Seiji Akimoto

Dark-heterotrophic growth allows photosynthetic pigment–protein complexes to be examined when photosynthetic energy conversion is not required for growth. A dark-adapted variant, dg211, of Leptolyngbya boryana has been reported to exhibit altered carotenoid composition, but its effect on excitation-energy transfer remains unclear. Here, we analyzed light-grown wild-type (WT-light) and dg211 (dg211-light), and dark-grown dg211 (dg211-dark) cells using immunoblotting, pigment analysis, absorption spectroscopy, 77 K steady-state and time-resolved fluorescence spectroscopy, and fluorescence decay-associated spectra. β-Carotene was detected in the WT-light and dg211-light cells but not in the dg211-dark cells, although chlorophyll a and phycobilisome-related spectral features were retained. In the dg211-dark cells, D1 accumulation was markedly reduced, whereas PsaA/B remained detectable. PSII-delayed fluorescence disappeared, while PSI-associated 729–730 nm fluorescence persisted, indicating a severe depletion of functional PSII reaction-center units with retention of PSI-associated features. Pronounced 687 nm nanosecond fluorescence was attributed mainly to prolonged excitation-energy retention in phycobilisome-related emitting states. These findings reveal substantial reorganization of photosynthetic excitation-energy transfer in dark-grown dg211.

Diatoms are red-lineage algae that utilize the light-harvesting complex (LHC) subfamily Lhcx for photoprotection via nonphotochemical quenching (NPQ); however, its evolutionary origin and molecular mechanism remain poorly understood. Through molecular phylogenetic analysis, we show that diatom Lhcxs and green algal Lhcsrs evolved from a common ancestor, with green plants subsequently acquiring them via horizontal gene transfer. To investigate the functional role of Lhcx1, we generated knockout mutants of Chaetoceros gracilis, a diatom with low Lhcx redundancy. The lhcx1 mutants nearly abolished NPQ, and time-resolved fluorescence measurements revealed that Lhcx1-mediated quenching occurs in energetically detached antenna complexes. Clear-native PAGE with Amphipol further indicated that CgLhcx1 interacts with the FCP L-dimer, functioning as a peripheral antenna for the C2S2M2 PSII–FCPII supercomplex. Notably, under high-light acclimation, lhcx1 mutants exhibited higher PSII effective quantum yields than wild type, attributable to reduced antenna size and enhanced carbon fixation capacity. The absence of NPQ accelerated high-light acclimation and was accompanied by increased xanthophyll accumulation, indicating that compensatory mechanisms can enhance overall photosynthetic efficiency. Together, these findings reveal the evolutionary origin of Lhcx/Lhcsr proteins and define the molecular basis of Lhcx1-mediated photoprotection in diatoms, providing fundamental insights into LHC-based photoprotection across photosynthetic lineages.

We analyzed the optical spectra of an electron donor-acceptor system undergoing intramolecular charge transfer (ICT) in the electronically excited state to construct the Gibbs energy surfaces associated with the ICT process. The molecular systems examined were 3,6-bis(dimethylamino)-9-(4-cyanophenyl)carbazole (BANCC) and its electron-donor part, 3,6-bis(dimethylamino)carbazole (BAC), in a series of solvents. The spectral analyses employed a model in which the reaction coordinate is governed primarily by solvent polarization, while the vibronic structure is assumed to be independent of the solvation coordinate. Two key parameters describing the Gibbs energy surfaces, i.e., the solvent reorganization energy and the Gibbs energy for reaction, were extracted from the spectral analysis. Their solvent dependence allowed us to estimate the dipole-moment difference between the locally excited and charge-transfer (CT) states, yielding values in good agreement with density functional theory predictions. We note that BANCC exhibits a direct CT absorption band in the visible region, with only a weak dependence on solvent polarity. This unusual behavior originates from a fortuitous cancellation between the solvent reorganization energy and the Gibbs energy change.

