Combined effects of hypoxia and thermal stress on hemocyte response in Pacific oysters (Crassostrea gigas): Insights from transcriptomic and proteomic analyses

Hypoxia and elevated seawater temperatures are increasingly prevalent stressors in marine ecosystems, significantly impacting the physiology of marine organisms. This study investigates the transcriptomic and proteomic responses of Pacific oyster (Crassostrea gigas) hemocytes to hypoxia alone (water temperature, 23 °C; dissolved oxygen [DO] level, 1 mg O₂/L) and combined hypoxia with high temperature (water temperature, 28 °C; DO level, 1 mg O₂/L) over a 10-day exposure period. Using RNA sequencing and liquid chromatography-mass spectrometry, we identified distinct molecular responses to these stressors. Hypoxia alone upregulated genes associated with DNA replication and protein localization, while suppressing those involved in metabolic activity, reactive oxygen species (ROS) generation, and phagosome function. Under combined hypoxia and high temperature stress, these effects were more pronounced, with enhanced downregulation of hydrolase activity and tyrosine metabolism, indicating synergistic impacts on oyster physiology. Notably, prostaglandin (PG) synthesis was specifically induced under combined stress conditions, suggesting its potential as a biomarker for environmental stress. These findings highlight how hypoxia and thermal stress interact to compromise immune function and energy metabolism in C. gigas, providing critical insights into the physiological vulnerabilities of commercially important bivalves under climate change.