Biology subjects

Tolleter, D.

Publications and source records attributed to Tolleter, D..

8 recordsLinked to original sources

bioRxiv2026

Thylakoid plasticity primes photoprotection in diatoms

For many millions of years, diatoms have been major primary producers in marine ecosystems, supporting food webs and contributing to carbon sequestration through the export of photosynthetically fixed carbon to the deep ocean, mitigating atmospheric carbon dioxide levels and global climate. A key factor in their ecological success is their ability to rapidly adapt to fluctuating light, balancing efficient light harvesting under dim conditions with energy dissipation under excess light. However, the mechanisms behind this regulation have not been fully elucidated. This study shows that several processes contribute to dissipating excess excitation energy in diatoms exposed to light stress: (i) the detachment of the oxygen-evolving complex proteins from the PSII core, leading to energy dissipation via charge recombination, (ii) Non-Photochemical Quenching (NPQ) in the antenna, i.e., thermal dissipation of excited states and (iii) excitation energy transfer from PSII to PSI via energy spillover. The latter process is observed in parallel with high light-induced topological changes of the thylakoids that may favor transient interactions between the photosystems. Along with the accumulation of light-stress-protecting carotenoids on longer time-scales, this ensemble of photoprotective mechanisms provides a dynamic and flexible response to the light challenges expected in coastal marine habitats.

biochemistry↗

bioRxiv2026

Dual-targeted NADK2 Links Mitochondrial Redox Homeostasis to Carbon Partitioning and Heterotrophic Growth in Chlamydomonas reinhardtii

NAD kinases (NADKs) can modulate NADP(H) levels across shifting environmental conditions. In Chlamydomonas reinhardtii, we localized CreNADK1 to the chloroplast, CreNADK3 to the cytoplasm, and CreNADK2 to both the chloroplast and mitochondria. Importantly, during mixotrophic (light + acetate) and photoautotrophic (light) growth, most CreNADK2 is chloroplast localized whereas an increased proportion resides in mitochondria when the cells are grown heterotrophically (dark + acetate). In Crenadk2 mutants, a diminished ability to synthesize mitochondrial NADP(H) inhibits the TCA cycle and markedly retards heterotrophic growth. In the light, photosynthesis becomes the primary energy source, and the presence of chloroplastic CreNADK1 enables near-normal growth in mutant cells, with inhibition of TCA cycle activity partially ameliorated via rerouting of acetate metabolism through the glyoxylate shunt, bypassing the TCA cycle and diminishing mitochondrial ROS production. These findings demonstrate spatial plasticity of NAD(H)Ks and the implementation of bypass metabolism to enable growth of Crenadk2 mutants.

cell biology↗

bioRxiv2026

CO2 fixation mediated by the carbon concentrating mechanism enables a rapid response to nitrogen deprivation in cyanobacteria

Cyanobacteria are leading biomass producers of the ocean whose ecological success relies on their ability to respond to dynamic availability of nutrients like CO2 and nitrogen, which require distinct adaptive mechanisms. To survive nitrogen deprivation, cyanobacteria undergo a reversible transition to a dormant mode. Under low CO2 levels, a CO2 concentrating mechanism (CCM) supports their CO2 fixation. While the CCM and nitrogen assimilation have been shown to share some regulatory pathways, how the CCM impacts the response to nitrogen deprivation remains underexplored. In this study, by using mutants of the coastal cyanobacteria Synechococcus sp. PCC 7002 lacking a CCM component, we show that the high rate of carbon fixation mediated by the CCM tunes the speed of the nitrogen deprivation response in {beta}-cyanobacteria. We first show that CCM mutants are deficient in inducing their typical nitrogen deprivation response under atmospheric CO2. However, at higher CO2 concentrations, the CCM mutants induce the nitrogen deprivation response. By combining Rubisco kinetics modeling with measurement of the response speed to nitrogen in various CO2 concentrations, we show that the speed of the nitrogen deprivation response increases linearly with Rubiscos carboxylation rate. We further reveal that the regulation of nitrogen response by the CCM is also present in the distantly related freshwater cyanobacteria Synechococcus elongatus PCC 7942, suggesting a widespread role of this regulation across {beta}-cyanobacteria. This study demonstrates that CO2 fixation by the cyanobacterial CCM is a key regulator of the nitrogen deprivation response, favoring a rapid response to dynamic environments.

plant biology↗

bioRxiv2025

A genome-wide CRISPR screen reveals how diatoms thrive in dynamic light

Diatoms are a highly diverse group of phytoplankton that have a large impact on global primary production and carbon sequestration in the ocean1,2. However, they are evolutionarily divergent from model phototrophs of the green lineage, and limited screening tools have hampered discovery of unique diatom biology. To address this challenge, we developed a genome-wide CRISPR/Cas9 screen in the model marine diatom, Phaeodactylum tricornutum. The screen was applied to identify genes required for survival in different light regimes, including both high light and fluctuating light. We identified a broad set of uncharacterized genes, providing the foundation for mechanistic studies of diatom adaptation to dynamic light. Among these genes, we demonstrated that the red lineage-exclusive gene STROBE1 is a new potentiator of cyclic electron flow (CEF) required for CEF to generate a trans-thylakoid proton gradient3,4. As dynamic light conditions are common in marine environments, STROBE1 and other genes identified in this screen may contribute to the broad ecological success of diatoms5,6. This genome-wide genetic screen in P. tricornutum will accelerate the unbiased discovery of novel gene functions in these ecologically important organisms.

