Long-term light adaptation of light-harvesting and energy-transfer processes in the glaucophyte Cyanophora paradoxa under different light conditions.

In response to fluctuation in light intensity and quality, oxygenic photosynthetic organisms modify their light-harvesting and excitation energy-transfer processes to maintain optimal photosynthetic activity. Glaucophytes, which are a group of primary symbiotic algae, possess light-harvesting antennas called phycobilisomes (PBSs) consistent with cyanobacteria and red algae. However, compared with cyanobacteria and red algae, glaucophytes are poorly studied and there are few reports on the regulation of photosynthesis in the group. In this study, we examined the long-term light adaptation of light-harvesting functions in a glaucophyte, Cyanophora paradoxa, grown under different light conditions. Compared with cells grown under white light, the relative number of PBSs to photosystems (PSs) increased in blue-light-grown cells and decreased in green-, yellow-, and red-light-grown cells. Moreover, the PBS number increased with increment in the monochromatic light intensity. More energy was transferred from PBSs to PSII than to PSI under blue light, whereas energy transfer from PBSs to PSII was reduced under green and yellow lights, and energy transfer from the PBSs to both PSs decreased under red light. Decoupling of PBSs was induced by intense green, yellow, and red lights. Energy transfer from PSII to PSI (spillover) was observed, but the contribution of the spillover did not distinctly change depending on the culture light intensity and quality. These results suggest that the glaucophyte C. paradoxa modifies the light-harvesting abilities of both PSs and excitation energy-transfer processes between the light-harvesting antennas and both PSs during long-term light adaption.

Biochemical and spectroscopic characterization of PSI-LHCI from the red alga Cyanidium caldarium.

Light-harvesting complexes (LHCs) have been diversified in oxygenic photosynthetic organisms, and play an essential role in capturing light energy which is transferred to two types of photosystem cores to promote charge-separation reactions. Red algae are one of the groups of photosynthetic eukaryotes, and their chlorophyll (Chl) a-binding LHCs are specifically associated with photosystem I (PSI). In this study, we purified three types of preparations, PSI-LHCI supercomplexes, PSI cores, and isolated LHCIs, from the red alga Cyanidium caldarium, and examined their properties. The polypeptide bands of PSI-LHCI showed characteristic PSI and LHCI components without contamination by other proteins. The carotenoid composition of LHCI displayed zeaxanthins, ß-cryptoxanthins, and ß-carotenes. Among the carotenoids, zeaxanthins were enriched in LHCI. On the contrary, both zeaxanthins and ß-cryptoxanthins could not be detected from PSI, suggesting that zeaxanthins and ß-cryptoxanthins are bound to LHCI but not PSI. A Qy peak of Chl a in the absorption spectrum of LHCI was shifted to a shorter wavelength than those in PSI and PSI-LHCI. This tendency is in line with the result of fluorescence-emission spectra, in which the emission maxima of PSI-LHCI, PSI, and LHCI appeared at 727, 719, and 677 nm, respectively. Time-resolved fluorescence spectra of LHCI represented no 719 and 727-nm fluorescence bands from picoseconds to nanoseconds. These results indicate that energy levels of Chls around/within LHCIs and within PSI are changed by binding LHCIs to PSI. Based on these findings, we discuss the expression, function, and structure of red algal PSI-LHCI supercomplexes.

Structure of a monomeric photosystem I core associated with iron-stress-induced-A proteins from Anabaena sp. PCC 7120.

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions. The cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, their binding property and functional roles in PSI are still missing. We analyzed a cryo-electron microscopy structure of a PSI-IsiA supercomplex isolated from Anabaena grown under an iron-deficient condition. The PSI-IsiA structure contains six IsiA subunits associated with the PsaA side of a PSI core monomer. Three of the six IsiA subunits were identified as IsiA1 and IsiA2. The PSI-IsiA structure lacks a PsaL subunit; instead, a C-terminal domain of IsiA2 occupies the position of PsaL, which inhibits the oligomerization of PSI, leading to the formation of a PSI monomer. Furthermore, excitation-energy transfer from IsiAs to PSI appeared with a time constant of 55 ps. These findings provide insights into both the molecular assembly of the Anabaena IsiA family and the functional roles of IsiAs.

Reversible down-regulation of photosystems I and II leads to fast photosynthesis recovery after long-term drought in Jatropha curcas.

