Fluctuating seawater pCO2/pH induces opposing interactions with copper toxicity for two intertidal invertebrates.

Global ocean pCO2 is increasing as a result of anthropogenic CO2 emissions, driving a decline in seawater pH. However, coastal waters already undergo fluctuations in pCO2/pH conditions over far shorter timescales, with values regularly exceeding those predicted for the open ocean by the year 2100. The speciation of copper, and therefore its potential toxicity, is affected by changing seawater pH, yet little is known concerning how present-day natural fluctuations in seawater pH affect copper toxicity to marine biota. Here, we test the hypothesis that a fluctuating seawater pCO2/pH regime will alter the responses of the mussel Mytilus edulis and the ragworm Alitta virens to sub-lethal copper, compared to a static seawater pCO2/pH scenario. Mussels and worms were exposed to 0.1 and 0.25 µM copper respectively, concentrations determined to produce comparable toxicity responses in these species, for two weeks under a fluctuating 12-hour pCO2/pH cycle (pH 8.14-7.53, pCO2 445-1747 µatm) or a static pH 8.14 (pCO2 432 µatm) treatment. Mussels underwent a haemolymph acidosis of 0.1-0.2 pH units in the fluctuating treatments, alongside two-fold increases in the superoxide dismutase activity and DNA damage induced by copper, compared to those induced by copper under static pH conditions. Conversely, ragworms experienced an alkalosis of 0.3 pH units under fluctuating pH/pCO2, driven by a two-fold increase in coelomic fluid bicarbonate. This mitigated the copper-induced oxidative stress to slightly reduce both antioxidant activity and DNA damage, relative to the static pH + copper treatment. These opposing responses suggest that differences in species acid-base physiology were more important in determining toxicity responses than the pH-induced speciation change. With variability in seawater chemistry predicted to increase as climate change progresses, understanding how fluctuating conditions interact with the toxicity of pH-sensitive contaminants will become more crucial in predicting their risk to coastal biota.

Sea urchin reproductive performance in a changing ocean: poor males improve while good males worsen in response to ocean acidification.

Ocean acidification (OA) is predicted to be a major driver of ocean biodiversity change. At projected rates of change, sensitive marine taxa may not have time to adapt. Their persistence may depend on pre-existing inter-individual variability. We investigated individual male reproductive performance under present-day and OA conditions using two representative broadcast spawners, the sea urchins Lytechinus pictus and Heliocidaris erythrogramma. Under the non-competitive individual ejaculate scenario, we examined sperm functional parameters (e.g. swimming speed, motility) and their relationship with fertilization success under current and near-future OA conditions. Significant inter-individual differences in almost every parameter measured were identified. Importantly, we observed strong inverse relationships between individual fertilization success rate under current conditions and change in fertilization success under OA. Individuals with a high fertilization success under current conditions had reduced fertilization under OA, while individuals with a low fertilization success under current conditions improved. Change in fertilization success ranged from -67% to +114% across individuals. Our results demonstrate that while average population fertilization rates remain similar under OA and present-day conditions, the contribution by different males to the population significantly shifts, with implications for how selection will operate in a future ocean.

Ocean acidification buffers the physiological responses of the king ragworm Alitta virens to the common pollutant copper.

Ocean acidification (OA) has the potential to alter the bioavailability of pH sensitive metals contaminating coastal sediments, particularly copper, by changing their speciation in seawater. Hence OA may drive increased toxicity of these metals to coastal biota. Here, we demonstrate complex interactions between OA and copper on the physiology and toxicity responses of the sediment dwelling polychaete Alitta virens. Worm coelomic fluid pCO2 was not increased by exposure to OA conditions (pHNBS 7.77, pCO2 530 µatm) for 14 days, suggesting either physiological or behavioural responses to control coelomic fluid pCO2. Exposure to 0.25 µM nominal copper caused a decrease in coelomic fluid pCO2 by 43.3% and bicarbonate ions by 44.6% but paradoxically this copper-induced effect was reduced under near-future OA conditions. Hence OA appeared to 'buffer' the copper-induced acid-base disturbance. DNA damage was significantly increased in worms exposed to copper under ambient pCO2 conditions, rising by 11.1% compared to the worms in the no copper control, but there was no effect of OA conditions on the level of DNA damage induced by copper when exposed in combination. These interactions differ from the increased copper toxicity under OA conditions reported for several other invertebrate species. Hence this new evidence adds to the developing paradigm that species' physiology is key in determining the interactions of these two stressors rather than it purely being driven by the changes in metal chemistry under lower seawater pH.