Ocean acidification impacts on sperm mitochondrial membrane potential bring sperm swimming behaviour near its tipping point

Broadcast spawning marine invertebrates are susceptible to environmental stressors such as climate change, as their reproduction depends on the successful meeting and fertilization of gametes in the water column. Under near-future scenarios of ocean acidification, the swimming behaviour of marine invertebrate sperm is altered. We tested whether this was due to changes in sperm mitochondrial activity by investigating the effects of ocean acidification on sperm metabolism and swimming behaviour in the sea urchin Centrostephanus rodgersii. We used a fluorescent molecular probe (JC-1) and flow cytometry to visualize mitochondrial activity (measured as change in mitochondrial membrane potential, MMP). Sperm MMP was significantly reduced in delta pH -0.3 (35% reduction) and delta pH -0.5 (48% reduction) treatments, whereas sperm swimming behaviour was less sensitive with only slight changes (up to 11% decrease) observed overall. There was significant inter-individual variability in responses of sperm swimming behaviour and MMP to acidified seawater. We suggest it is likely that sperm exposed to these changes in pH are close to their tipping point in terms of physiological tolerance to acidity. Importantly, substantial inter-individual variation in responses of sperm swimming to ocean acidification may increase the scope for selection of resilient phenotypes, which, if heritable, could provide a basis for adaptation to future ocean acidification.

散播式产卵的海洋无脊椎动物极易受到气候变化等环境胁迫因子的影响，因其繁殖依赖于配子在水层中的成功相遇与受精。在近未来海洋酸化的情景下，海洋无脊椎动物精子的游泳行为会发生改变。为验证该现象是否由精子线粒体活性变化所介导，本研究以海胆（Centrostephanus rodgersii）为实验材料，探究了海洋酸化对其精子代谢与游泳行为的影响。本研究采用荧光分子探针（JC-1）与流式细胞术，通过检测线粒体膜电位（mitochondrial membrane potential, MMP）的变化来表征线粒体活性。在pH变化量为-0.3（MMP降幅达35%）与-0.5（MMP降幅达48%）的处理组中，精子的MMP水平显著下降；而精子游泳行为的敏感性较低，整体仅出现小幅变化（最大降幅达11%）。精子游泳行为与MMP对酸化海水的响应存在显著的个体间差异。本研究推测，暴露于该pH变化环境中的精子，在酸度生理耐受方面可能已接近其临界阈值。值得注意的是，精子游泳行为对海洋酸化的响应存在显著个体差异，这可能拓宽了抗逆表型的选择空间；若该差异可遗传，则可为未来海洋酸化的适应性进化提供基础。