Dicer-dependent heterochromatic small RNAs in the model diatom species Phaeodactylum tricornutum.

Diatoms are eukaryotic microalgae responsible for nearly half of the marine productivity. RNA interference (RNAi) is a mechanism of regulation of gene expression mediated by small RNAs (sRNAs) processed by the endoribonuclease Dicer (DCR). To date, the mechanism and physiological role of RNAi in diatoms are unknown. We mined diatom genomes and transcriptomes for key RNAi effectors and retraced their phylogenetic history. We generated DCR knockout lines in the model diatom species Phaeodactylum tricornutum and analyzed their mRNA and sRNA populations, repression-associated histone marks, and acclimatory response to nitrogen starvation. Diatoms presented a diversification of key RNAi effectors whose distribution across species suggests the presence of distinct RNAi pathways. P. tricornutum DCR was found to process 26-31-nt-long double-stranded sRNAs originating mostly from transposons covered by repression-associated epigenetic marks. In parallel, P. tricornutum DCR was necessary for the maintenance of the repression-associated histone marks H3K9me2/3 and H3K27me3. Finally, PtDCR-KO lines presented a compromised recovery post nitrogen starvation suggesting a role for P. tricornutum DCR in the acclimation to nutrient stress. Our study characterized the molecular function of the single DCR homolog of P. tricornutum suggesting an association between RNAi and heterochromatin maintenance in this model diatom species.

Isoprenoid biosynthesis in the diatom Haslea ostrearia.

Diatoms are eukaryotic, unicellular algae that are responsible for c. 20% of the Earth's primary production. Their dominance and success in contemporary oceans have prompted investigations on their distinctive metabolism and physiology. One metabolic pathway that remains largely unexplored in diatoms is isoprenoid biosynthesis, which is responsible for the production of numerous molecules with unique features. We selected the diatom species Haslea ostrearia because of its characteristic isoprenoid content and carried out a comprehensive transcriptomic analysis and functional characterization of the genes identified. We functionally characterized one farnesyl diphosphate synthase, two geranylgeranyl diphosphate synthases, one short-chain polyprenyl synthase, one bifunctional isopentenyl diphosphate isomerase - squalene synthase, and one phytoene synthase. We inferred the phylogenetic origin of these genes and used a combination of functional analysis and subcellular localization predictions to propose their physiological roles. Our results provide insight into isoprenoid biosynthesis in H. ostrearia and propose a model of the central steps of the pathway. This model will facilitate the study of metabolic pathways of important isoprenoids in diatoms, including carotenoids, sterols and highly branched isoprenoids.