Molecular Responses to Thermal and Osmotic Stress in Arctic Intertidal Mussels (Mytilus edulis): The Limits of Resilience

Increases in Arctic temperatures have accelerated melting of the Greenland icesheet, exposing intertidal organisms, such as the blue mussel Mytilus edulis, to high air temperatures and low salinities in summer. However, the interaction of these combined stressors is poorly described at the transcriptional level. Comparing expression profiles of M. edulis from experimentally warmed (30 °C and 33 °C) animals kept at control (23‰) and low salinities (15‰) revealed a significant lack of enrichment for Gene Ontology terms (GO), indicating that similar processes were active under all conditions. However, there was a progressive increase in the abundance of upregulated genes as each stressor was applied, with synergistic increases at 33 °C and 15‰, suggesting combined stressors push the animal towards their tolerance thresholds. Further analyses comparing the effects of salinity alone (23‰, 15‰ and 5‰) showed high expression of stress and osmoregulatory marker genes at the lowest salinity, implying that the cell is carrying out intracellular osmoregulation to maintain the cytosol as hyperosmotic. Identification of aquaporins and vacuolar-type ATPase transcripts suggested the cell may use fluid-filled cavities to excrete excess intracellular water, as previously identified in embryonic freshwater mussels. These results indicate that M. edulis has considerable resilience to heat stress and highly efficient mechanisms to acclimatise to lowered salinity in a changing world.

Discharge of drill cuttings in the Arctic seas and responses of bottom fauna: bivalve Mytilus edulis L.

Drill cuttings are the largest solid waste of drilling operations. Onsite discharge of drill cuttings into the marine environment is relevant and can be permitted under certain conditions and regulations. Environmental impact of the on-site discharge of drill cuttings is poorly understood for the Arctic shelf. A risk assessment of the discharge is required and the related biological effects are estimated in bioassays on various local species. The sublethal screening tests of water-soluble fraction of drill cutting (WSF DC) were performed on a test-organism of the Barents Sea coast, blue mussel, Mytilus edulis. Physiological (oxygen consumption and filtration rates) and behavioral (shell gaping) activity of mussels was affected by the tested WSF DC above the certain threshold concentration. In general, biological effects of WSF DC might be assumed as a toxic stress of low intensity. Behavioral responses are the most sensitive in blue mussels and behavior of this sentinel organism is to be applied as a reliable indicator of environmental stress. Environmental impact from the drill cuttings of water based drilling fluid appears to be less toxic-stressful for M. edulis and many other bottom invertebrates than it might be expected from release of some drilling fluids.