Jatropha curcas is a drought-tolerant plant that maintains its photosynthetic pigments under prolonged drought, and quickly regains its photosynthetic capacity when water is available. It has been reported that drought stress leads to increased thermal dissipation in PSII, but that of PSI has been barely investigated, perhaps due to technical limitations in measuring the PSI absolute quantum yield. In this study, we combined biochemical analysis and spectroscopic measurements using an integrating sphere, and verified that the quantum yields of both photosystems are temporarily down-regulated under drought. We found that the decrease in the quantum yield of PSII was accompanied by a decrease in the core complexes of PSII while light-harvesting complexes are maintained under drought. In addition, in drought-treated plants, we observed a decrease in the absolute quantum yield of PSI as compared with the well-watered control, while the amount of PSI did not change, indicating that non-photochemical quenching occurs in PSI. The down-regulation of both photosystems was quickly lifted in a few days upon re-watering. Our results indicate, that in J. curcas under drought, the down-regulation of both PSII and PSI quantum yield protects the photosynthetic machinery from uncontrolled photodamage.

Structural basis for the absence of low-energy chlorophylls in a photosystem I trimer from Gloeobacter violaceus.

Photosystem I (PSI) is a multi-subunit pigment-protein complex that functions in light-harvesting and photochemical charge-separation reactions, followed by reduction of NADP to NADPH required for CO2 fixation in photosynthetic organisms. PSI from different photosynthetic organisms has a variety of chlorophylls (Chls), some of which are at lower-energy levels than its reaction center P700, a special pair of Chls, and are called low-energy Chls. However, the sites of low-energy Chls are still under debate. Here, we solved a 2.04-Å resolution structure of a PSI trimer by cryo-electron microscopy from a primordial cyanobacterium Gloeobacter violaceus PCC 7421, which has no low-energy Chls. The structure shows the absence of some subunits commonly found in other cyanobacteria, confirming the primordial nature of this cyanobacterium. Comparison with the known structures of PSI from other cyanobacteria and eukaryotic organisms reveals that one dimeric and one trimeric Chls are lacking in the Gloeobacter PSI. The dimeric and trimeric Chls are named Low1 and Low2, respectively. Low2 is missing in some cyanobacterial and eukaryotic PSIs, whereas Low1 is absent only in Gloeobacter. These findings provide insights into not only the identity of low-energy Chls in PSI, but also the evolutionary changes of low-energy Chls in oxyphototrophs.

Structure of a tetrameric photosystem I from a glaucophyte alga Cyanophora paradoxa.

Photosystem I (PSI) is one of the two photosystems functioning in light-energy harvesting, transfer, and electron transfer in photosynthesis. However, the oligomerization state of PSI is variable among photosynthetic organisms. We present a 3.8-Å resolution cryo-electron microscopic structure of tetrameric PSI isolated from the glaucophyte alga Cyanophora paradoxa, which reveals differences with PSI from other organisms in subunit composition and organization. The PSI tetramer is organized in a dimer of dimers with a C2 symmetry. Unlike cyanobacterial PSI tetramers, two of the four monomers are rotated around 90°, resulting in a completely different pattern of monomer-monomer interactions. Excitation-energy transfer among chlorophylls differs significantly between Cyanophora and cyanobacterial PSI tetramers. These structural and spectroscopic features reveal characteristic interactions and excitation-energy transfer in the Cyanophora PSI tetramer, suggesting that the Cyanophora PSI could represent a turning point in the evolution of PSI from prokaryotes to eukaryotes.

Characterization of photosystem II assembly complexes containing ONE-HELIX PROTEIN1 in Arabidopsis thaliana.

