ONT sequencing of H3K27me3 immunoprecipitated and Input DNA from Fragilariopsis cylindrus
Polar microalgae survive extreme light and temperature conditions. The molecular mechanisms underlying these outstanding adaptive capacities remain poorly understood, including the signal integration pathways mediating genome expression regulation. In particular, epigenome control has not been explored in polar microeukaryotes. In this study, we combined time-resolved genome-wide profiling of histone and DNA modifications with transcriptomic analyses of the marine polar diatom Fragilariopsis cylindrus upon entry in and exit out of a 3-month-long polar night.  Histone H3 lysine 27 trimethylation (H3K27me3) and DNA cytosine methylation, two chromatin hallmarks of epigenetic silencing, are stably enriched at transposable elements and specific repressed gene sets throughout the transitions. Using Nanopore sequencing of H3K27me3-immunoprecipitated DNA, we provide direct physical proof that histone H3K27me3 and methylated cytosines co-occur on the same nucleosomes, likely reflecting a dual locking system that enables F. cylindrus cells to cope with the invasive nature of transposable elements in stress conditions. These findings provide the first insights into genome and epigenome regulation in polar organisms and open new avenues for exploring the role played by chromatin-level regulation in the unique evolutionary history of polar microalgae.