Diclofenac stress responses and biotransformation pathways in the marine diatom Phaeodactylum tricornutum

Effects of organic contaminants (OCs) on phytoplankton physiology were extensively studied in the last years while a knowledge gap exists regarding the ability of phytoplankton to transform OCs. Knowledge about biotransformation pathways in these organisms lag far behind that of other microorganisms. A better understanding of biotransformation pathways would help identify biomarkers of contaminants exposure, improve microalgae-based water remediation strategies and help better assess contaminants persistence and trophic transfer in natural aquatic environments. The present study investigated diclofenac (DCF) physiological effects, transcriptional responses and metabolism in the marine diatom Phaeodactylum tricornutum with the aim of getting an insight on detoxifications pathways. P. tricornutum did not result in significant removal capacity of DCF from the exposure medium. Bioconcentration factors varied depending on the exposure concentration (3.9 and 2.7 for 1.5 mg L ^-1 and 10 mg L ^-1 DCF respectively) but remained relatively low. DCF resulted in mild physiological effects on P. tricornutum but gene expression analysis indicated that multiple molecular functions and biological processes were altered by DCF exposure. Transcriptomic analysis suggested increased nutrients and energy requirements possibly associated with the contaminant stress and detoxification metabolism. CYP gene expression was not significantly regulated upon DCF exposure but 4’-Hydroxy Diclofenac (OH-DCF), a metabolite generally associated with CYP enzymatic activity, was detected. However, CYP gene expression was not significantly regulated upon DCF exposure. Five additional DCF metabolites with high molecular weight were detected. These metabolites were not previously described in the literature and were suggested to be generated via amino acid (or peptides) conjugation. Gene ontology analysis indicated that amino acid and peptide biosynthetic pathways were regulated upon DCF exposure supporting a possible correlation between organic contaminants detoxification responses and amino acid and protein content in phytoplankton cells. Our findings contribute to highlighting the diversity of biotransformation pathways in phytoplankton and provide mechanistic information about contaminants detoxification.