The assembly process of photosystem II (PSII) requires several auxiliary proteins to form assembly intermediates. In plants, early assembly intermediates comprise D1 and D2 subunits of PSII together with a few auxiliary proteins including at least ONE-HELIX PROTEIN1 (OHP1), OHP2, and HIGH-CHLOROPHYLL FLUORESCENCE 244 (HCF244) proteins. Herein, we report the basic characterization of the assembling intermediates, which we purified from Arabidopsis transgenic plants overexpressing a tagged OHP1 protein and named the OHP1 complexes. We analyzed two major forms of OHP1 complexes by mass spectrometry, which revealed that the complexes consist of OHP1, OHP2, and HCF244 in addition to the PSII subunits D1, D2, and cytochrome b559. Analysis of chlorophyll fluorescence showed that a major form of the complex binds chlorophyll a and carotenoids and performs quenching with a time constant of 420 ps. To identify the localization of the auxiliary proteins, we solubilized thylakoid membranes using a digitonin derivative, glycodiosgenin, and separated them into three fractions by ultracentrifugation, and detected these proteins in the loose pellet containing the stroma lamellae and the grana margins together with two chlorophyll biosynthesis enzymes. The results indicated that chlorophyll biosynthesis and assembly may take place in the same compartments of thylakoid membranes. Inducible suppression of the OHP2 mRNA substantially decreased the OHP2 protein in mature Arabidopsis leaves without a significant reduction in the maximum quantum yield of PSII under low-light conditions, but it compromised the yields under high-light conditions. This implies that the auxiliary protein is required for acclimation to high-light conditions.

Excitation-energy transfer in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120 as studied by time-resolved fluorescence spectroscopy.

Heterocysts are formed in filamentous heterocystous cyanobacteria under nitrogen-starvation conditions, and possess a very low amount of photosystem II (PSII) complexes than vegetative cells. Molecular, morphological, and biochemical characterizations of heterocysts have been investigated; however, excitation-energy dynamics in heterocysts are still unknown. In this study, we examined excitation-energy-relaxation processes of pigment-protein complexes in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120. Thylakoid membranes from the heterocysts showed no oxygen-evolving activity under our experimental conditions and no thermoluminescence-glow curve originating from charge recombination of S2QA-. Two dimensional blue-native/SDS-PAGE analysis exhibits tetrameric, dimeric, and monomeric photosystem I (PSI) complexes but almost no dimeric and monomeric PSII complexes in the heterocyst thylakoids. The steady-state fluorescence spectrum of the heterocyst thylakoids at 77 K displays both characteristic PSI fluorescence and unusual PSII fluorescence different from the fluorescence of PSII dimer and monomer complexes. Time-resolved fluorescence spectra at 77 K, followed by fluorescence decay-associated spectra, showed different PSII and PSI fluorescence bands between heterocysts and vegetative thylakoids. Based on these findings, we discuss excitation-energy-transfer mechanisms in the heterocysts.

Proteome analysis of response to different spectral light irradiation in Synechocystis sp. PCC 6803.

In cyanobacteria, it is known that the excitation ratios of photosystem (PS) I and PSII changes with the wavelength of irradiated light due to mobile phycobilisome (PBS) and spillover, affecting the photosynthetic ATP/NADPH synthesis ratio and metabolic flux state. However, the mechanisms by which these changes are controlled have not been well studied. In this study, we performed a targeted proteomic analysis of Synechocystis sp. PCC 6803 under different spectral light conditions to clarify the regulation mechanisms of mobile PBS, spillover and metabolisms under different light qualities at the protein level. The results showed an increase in the amount of proteins mainly involved in CO2 fixation under Red1 light conditions with a high specific growth rate, suggesting that the rate of intracellular metabolism is controlled by the rate of carbon uptake, not by changes in the amount of each enzyme. Correlation analysis between protein levels and PSI/PSII excitation ratios revealed that PsbQUY showed high correlations and significantly increased under Blue and Red2 light conditions, where the PSI/PSII excitation ratio was higher due to spillover. In the strains lacking the genes encoding these proteins, a decrease in the PSI/PSII excitation ratio was observed, suggesting that PsbQUY contribute to spillover occurrence. SIGNIFICANCE: In cyanobacteria, the photosynthetic apparatus's responses, such as state transition [mobile PBS and spillover], occur due to the intensity and wavelength of irradiated light, resulting in changes in photosynthetic electron transport and metabolic flux states. Previous studies have analyzed the response of Synechocystis sp. PCC 6803 to light intensity from various directions, but only spectroscopic analysis of the photosynthetic apparatus has been done on the response to changes in the wavelength of irradiated light. This study analyzed the response mechanisms of mobile PBS, spillover, photosynthetic, and metabolic systems in Synechocystis sp. PCC 6803 under six different spectral light conditions by a targeted proteomic analysis. As a result, many proteins were successfully quantified, and the metabolic enzymes and photosynthetic apparatus were analyzed using an integrated approach. Principal component and correlation analyses and volcano plots revealed that the PSII subunits PsbQ, PsbU, and PsbY have a strong correlation with the PSI/PSII excitation ratio and contribute to spillover occurrence. Thus, statistical analysis based on proteome data revealed that PsbQ, PsbU, and PsbY are involved in spillover, as revealed by spectroscopic analysis.

