Contents lists available at ScienceDirect Marine Genomics journal homepage: www.elsevier.com/locate/margen Molecular effects of a variable environment on Sydney rock oysters, Saccostrea glomerata: Thermal and low salinity stress, and their synergistic effect Nicole G. Ertl a,b , Wayne A. O'Connor a,c , Abigail Elizur a, ⁎ a University of the Sunshine Coast, Sippy Downs, Queensland, Australia b Australian Seafood Cooperative Research Centre, South Australia, Australia c Department of Primary Industries, New South Wales, Australia ARTICLE INFO Keywords: Mollusc RNA-Seq Stress Immunity Transporters ABSTRACT Bivalves are frequently exposed to salinity and temperature fluctuations in the estuary. This study explored the molecular effect of these fluctuations by exposing Sydney rock oysters, (Saccostrea glomerata), native to Australia, to either low salinity, elevated temperature or a combined salinity and temperature stress. Following the exposures, RNA-Seq was carried out on the collected oyster tissues. Differential transcript analysis resulted in a total of 1473, 1232 and 2571 transcripts, which were differen- tially expressed in S. glomerata exposed to low salinity (10 ppt), elevated temperature (30 °C) or the combined stressor (15 ppt and 30 °C), respectively, when compared to control oysters. All stress treatments had some effect on molecular processes such as innate immune response or respiration, with overall the strongest effects seen in S. glomerata exposed to the combined stressor. Additionally, most transporters putatively involved in osmor- egulation were found to be suppressed in response to the combined stressor and the low salinity exposure. This study provides insight into the oyster's responses to both, single and dual stressors commonly found in an estuarine environment. 1. Introduction During their life-time, oysters are exposed to a varied environment, with both, water temperature and salinity fluctuating daily and sea- sonally in their estuarine habitat (Lannig et al., 2006; Gagnaire et al., 2006; Pörtner et al., 2006). These fluctuations can arise due to tidal cycles, rainfall or terrestrial run-off (Gagnaire et al., 2006; Pörtner et al., 2006) and can have serious effects on marine invertebrates. For instance, temperature affects behaviour (e.g. valve closure), as well as molecular, biochemical and physiological processes (Pörtner et al., 2006; Boutet et al., 2009; Stillman & Somero, 2000; Anesti et al., 2007), affecting organ function and membrane fluidity, altering antioxidant enzyme activity and causing protein damage (Boutet et al., 2009; Harley et al., 2006; Monari et al., 2007). Changes in temperature and salinity have also been shown to modulate innate immunity of various marine invertebrates, such as shrimp, oysters, mussel and clams (Gagnaire et al., 2006; Monari et al., 2007; Cheng et al., 2005; Butt et al., 2006; Wang et al., 2011; Paillard et al., 2004), potentially af- fecting the resilience of these marine organisms to invading pathogens. In addition, decreased salinity and increased temperature can impact growth rate and functions such as heart rate and respiration (Heilmayer et al., 2008). These effects might be exacerbated by climate change through the predicted increases in sea surface temperatures and ex- treme rainfall events (e.g. floods), as well as changes in rainfall patterns (Przeslawski et al., 2008; CSIRO, 2014). Increases in temperature might also lead to hypoxia in marine or- ganisms such as bivalves and at the temperature extremes, these ani- mals might transit from aerobic metabolism to anaerobic metabolism due to a decrease in oxygen uptake (Lannig et al., 2006; Przeslawski et al., 2008). In regards to the effect of salinity, research has shown that bivalves such as S. glomerata are hyperosmotic to their environment and are able to osmoconform, however are unable to osmoregulate their extracellular fluid (Bedford & Anderson, 1972; Pierce, 1970; Eierman & Hare, 2014; Nell & Dunkley, 1984). During a decrease in salinity, an osmotic gradient is produced between the extracellular fluid of the oyster and the external environment that would lead to an influx in water, causing a temporary swelling of the oyster tissue cells. To counteract this, osmotic solutes and water are expelled from the cells https://doi.org/10.1016/j.margen.2018.10.003 Received 11 February 2018; Received in revised form 7 October 2018; Accepted 18 October 2018 ⁎ Corresponding author at: 90 Sippy Downs Drive, Sippy Downs QLD 4556, Australia. E-mail addresses: nertl@usc.edu.au (N.G. Ertl), AElizur@usc.edu.au (A. Elizur). Marine Genomics 43 (2019) 19–32 Available online 23 November 2018 1874-7787/ Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved. T