is reduced and that from the red-shifted Fxs is minimal. Therefore, Chl c and the blue-shifted Fxs are the only contributors to the energy transfer pathways under HL and the blue- to red-shifted Fxs energy transfer pathway characteristic of the LL is inactive. The results indicate that T. pseudonana can redirect its function from light harvesting to energy-quenching state, and reversibly to light-harvesting upon subsequent illumination to LL by reproducing the red-shifted Fxs and decrease the number of Dds. The LL to HL reversible transitions are accompanied by structural modifications of Chl a/c and the lack of the red-shifted Fxs.

A reversible light-intensity behavior of Dds and Fxs composition in the cells of T. pseudonana. The observed LL to HL reversible transitions are accompanied by structural modifications of Chls a/c and the lack of the red-shifted Fxs.

The Critical Studies of Fucoxanthin Research Trends from 1928 to June 2021: A Bibliometric Review

Fucoxanthin is a major carotenoid in brown macroalgae and diatoms that possesses a broad spectrum of health benefits. This review evaluated the research trends of the fucoxanthin field from 1928 to June 2021 using the bibliometric method. The present findings unraveled that the fucoxanthin field has grown quickly in recent years with a total of 2080 publications. Japan was the most active country in producing fucoxanthin publications. Three Japan institutes were listed in the top ten productive institutions, with Hokkaido University being the most prominent institutional contributor in publishing fucoxanthin articles. The most relevant subject area on fucoxanthin was the agricultural and biological sciences category, while most fucoxanthin articles were published in Marine Drugs. A total of four research concepts emerged based on the bibliometric keywords analysis: “bioactivities”, “photosynthesis”, “optimization of process’’, and “environment”. The “bioactivities” of fucoxanthin was identified as the priority in future research. The current analysis highlighted the importance of collaboration and suggested that global collaboration could be the key to valorizing and efficiently boosting the consumer acceptability of fucoxanthin. The present bibliometric analysis offers valuable insights into the research trends of fucoxanthin to construct a better future development of this treasurable carotenoid.

Regulation of Phaeodactylum plastid gene transcription by redox, light, and circadian signals

Diatoms are a diverse group of photosynthetic unicellular algae with a plastid of red-algal origin. As prolific primary producers in the ocean, diatoms fix as much carbon as all rainforests combined. The molecular mechanisms that contribute to the high photosynthetic productivity and ecological success of diatoms are however not yet fully understood. Using the model diatom Phaeodactylum tricornutum, here we show rhythmic transcript accumulation of plastid psaA, psbA, petB, and atpB genes as driven by a free running circadian clock. Treatment with the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea overrides the circadian signal by markedly downregulating transcription of psaA, petB, and atpB genes but not the psbA gene. Changes in light quantity produce little change in plastid gene transcription while the effect of light quality seems modest with only the psaA gene responding in a pattern that is dependent on the redox state of the plastoquinone pool. The significance of these plastid transcriptional responses and the identity of the underlying genetic control systems are discussed with relevance to diatom photosynthetic acclimation.

The mechanism of regulation of photosystem I cross-section in the pennate diatom Phaeodactylum tricornutum

Photosystems possess distinct fluorescence emissions at low (77K) temperature. PSI emits in the long-wavelength region at ~710-740 nm. In diatoms, a successful clade of marine primary producers, the contribution of PSI-associated emission (710-717 nm) has been shown to be relatively small. However, in the pennate diatom Phaeodactylum tricornutum, the source of the long-wavelength emission at ~710 nm (F710) remains controversial. Here, we addressed the origin and modulation of F710 fluorescence in this alga grown under continuous and intermittent light. The latter condition led to a strong enhancement in F710. Biochemical and spectral properties of the photosynthetic complexes isolated from thylakoid membranes were investigated for both culture conditions. F710 emission appeared to be associated with PSI regardless of light acclimation. To further assess whether PSII could also contribute to this emission, we decreased the concentration of PSII reaction centres and core antenna by growing cells with lincomycin, a chloroplast protein synthesis inhibitor. The treatment did not diminish F710 fluorescence. Our data suggest that F710 emission originates from PSI under the conditions tested and is enhanced in intermittent light-grown cells due to increased energy flow from the FCP antenna to PSI.

Red Light Variation an Effective Alternative to Regulate Biomass and Lipid Profiles in Phaeodactylum tricornutum

Marine water diatom Phaeodactylum tricornutum is a photosynthetic organism that is known to respond to the changing light environment and adapt to different temperatures to prevent photoinhibition and maintain its metabolic functions. The objective of the present study was to test whether light shift variations in different growth phases impact the growth and lipid metabolism of P. tricornutum. Thus, we investigated R exposure in different growth phases to find the most effective light shift condition. The results showed that substituting white light (W) by red light (R) under autotrophic conditions, a condition called red shift (RS), increased biomass and lipid content compared to levels found under continuous W or R exposure alone. We observed an increase by 2-fold biomass and 2.3-fold lipid content in RS as compared to W. No significant change was observed in the morphology of lipid droplets, but the fatty acid (FA) composition was altered. Specifically, polyunsaturated FAs were increased, whereas monounsaturated FAs decreased in P. tricornutum grown in RS compared to W control. Therefore, we propose that a light shift during the beginning of the stationary phase is a low-cost cultivation strategy to boost the total biomass and lipids in P. tricornutum.