Laboratory studies on the physiological effects of acidification, deoxygenation and microplastics in mussel (Mytilus galloprovincialis)

Since the industrial revolution, ocean surface water acidity has increased by about 30% (0.10 to 0.15 pH units reduction) due to anthropogenic CO2 emissions. It is estimated that the acidification in the oceans can reach 0.3 to 0.4 pH units by the end of this century. On the other hand, the ongoing global warming since the industrial revolution causes low dissolved oxygen (DO) by both decreasing the oxygen holding capacity of seawater and increasing its stratification. On a regional scale, human-induced coastal eutrophication and mucilage increase the frequency and size of coastal hypoxia events. In addition to acidification and hypoxia, another abiotic stress factor for marine organisms is microplastic pollution. Microplastics (MP) are one of the most abundant and prevalent abiotic particulate matter in the marine environment and have been shown to accumulate in the bodies of marine organisms and have toxic effects. The biological responses and adaptations of marine organisms to the combined effects of these stress factors (multiple stress) are not clearly known. The biological responses of marine species living in our country's seas to these three stressors have not yet been addressed in a laboratory study. The edible Mediterranean mussel Mytilus galloprovincialis plays a significant role in biodiversity and is a crucial bivalve species for preventing uncontrolled algal growth. In this study, the physiological responses of the mussel were investigated in low pH, low DO and MP stresses as single, double or triple for 15 days. Mussels were exposed in the laboratory to two levels of the aforementioned variables: normal and low pHT (8.09 and 7.35), normal and low DO (7.65 mg L-1 and 1.91 mg L-1), two MP levels (0 mg L-1 and 0.026 mg L-1). Those ranges are consistent with ocean acidification scenarios and coastal pH, DO, and MP concentration levels observed in the Sea of Marmara. As the experiment included two levels of these three variables, it was performed in a total of 8 aquariums and 3 replicates (8×3). DO and pH adjustment were achieved by IKS Aquastar aquarium control unit and its probes. The nutrient absorption rate was determined by the ashing method of food (microalgae) and faeces, the respiration rate was determined by the dissolved oxygen measurement in an airtight respiration chamber, the filtration rate was determined by measuring the feeding rate of mussels on microalgae, and the excretion rate was determined by spectrophotometric NH4-N measurement. Low pH, low DO and MP did not increase mortality in mussels. Low pH, low DO and MP did not cause any change in absorption rate and respiration rate. According to the regression models created; there were interactions between pH, DO and MP in respect to their effects on excretion and filtration. Either alone or in combination, low pH, low DO and MP caused significant reductions in filtration rates. In the control group (normal pH, normal DO, 0 mg L-1 MP) the filtration rate was 49.3 ± 10.9 L s-1 g-1 while in the triple stress group (low pH, low DO, 0.026 mg L-1 MP) it decreased to 14.0 ± 7.6 L s-1 g-1. As a result, it has been observed that three stress factors cause a significant decrease in feeding rate of mussels, but not in their overall physiology.