High-light modification of excitation-energy-relaxation processes in the green flagellate Euglena gracilis.

Photosynthetic organisms finely tune their photosynthetic machinery including pigment compositions and antenna systems to adapt to various light environments. However, it is poorly understood how the photosynthetic machinery in the green flagellate Euglena gracilis is modified under high-light conditions. In this study, we examined high-light modification of excitation-energy-relaxation processes in Euglena cells. Oxygen-evolving activity in the cells incubated at 300 µmol photons m-2 s-1 (HL cells) cannot be detected, reflecting severe photodamage to photosystem II (PSII) in vivo. Pigment compositions in the HL cells showed relative increases in 9'-cis-neoxanthin, diadinoxanthin, and chlorophyll b compared with the cells incubated at 30 µmol photons m-2 s-1 (LL cells). Absolute fluorescence spectra at 77 K exhibit smaller intensities of the PSII and photosystem I (PSI) fluorescence in the HL cells than in the LL cells. Absolute fluorescence decay-associated spectra at 77 K of the HL cells indicate suppression of excitation-energy transfer from light-harvesting complexes (LHCs) to both PSI and PSII with the time constant of 40 ps. Rapid energy quenching in LHCs and PSII in the HL cells is distinctly observed by averaged Chl-fluorescence lifetimes. These findings suggest that Euglena modifies excitation-energy-relaxation processes in addition to pigment compositions to deal with excess energy. These results provide insights into the photoprotection strategies of this alga under high-light conditions.

Role of type I NADH dehydrogenase in Synechocystis sp. PCC 6803 under phycobilisome excited red light.

Cyanobacterial type I NADH dehydrogenase (NDH-1) is involved in various bioenergetic reactions including respiration, cyclic electron transport (CET), and CO2 uptake. The role of NDH-1 is usually minor under normal growth conditions and becomes important under stress conditions. However, in our previous study, flux balance analysis (FBA) simulation predicted that the drive of NDH-1 as CET pathway with a photosystem (PS) I/PSII excitation ratio around 1.0 contributes to achieving an optimal specific growth rate. In this study, to experimentally elucidate the predicted functions of NDH-1, first, we measured the PSI/PSII excitation ratios of Synechocystis sp. PCC 6803 grown under four types of spectral light conditions. The specific growth rate was the highest and PSI/PSII excitation ratio was with 0.88 under the single-peak light at 630 nm (Red1). Considering this measured excitation ratios, FBA simulation predicted that NDH-1-dependent electron transport was the major pathway under Red1. Moreover, in actual culture, an NDH-1 deletion strain had slower growth rate than that of wild type only under Red1 light condition. Therefore, we experimentally demonstrated that NDH-1 plays an important role under optimal light conditions such as Red1 light, where Synechocystis exhibits the highest specific growth rate and PSI/PSII excitation ratio of around 1.0.

Molecular organizations and function of iron-stress-induced-A protein family in Anabaena sp. PCC 7120.

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions, and surround photosystem I (PSI) trimer with a ring formation. A cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, it is unknown how the IsiAs are associated with PSI. Here we report on molecular organizations and function of the IsiAs in this cyanobacterium. A deletion mutant of the isiA1 gene was constructed, and the four types of thylakoids were prepared from the wild-type (WT) and ΔisiA1 cells under iron-replete (+Fe) and iron-deficient (-Fe) conditions. Immunoblotting analysis exhibits a clear expression of the IsiA1 in the WT-Fe. The PSI-IsiA1 supercomplex is found in the WT-Fe, and excitation-energy transfer from IsiA1 to PSI is verified by time-resolved fluorescence analyses. Instead of the IsiA1, both IsiA2 and IsiA3 are bound to PSI monomer in the ΔisiA1-Fe. These findings provide insights into multiple-expression system of the IsiA family in this cyanobacterium.