microbiology↗

bioRxiv2025

Sustaining the cell energy in dynamic environments requires photosynthetic electron flows with diverse bandwidths

All living cells depend on the dynamic balance between their energy supply and demand to survive and thrive in dynamic environments. In extreme cases, like for photosynthetic organisms, their energy source, light, can fluctuate dramatically in intensity over timescales of seconds to hours. While various photosynthetic electron flows (EF) are crucial for maintaining bioenergetic homeostasis, how EFs are modulated to respond to dynamic energy intake remains unclear. Here, we show in the model green alga Chlamydomonas reinhardtii that each EF is best suited to a specific domain of energetic fluctuation periodicity for which it can support the cells energetic needs, which we term bandwidth. By systematically exposing cells to a range of light periodicities, we show that while cyclic EF has a large bandwidth, pseudo-cyclic EF (PCEF) can only sustain the cells energetic needs for fast light fluctuations, and that the interplay between the chloroplast and the mitochondria (CMEF) has a limited bandwidth. We further show that the bandwidths of PCEF and CMEF, specialized for dynamic lights, are related to their capacity to generate ATP and protect the photosynthetic apparatus. Finally, we show that in wild-type cells, the activity level of PCEF matches its bandwidth, and we propose that cells tune the relative activity of AEFs depending on the light fluctuation frequency. Our work opens an avenue of research to characterize the molecular mechanisms that can sustain phototrophic growth in complex and dynamic energetic landscapes. It further provides a generalizable framework for understanding the physiological importance of molecular mechanisms in a dynamic environment.

plant biology↗

bioRxiv2024

Photoreceptor-induced LHL4 protects photosystem II in Chlamydomonas reinhardtii

Photosynthesis, the fundamental process using light energy to convert CO2 to organic matter, is vital for life on Earth. It relies on capturing light through light-harvesting complexes in evolutionarily well-conserved photosystems (PS) I and II and on the conversion of light energy into chemical energy. Composition and organization of both photosystem core complexes are well conserved across evolution. PSII is particularly sensitive to photodamage but benefits from a large diversity of photoprotective mechanisms, finely tuned for the specific light conditions. Light Harvesting Complex protein family members (LHC and LHC-like families) have acquired a dual function during evolution. Members of the LHC antenna complexes of photosystems capture light energy whereas others dissipate excess energy that cannot be harnessed for photosynthesis. This process mainly occurs through non photochemical quenching (NPQ). In this work, we focus on the LHL4 protein, which is a LHC-like protein induced by UV-B and blue light photoreceptor signaling pathways in the model green microalgae Chlamydomonas reinhardtii. We demonstrate that alongside established NPQ effectors, LHL4 plays a key role in photoprotection, preventing singlet oxygen accumulation in PSII and promoting cell survival upon light stress. LHL4 protective function is distinct from that of NPQ-related proteins, as it specifically and uniquely binds to the transient monomeric form of the core PSII complex, safeguarding its integrity. LHL4 characterization expands our understanding of the interplay between light harvesting and photoprotection mechanisms upon light stress in photosynthetic microalgae.

plant biology↗

bioRxiv2021

Photosynthetic light harvesting and thylakoid organization in a CRISPR/Cas9 Arabidopsis thaliana LHCB1 knockout mutant.

Light absorbed by chlorophylls of photosystem II and I drives oxygenic photosynthesis. Light-harvesting complexes increase the absorption cross-section of these photosystems. Furthermore, these complexes play a central role in photoprotection by dissipating the excess of absorbed light energy in an inducible and regulated fashion. In higher plants, the main light-harvesting complex is the trimeric LHCII. In this work, we used CRISPR/Cas9 to knockout the five genes encoding LHCB1, which is the major component of the trimeric LHCII. In absence of LHCB1 the accumulation of the other LHCII isoforms was only slightly increased, thereby resulting in chlorophyll loss leading to a pale green phenotype and growth delay. Photosystem II absorption cross-section was smaller while photosystem I absorption cross-section was unaffected. This altered the chlorophyll repartition between the two photosystems, favoring photosystem I excitation. The equilibrium of the photosynthetic electron transport was partially maintained by a lower photosystem I over photosystem II reaction center ratio and by the dephosphorylation of LHCII and photosystem II. Loss of LHCB1 altered the thylakoid structure, with less membrane layers per grana stack and reduced grana width. Stable LHCB1 knock out lines allow characterizing the role of this protein in light harvesting and acclimation and pave the way for future in vivo mutational analyses of LHCII.

plant biology↗

bioRxiv2020

Rerouting of ribosomal proteins into splicing in plant organelles

Production and expression of RNAs requires the action of multiple RNA-binding proteins (RBPs). New RBPs are most often created by novel combinations of dedicated RNA binding modules. However, recruiting existing genes to create new RBPs is also an important evolutionary strategy. In this report, we analysed the 8-member uL18 ribosomal protein family in Arabidopsis. uL18 proteins share a short structurally conserved domain that binds the 5S rRNA and allow its incorporation into ribosomes. Our results indicate that Arabidopsis uL18-like proteins are targeted to either mitochondria or chloroplasts. While two members of the family are found in organelle ribosomes, we reveal that two uL18-type proteins correspond to splicing factors that are necessary for the elimination of certain mitochondrial and plastid group II introns. These two proteins do not co-sediment with mitochondrial or plastid ribosomes but associate with the introns whose splicing they promote. Our study thus reveals that the RNA binding capacity of uL18 ribosomal proteins has been detoured to create factors facilitating the elimination of organellar introns.

molecular biology↗