Photosynthetic organisms in aquatic environments experience rapid fluctuations in light intensity, light quality, and carbon availability, requiring tight regulation of photosynthetic energy conversion. In marine diatoms, non-photochemical quenching (NPQ), particularly energy-dependent quenching (qE), plays a central role in dissipating excess excitation energy as heat. However, excessive NPQ can reduce photosynthetic efficiency under light-limiting or carbon-rich conditions, and how this trade-off is regulated remains poorly understood. Here, we identify CgLhcf9, a previously uncharacterized light-harvesting complex (LHC) protein, as a negative regulator of qE-type NPQ in the centric diatom Chaetoceros gracilis . Expression of CgLhcf9 is induced under low red-light and high CO ₂ conditions and strongly suppressed by blue light, indicating regulation by both light quality and carbon availability. Functional analyses using CgLhcf9 knockout and overexpression lines reveal that CgLhcf9 suppresses qE: NPQ induction is enhanced in the absence of CgLhcf9, whereas its accumulation downregulates NPQ without affecting other established qE effectors, including Lhcx1 or xanthophyll cycle pigments. Notably, CgLhcf9 accumulation improves cellular growth under light-limiting conditions. These results identify CgLhcf9 as a novel LHC-type regulator that fine-tunes photosynthetic energy dissipation in response to environmental signals. Our findings establish a regulatory mechanism that balances photoprotection, electron transport, and carbon fixation, advancing our understanding of how marine diatoms optimize photosynthesis under fluctuating light and CO ₂ conditions. Significance statement: Photosynthetic microalgae must balance light-driven electron transport with carbon fixation to maximize growth under fluctuating light and CO ₂ conditions. While non-photochemical quenching (NPQ) protects photosystems from excess light, excessive NPQ can limit photosynthetic efficiency when light or carbon is limiting. Here, we identify the antenna protein CgLhcf9 as a negative regulator of energy-dependent NPQ in the marine diatom Chaetoceros gracilis . CgLhcf9 integrates light-quality and CO ₂ signals to suppress NPQ without altering canonical quenching effectors, thereby improving growth under light-limiting conditions. This study reveals a regulatory role for a light-harvesting complex protein in tuning the balance between photoprotection and photosynthetic efficiency, providing insight into how marine diatoms coordinate electron transport and carbon fixation in dynamic environments.

Acaryochloris marina (A. marina) is a distinctive cyanobacterium that contains rod-shaped phycobiliproteins (PBPs) and uses chlorophyll (Chl) d as its principal photosynthetic pigment, enabling it to absorb far-red light for photosynthesis. Under varying environmental light conditions, several cyanobacteria have been reported to exhibit decreased PBP content under red light and increased levels under green light. For A. marina, a reduction in PBP has also been observed under red and far-red light. However, the changes in light-harvesting and energy-transfer processes of A. marina in response to different light qualities remain insufficiently characterized. In the present study, A. marina cells were cultured under white, blue, green, yellow, red, or far-red LEDs. Compared with cultures grown under white-LEDs, blue-, red-, and far-red-LED-grown cells showed a decrease in the PBP/Chl d ratio, whereas green- and yellow-LED-grown cells showed an increase. The energy-transfer process of green-LED-grown cells at 77 K exhibited pronounced alterations in photosystem II. Based on these findings, we discuss the variations in light-harvesting and energy-transfer processes of A. marina under different light conditions.

Summary: Diatoms are red-lineage algae that utilize the light-harvesting complex (LHC) subfamily Lhcx for photoprotection via non-photochemical quenching (NPQ); however, its evolutionary origin and molecular mechanism remain poorly understood. Through molecular phylogenetic analysis, we show that diatom Lhcxs and green algal Lhcsrs evolved from a common ancestor, with green plants subsequently acquiring them via horizontal gene transfer. To investigate the functional role of Lhcx1, we generated knockout mutants of Chaetoceros gracilis , a diatom with low Lhcx redundancy. The lhcx1 mutants nearly abolished NPQ, and time-resolved fluorescence measurements revealed that Lhcx1-mediated quenching occurs in energetically detached antenna complexes. Clear-native PAGE with Amphipol further indicated that CgLhcx1 interacts with the FCP L-dimer, functioning as a peripheral antenna for the C ₂ S ₂ M ₂ PSII–FCPII supercomplex. Notably, under high-light acclimation, lhcx1 mutants exhibited higher PSII effective quantum yields than wild type, attributable to reduced antenna size and enhanced carbon fixation capacity. The absence of NPQ accelerated high-light acclimation and was accompanied by increased xanthophyll accumulation, indicating that compensatory mechanisms can enhance overall photosynthetic efficiency. Together, these findings reveal the evolutionary origin of Lhcx/Lhcsr proteins and define the molecular basis of Lhcx1-mediated photoprotection in diatoms, providing fundamental insights into LHC-based photoprotection across photosynthetic lineages.