Basic pH-induced modification of excitation-energy dynamics in fucoxanthin chlorophyll a/c-binding proteins isolated from a pinguiophyte, Glossomastix chrysoplasta.

Photosynthetic organisms have diversified light-harvesting complexes (LHCs) to collect solar energy efficiently, leading to an acquisition of their ecological niches. Herein we report on biochemical and spectroscopic characterizations of fucoxanthin chlorophyll a/c-binding protein (FCP) complexes isolated from a marine pinguiophyte Glossomastix chrysoplasta. The pinguiophyte FCP showed one subunit band in SDS-PAGE and one protein-complex band with a molecular weight at around 66 kDa in clear-native PAGE. By HPLC analysis, the FCP possesses chlorophylls a and c, fucoxanthin, and violaxanthin. To clarify excitation-energy-relaxation processes in the FCP, we measured time-resolved fluorescence spectra at 77 K of the FCP adapted to pH 5.0, 6.5, and 8.0. Fluorescence curves measured at pH 5.0 and 8.0 showed shorter lifetime components compared with those at pH 6.5. The rapid decay components at pH 5.0 and 8.0 are unveiled by fluorescence decay-associated (FDA) spectra; fluorescence decays occur in the 270 and 160-ps FDA spectra only at pH 5.0 and 8.0, respectively. In addition, energy-transfer pathways with time constants of tens of picoseconds are altered under the basic pH condition but not the acidic pH condition. These findings provide novel insights into pH-dependent energy-transfer and energy-quenching machinery in not only FCP family but also photosynthetic LHCs.

Enhancement of excitation-energy quenching in fucoxanthin chlorophyll a/c-binding proteins isolated from a diatom Phaeodactylum tricornutum upon excess-light illumination.

Photosynthetic organisms regulate pigment composition and molecular oligomerization of light-harvesting complexes in response to solar light intensities, in order to improve light-harvesting efficiency. Here we report excitation-energy dynamics and relaxation of fucoxanthin chlorophyll a/c-binding protein (FCP) complexes isolated from a diatom Phaeodactylum tricornutum grown under high-light (HL) illumination. Two types of FCP complexes were prepared from this diatom under the HL condition, whereas one FCP complex was isolated from the cells grown under a low-light (LL) condition. The subunit composition and oligomeric states of FCP complexes under the HL condition are different from those under the LL condition. Absorption and fluorescence spectra at 77 K of the FCP complexes also vary between the two conditions, indicating modifications of the pigment composition and arrangement upon the HL illumination. Time-resolved fluorescence curves at 77 K of the FCP complexes under the HL condition showed shorter lifetime components compared with the LL condition. Fluorescence decay-associated spectra at 77 K showed distinct excitation-energy-quenching components and alterations of energy-transfer pathways in the FCP complexes under the HL condition. These findings provide insights into molecular and functional mechanisms of the dynamic regulation of FCPs in this diatom under excess-light conditions.

Estimation of linear and cyclic electron flows in photosynthesis based on 13C-metabolic flux analysis.

Photosynthetic organisms produce ATP and NADPH using light as an energy source and further utilize these cofactors during metabolism. Photosynthesis involves linear and cyclic electron flows; as the cyclic electron flow produces ATP more effectively than the linear electron flow without NADPH, the cell efficiently adjusts ATP and NADPH production using the two different pathways. Nevertheless, direct measurement of ATP and NADPH production during photosynthesis has been difficult. In the present study, the photosynthetic ATP and NADPH production rates of Synechocystis sp. PCC 6803 under three different single peak wavelength lights (blue: 470 nm, R630: 630 nm, and R680: 680 nm) were evaluated based on 13C-metabolic flux analysis (13C-MFA) by considering the mass balance of ATP and NADPH between photosynthesis and metabolism. The ratios of ATP/NADPH production via photosynthesis were estimated as 3.13, 1.70, and 2.10 under blue, R630, and R680 light conditions, respectively. Moreover, the linear and cyclic electron flow ratios were estimated to be 1.1-2.2, 0.2-0.5, and 0.5-1.0 under blue, R630, and R680 light conditions, respectively. The predicted linear and cyclic electron flow ratios were consistent with the excitation ratio between photosystems I and II, as observed in the steady-state fluorescence spectra.

Structure of a cyanobacterial photosystem I surrounded by octadecameric IsiA antenna proteins.

Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Here we report a 2.7-Å resolution cryo-electron microscopic structure of a supercomplex between PSI core trimer and IsiA from a thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure showed that 18 IsiA subunits form a closed ring surrounding a PSI trimer core. Detailed arrangement of pigments within the supercomplex, as well as molecular interactions between PSI and IsiA and among IsiAs, were resolved. Time-resolved fluorescence spectra of the PSI-IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex. These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI-IsiA supercomplex.

Acidic pH-Induced Modification of Energy Transfer in Diatom Fucoxanthin Chlorophyll a/c-Binding Proteins.

pH influences excitation-energy-relaxation processes in photosynthetic light-harvesting complexes. Here, we report the excitation-energy dynamics by pH changes in fucoxanthin chlorophyll a/c-binding proteins (FCPs) isolated from a diatom Phaeodactylum tricornutum, probed by time-resolved fluorescence spectroscopy at 77 K. The fluorescence curve measured at pH 5.0 showed a shorter lifetime component than that measured at pH 6.5 and 8.0. The rapid decay component at pH 5.0 is supported by fluorescence decay-associated (FDA) spectra, where strong fluorescence decays relative to fluorescence rises appear in the pH-5.0 FDA spectrum with 70 ps. These results indicate that the diatom FCPs switch their function from light-harvesting to energy-quenching via arrangements of the energy-transfer pathways under acidic pHs. Based on the crystal structure of the diatom FCPs, we propose a model for the energy-quenching machinery through structural changes of the pigment environments, thus providing insights into the pH-dependent light-harvesting strategy in the diatom FCPs.

Photoprotection mechanisms under different CO2 regimes during photosynthesis in a green alga Chlorella variabilis.

Oxygenic photosynthesis converts light energy into chemical energy via electron transport and assimilates CO2 in the Calvin-Benson cycle with the chemical energy. Thus, high light and low CO2 conditions induce the accumulation of electrons in the photosynthetic electron transport system, resulting in the formation of reactive oxygen species. To prevent the accumulation of electrons, oxygenic photosynthetic organisms have developed photoprotection mechanisms, including non-photochemical quenching (NPQ) and alternative electron flow (AEF). There are diverse molecular mechanisms underlying NPQ and AEF, and the corresponding molecular actors have been identified and characterized using a model green alga Chlamydomonas reinhardtii. In contrast, detailed information about the photoprotection mechanisms is lacking for other green algal species. In the current study, we examined the photoprotection mechanisms responsive to CO2 in the green alga Chlorella variabilis by combining the analyses of pulse-amplitude-modulated fluorescence, O2 evolution, and the steady-state and time-resolved fluorescence spectra. Under the CO2-limited condition, ΔpH-dependent NPQ occurred in photosystems I and II. Moreover, O2-dependent AEF was also induced. Under the CO2-limited condition with carbon supplementation, NPQ was relaxed and light-harvesting chlorophyll-protein complex II was isolated from both photosystems. In C. variabilis, the O2-dependent AEF and the mechanisms that instantly convert the light-harvesting functions of both photosystems may be important for maintaining efficient photosynthetic activities under various CO2 conditions.

Effects of CO2 and temperature on photosynthetic performance in the diatom Chaetoceros gracilis.

CO2 concentration and temperature for growth of photosynthetic organisms are two important factors to ensure better photosynthetic performance. In this study, we investigated the effects of CO2 concentration and temperature on the photosynthetic performance in a marine centric diatom Chaetoceros gracilis. Cells were grown under four different conditions, namely, at 25 °C with air bubbling, at 25 °C with a supplementation of 3% CO2, at 30 °C with air bubbling, and at 30 °C with the CO2 supplementation. It was found that the growth rate of cells at 30 °C with the CO2 supplementation is faster than those at other three conditions. The pigment compositions of cells grown under the different conditions are altered, and fluorescence spectra measured at 77 K also showed different peak positions. A novel fucoxanthin chlorophyll a/c-binding protein complex is observed in the cells grown at 30 °C with the CO2 supplementation but not in the other three types of cells. Since oxygen-evolving activities of the four types of cells are almost unchanged, it is suggested that the CO2 supplementation and growth temperature are involved in the regulation of photosynthetic light-harvesting apparatus in C. gracilis at different degrees. Based on these observations, we discuss the favorable growth conditions for C. gracilis.