Seawater carbonate chemistry and survival, health, growth, and meat quality of black sea bream (Acanthopagrus schlegelii)

Acidification (OA), a global threat to the world's oceans, is projected to significantly grow if CO2 continues to be emitted into the atmosphere at high levels. This will result in a slight decrease in pH. Since the latter is a logarithmic scale of acidity, the higher acidic seawater is expected to have a tremendous impact on marine living resources in the long-term. An 8-week laboratory experiment was designed to assess the impact of the projected pH in 2100 and beyond on fish survival, health, growth, and fish meat quality. Two projected scenarios were simulated with the control treatment, in triplicates. The control treatment had a pH of 8.10, corresponding to a pCO2 of 321.37 ± 11.48 µatm. The two projected scenarios, named Predict_A and Predict_B, had pH values of 7.80-pCO2 = 749.12 ± 27.03 and 7.40-pCO2 = 321.37 ± 11.48 µatm, respectively. The experiment was preceded by 2 weeks of acclimation. After the acclimation, 20 juvenile black sea breams (Acanthopagrus schlegelii) of 2.72 ± 0.01 g were used per tank. This species has been selected mainly due to its very high resistance to diseases and environmental changes, assuming that a weaker fish resistance will also be susceptibly affected. In all tanks, the fish were fed with the same commercial diet. The seawater's physicochemical parameters were measured daily. Fish samples were subjected to physiological, histological, and biochemical analyses. Fish growth, feeding efficiency, protein efficiency ratio, and crude protein content were significantly decreased with a lower pH. Scanning electron microscopy revealed multiple atrophies of microvilli throughout the small intestine's brush border in samples from Predict_A and Predict_B. This significantly reduced nutrient absorption, resulting in significantly lower feed efficiency, lower fish growth, and lower meat quality. As a result of an elevated pCO2 in seawater, the fish eat more than normal but grow less than normal. Liver observation showed blood congestion, hemorrhage, necrosis, vacuolation of hepatocytes, and an increased number of Kupffer cells, which characterize liver damage. Transmission electron microscopy revealed an elongated and angular shape of the mitochondrion in the liver cell, with an abundance of peroxisomes, symptomatic of metabolic acidosis.

海洋酸化（Ocean Acidification, OA）是威胁全球海洋的重大环境问题，若持续向大气高强度排放二氧化碳，该问题预计将显著加剧。这将导致海水pH值小幅下降。由于pH是酸度的对数标尺，从长期来看，酸性更强的海水预计会对海洋生物资源造成深远的不利影响。本研究设计了一项为期8周的室内受控实验，以评估2100年及之后的预测海水pH值对鱼类存活、健康状况、生长性能及鱼肉品质的影响。实验设置了对照组与两种预测情景组，每组均设置3个生物学重复。对照组的pH值为8.10，对应的二氧化碳分压（partial pressure of carbon dioxide, pCO2）为321.37 ± 11.48 µatm。两种预测情景组分别命名为Predict_A与Predict_B，其pH值与对应pCO2分别为7.80（pCO2=749.12 ± 27.03 µatm）和7.40（pCO2=321.37 ± 11.48 µatm）。实验正式开展前设有2周的驯化适应期。驯化结束后，每个养殖缸投放20尾初始体重为2.72 ± 0.01 g的黑鲷幼鱼（Acanthopagrus schlegelii）。选择该鱼种主要因其对疾病及环境变化具备极高的耐受性，由此可佐证：即便自身抗性较弱的鱼类也会受到酸化的显著不利影响。所有养殖缸中的鱼类均投喂同一款商用配合饲料。每日测定海水的理化参数指标。对采集的鱼类样本开展生理学、组织学及生化分析。实验结果显示，随着海水pH值降低，鱼类生长速率、饲料转化率、蛋白质效率比及肌肉粗蛋白含量均显著下降。扫描电子显微镜（Scanning Electron Microscopy, SEM）观察发现，Predict_A与Predict_B组的样本小肠刷状缘微绒毛出现多处萎缩，这一结构损伤显著降低了肠道营养物质吸收效率，进而导致饲料转化率下降、鱼类生长迟缓及鱼肉品质降低。受海水pCO2升高的影响，鱼类摄食量高于正常水平，但生长速度却低于正常水平。肝脏组织观察结果显示，实验组出现血液充血、出血、坏死、肝细胞空泡化以及库普弗细胞（Kupffer cells）数量增多等典型肝脏损伤特征。透射电子显微镜（Transmission Electron Microscopy, TEM）观察发现，肝细胞内线粒体呈细长棱角状，且过氧化物酶体大量富集，这是代谢性酸中毒的典型病理表征。