Combined exposure to pyrene and fluoranthene and their molecular effects on the Sydney rock oyster, Saccostrea glomerata.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously detected in the water column, associated with particulate matter or in the tissue of marine organisms such as molluscs. PAH exposure and their resultant bioaccumulation in molluscs can cause a range of serious physiological effects in the affected animals. To examine the molecular response of these xenobiotics in bivalves, Sydney rock oysters (Saccostrea glomerata) were exposed to pyrene and fluoranthene for seven days. Chemical analysis of the soft-tissue of PAH stressed S. glomerata confirmed that pyrene and fluoranthene could be bioaccumulated by these oysters. RNA-Seq analysis of PAH-exposed S. glomerata showed a total of 765 transcripts differentially expressed between control and PAH-stressed oysters. Closer examination of the transcripts revealed a range genes encoding enzymes involved in PAH detoxification (e.g. cytochrome P450), innate immune responses (e.g. pathogen recognition, phagocytosis) and protein synthesis. Overall, pyrene and fluoranthene exposure appears to have resulted in a suppression of pathogen recognition and some protein synthesis processes, whereas transcripts of genes encoding proteins involved in clearance of cell debris and some transcripts of genes involved in PAH detoxification were induced in response to the stressors. Pyrene and fluoranthene exposure thus invoked a complex molecular response in S. glomerata, with results suggesting that oysters focus on removing the stressors from their system and dealing with the downstream effects of PAH exposure, potentially at the exclusion of other, less immediate concerns (e.g. protection from infection).