Synergistic Effects of Temperature and Salinity on the Gene Expression and Physiology of Crassostrea virginica

Abstract

Crassostrea virginica, the eastern oyster, forms reefs that provide critical services and benefits to the resiliency of the surrounding ecosystem. Changes in environmental conditions, including salinity and temperature, can dramatically alter the services oysters provide by affecting their population dynamics. Climate warming may further exacerbate the effects of salinity changes as precipitation events increase in frequency, intensity, and duration. Temperature and salinity independently and synergistically influence gene expression and physiology in marine organisms. We used comparative transcriptomics, physiology, and a field assessment experiment to investigate whether Louisianan oyster are changing their phenotypes to cope with increased temperature and salinity stress in Gulf of Mexico. Oysters from Sister Lake, Louisiana were exposed to fully crossed temperature (20°C and 30°C) and salinity (25ppt, 15ppt, and 7ppt) treatments. We found a higher number of genes were differentially expressed (downregulated) in response to low salinity at warmer temperatures – suggesting metabolic suppression. Gene ontology terms for ion transport and microtubule based processes were significantly enriched among upregulated genes in response to low salinity. Ion transport plays a role in osmolyte regulation which is important to maintain cell volume during low salinity. Microtubule based processes play a role in ciliary action which can improve fluid transport, prolonging aerobic metabolism and survival at low salinities. Oyster respiration rate significantly increased between 20°C and 30°C but, despite the higher energetic demands the clearance rate did not comparably increase. To investigate transcriptional differences in wild populations, we collected tissue from three locations across the Louisiana Gulf. We determined the expression levels of seven target genes and found an upregulation of genes that function in osmolyte transport, oxidative stress mediation, apoptosis, and protein synthesis at our low salinity site and sampling time point. Overall, oysters altered their phenotype more in response to low salinity at higher temperatures as evidenced by a higher number of differentially expressed genes during laboratory exposure, increased respiration (higher energetic demands), differentially expression by season and location. Warm temperatures lower the eastern oyster’s ability to cope with low salinities; the timing and length of low salinity exposure is important for understanding oyster recruitment, mortality, and growth.