Transcriptome-wide responses of marine snow aggregates of the diatom Odontella aurita to oil

Diatom aggregates played an important role in the formation of marine oil snow after the Deepwater Horizon oil spill, and the transport of oil to sediments in the Gulf of Mexico. Although many factors are thought to influence the formation of marine oil snow by diatoms, the molecular response of phytoplankton aggregates to oil is poorly characterized. Here we use RNAseq to analyze the transcriptome-wide responses of aggregates of the marine diatom Odontella aurita exposed to the water accommodated fraction of two different types of oil, Macondo surrogate and Refugio Beach oil, in a roller table experiment. We identified a common set of 353 genes that are differentially expressed under both Macondo and Refugio oil exposure relative to the control. Oil exposure leads to a down-regulation of genes associated with photosynthetic processes, the photosynthetic apparatus, nuclear and ribosomal processes and an up-regulation of genes related to cellular stress and potentially the production of exopolymeric substances. Differential expression was often higher in magnitude in the Refugio than the Macondo oil treatment, which may be due to differences in oil concentration in the treatments or the composition of the oils. Differential gene expression and gene ontology enrichment indicates that Refugio oil induces more severe nucleolar stress and more damage to chloroplasts than the lighter Macondo oil, while Macondo oil exposure triggers the up-regulation of more stress response genes. Better understanding of the responses to oil toxicity in aggregate-forming diatoms can help predict the fate of oil after a spill.