Biology subjects

Joli, N.

Publications and source records attributed to Joli, N..

3 recordsLinked to original sources

bioRxiv2026

Paradoxical energetics in the polar diatom Fragilariopsis cylindrus exposed to extreme low light

In the Arctic Ocean, diatoms initiate blooms under sea ice at extreme low-light levels, yet the limits and mechanisms behind this capability remain unknown. We investigated the steady-state physiological and molecular responses of the polar diatom Fragilariopsis cylindrus across a light gradient (0.1 to 30 {micro}mol photons m-2 s-1), representative of under-ice winter to early spring conditions, and reveals distinct strategies to cope with low light at both ends of this range. While cells optimize photon capture efficiency between 3 and 15 {micro}mol photons m-2 s-1 relative to 30 {micro}mol photons m-2 s-1, this strategy collapses below 1 {micro}mol photons m-2 s-1. In this dim-light regime, cells activate non-photochemical quenching and a sustained xanthophyll cycle, which indicates a paradoxical requirement for energy dissipation despite extreme photon scarcity. While cell division arrests at 0.18 {micro}mol photons m-2 s-1, photosynthetic electron transport seems to remain possible down to 0.1 {micro}mol photons m-2 s-1, which suggests an uncoupling between photosynthesis and biomass accumulation. Crucially, this low-light regime occurs without the consumption of reserves and represents a physiological state distinct from metabolic hypometabolism in prolonged darkness. We propose that this dim-light physiological state arises when residual light absorption exceeds the energetic requirements for cellular maintenance in the absence of division. The result is a regulated imbalance dissipated through heat and carbon excretion. This mechanism allows polar diatoms to maintain a primed photosynthetic metabolism to facilitate rapid growth recovery upon the return of light after the winter solstice.

plant biology↗

bioRxiv2025

Entry and exit of Fragilariopsis cylindrus diatoms from the polar night: evidence for chromatin-mediated genome control

Regulation of the epigenome landscape plays a crucial role in adaptation to environmental changes. However, the molecular mechanisms by which epigenome landscapes are established and tuned to adjust the cellular status to environmental cues remain poorly understood, especially in unicellular photosynthetic eukaryotes. Polar microalgae must adapt to extreme variations in light and temperature for survival and therefore represent excellent models to understand the diversity of gene regulatory mechanisms, yet chromatin-based mechanisms have never been explored in these organisms. In this study, we combined genome-wide profiling of histone and DNA modifications with transcriptomic analyses of the marine polar diatom Fragilariopsis cylindrus upon entry and exit of a 3-month-long polar night. We show that, in agreement with its established role in transcription facilitation, histone H2B monobiquitination (H2Bub) is modulated at active genes upon prolonged darkness to light re-exposure. In contrast, histone H3 lysine 27 trimethylation (H3K27me3) and DNA cytosine methylation (CG methylation), two modifications typically associated with the silencing of genes or transposable elements (TEs), remained stable throughout the transitions, reinforcing their role in transcriptional repression, particularly at TEs but also at a few genes. We further demonstrate that H3K27me3 and DNA methylation are physically associated at TEs, probably reflecting a dual locking system enabling F.cylindrus cells to cope with the invasive nature of these genetic elements under extreme environmental conditions. These findings enhance our understanding of genome and epigenome regulation in diatoms in response to environmental variability and open new avenues for exploring the role played by chromatin-level regulation in the unique evolutionary history of polar unicellular eukaryotes.

genomics↗

bioRxiv2023

Hypometabolism to survive the long polar night in the diatom Fragilariopsis cylindrus

Diatoms, the major eukaryotic phytoplankton in polar regions, are essential to sustain Arctic and Antarctic ecosystems. As such, it is fundamental to understand the physiological mechanisms and associated molecular basis of their resilience to the long polar night. Here, we report an integrative approach revealing that in prolonged darkness, diatom cells enter a state of quiescence associated with reduced metabolic and transcriptional activity during which no cell division occurs. We propose that minimal energy is provided by respiration and degradation of protein, carbohydrate, and lipid stores and that homeostasis is maintained by autophagy in prolonged darkness. We also report internal structural changes that manifest the morphological acclimation of cells to darkness. Our results further indicate that immediately following a return to light, diatom cells are able to use photoprotective mechanisms and rapidly resume photosynthesis. Cell division resumed rates similar to those before darkness. Our study demonstrates the remarkable robustness of polar diatoms to prolonged darkness at low temperatures. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=160 SRC="FIGDIR/small/524047v1_ufig1.gif" ALT="Figure 1"> View larger version (55K): org.highwire.dtl.DTLVardef@1da578corg.highwire.dtl.DTLVardef@1a9ac5borg.highwire.dtl.DTLVardef@729265org.highwire.dtl.DTLVardef@1e94c18_HPS_FORMAT_FIGEXP M_FIG C_FIG TeaserTo survive the long winter, polar diatoms slow down metabolism and express genes to assure survival following return to light.

ecology↗