Light-harvesting complexes (LHCs) are vital for photosynthesis, capturing light energy and transferring it to photosystems I and II. In diatoms, fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs) function as unique LHCs. In this study, we examined the spectral properties of untreated and aggregated FCP complexes (Untreated-FCP and Aggregated-FCP, respectively) from the diatom Phaeodactylum tricornutum. Fluorescence quantum yields and excitation-energy transfer pathways were evaluated using absolute fluorescence spectroscopy and fluorescence decay-associated (FDA) spectra. Aggregation of FCPs significantly enhanced excitation-energy quenching, with a marked decrease in fluorescence quantum yield from 37.6% in Untreated-FCP to 4.8% in Aggregated-FCP. The FDA spectra of Aggregated-FCP showed prominent fluorescence decays with relatively high amplitudes with time constants of 310 ps and 1.6 ns, reflecting distinct alterations in excitation-energy transfer among Chls upon aggregation. These changes were accompanied by long-wavelength shifts and broadening of the fluorescence-emission spectra, characteristics typically observed in aggregated LHCs in land plants. Our results suggest that the structural rearrangement of pigment molecules, driven by changes in Chl-Chl and Chl-Car interactions, underlies the observed excitation-energy quenching upon aggregation. This study provides key insights into the quenching mechanisms of diatom FCPs, offering broader implications for understanding energy regulation in photosynthetic systems.

Carbazol-3-olate photosensitizers with long-wavelength absorption and strong reducing ability have been developed.

Overwintering evergreen trees in boreal regions continuously convert absorbed light energy into heat through a process known as 'sustained thermal dissipation'. To better understand this mechanism, this study examined the alterations in the photosynthetic apparatus and transcriptomes of yew (Taxus cuspidata) leaves throughout the year, comparing sun-exposed and shaded leaves. The Y(II) parameter, conventionally used to estimate the quantum yield of photosystem II (PSII), suggests the occurrence of temperature-dependent thermal dissipation during winter. On the other hand, the levels of photosystem subunits, including the D1 subunit of the PSII reaction center, remain relatively stable year-round, suggesting that typical photoinhibition is unlikely to occur. Time-resolved chlorophyll fluorescence analysis revealed that heat dissipation at the PSII antenna is prominent in winter. Winter transcriptomes are notably characterized by a predominance of Elip transcripts encoding early light-induced protein (ELIP), which constitute 20% of the total transcripts, as deduced from RNA-seq analysis. Furthermore, ELIP protein concentration increases to nearly half that of the major light-harvesting complexes. The predicted structure of ELIP includes potential chlorophyll a and carotenoid binding sites. Considering a previous report showing ELIP's capacity for energy dissipation, these findings lead to a reevaluation of its significant role in sustained thermal dissipation.

Photosynthesis needs light energy, but that exceeding the maximal capacity of photosynthesis enhances formation of reactive oxygen species, which potentially causes photodamages. Therefore, light-harvesting complexes (Lhc) in phototrophs harbor various proteins and pigments to function in both light capture and energy dissipation. Diatom Lhcx proteins are reported to be a critical component for thermal dissipation of excess light energy, but the molecular mechanism of photoprotection is still not fully understood and the functions of each Lhcx isoform are not yet differentiated. Here, we focused on two types of Lhcx isoforms in Thalassiosira pseudonana : TpLhcx1/2, putative major components for energy-dependent fluorescence quenching (qE); and TpLhcx6_1, functionally unknown isoform uniquely conserved in Thalassiosirales. TpLhcx1/2 proteins accumulated more under high light than under low light, while the TpLhcx6_1 protein level was constitutive irrespective of light intensities and CO2 concentrations. High-light induced photodamage of photosystem II was increased in the genome-editing transformants of these Lhcx isoforms relative to the wild-type. Transformants lacking TpLhcx1/2 showed significantly lowered qE capacities, strongly suggesting that these proteins are important for the fast thermal energy dissipation. While in contrast, genome-editing transformants lacking the TpLhcx6_1 protein rather increased the qE capacity. TpLhcx6_1 transformants were further evaluated by the low-temperature time-resolved chlorophyll fluorescence measurement, showing the longer fluorescence lifetime in transformants than that in the wild type cells even at the dark-acclimated state of these cells. These results suggest that TpLhcx6_1 functions in photoprotection through non-photochemical energy dissipation in the different way from qE. One sentence summary: The marine diatom Thalassiosira pseudonana dissipates excess light energy for photoprotection via two types of mechanisms supported by different Lhc isofoms.