Response of three krill species to hypoxia and warming: An experimental approach to oxygen minimum zones expansion in coastal ecosystems

To understand the adaptation of euphausiid (krill) species to oxygen minimum zones (OMZ), respiratory response and stress experiments combining hypoxia/reoxygenation exposure with warming were conducted. Experimental krill species were obtained from the Antarctic (South Georgia area), the Humboldt Current system (HCS, Chilean coast), and the Northern California Current system (NCCS, Oregon). Euphausia mucronata from the HCS shows oxyconforming or oxygen partial pressure (pO2)-dependent respiration below 80% air saturation (18 kPa). Normoxic subsurface oxygenation in winter posed a \"high oxygen stress\" for this species. The NCCS krill, Euphausia pacifica, and the Antarctic krill, Euphausia superba maintain respiration rates constant down to low critical pO2 values of 6 kPa (30% air saturation) and 11 kPa (55% air saturation), respectively. Antarctic krill had the lowest antioxidant enzyme activities, but the highest concentrations of the molecular antioxidant glutathione (GSH) and was not affected by 6 h exposure to moderate hypoxia. Temperate krill species had higher SOD (superoxide dismutase) values in winter than in summer, which relate to higher winter metabolic rate (E. pacifica). In all species, antioxidant enzyme activities remained constant during hypoxic exposure at habitat temperature. Warming by 7°C above habitat temperature in summer increased SOD activities and GSH levels in E. mucronata (HCS), but no oxidative damage occurred. In winter, when the NCCS is well mixed and the OMZ is deeper, +4°C of warming combined with hypoxia represents a lethal condition for E. pacifica. In summer, when the OMZ expands upwards (100 m subsurface), antioxidant defences counteracted hypoxia and reoxygenation effects in E. pacifica, but only at mildly elevated temperature (+2°C). In this season, experimental warming by +4°C reduced antioxidant activities and the hypoxia combination again caused mortality of exposed specimens. We conclude that a climate change scenario combining warming and hypoxia represents a serious threat to E. pacifica and, as a consequence, NCCS food webs.

为探究磷虾（euphausiid，krill）物种对最低氧区（oxygen minimum zone，OMZ）的适应机制，本研究开展了结合低氧/复氧暴露与升温的呼吸响应及胁迫实验。实验用磷虾样本采集于南极（南乔治亚地区）、洪堡洋流系统（Humboldt Current system，HCS，智利海岸）及北加州洋流系统（Northern California Current system，NCCS，俄勒冈州）。来自HCS的穆氏磷虾（Euphausia mucronata）在空气饱和度低于80%（18 kPa）时表现出氧顺应性或氧分压（oxygen partial pressure，pO₂）依赖性呼吸；冬季正常氧的次表层充氧对该物种构成“高氧胁迫”。NCCS的太平洋磷虾（Euphausia pacifica）及南极磷虾（Euphausia superba）分别可在临界低氧分压（pO₂）值低至6 kPa（空气饱和度30%）和11 kPa（空气饱和度55%）时维持呼吸速率稳定。南极磷虾的抗氧化酶活性最低，但分子抗氧化剂谷胱甘肽（glutathione，GSH）浓度最高，且暴露于中度低氧环境6小时未受影响。温带磷虾物种冬季的超氧化物歧化酶（superoxide dismutase，SOD）水平高于夏季，这与冬季代谢率升高有关（以太平洋磷虾为例）。在栖息地温度下，所有物种的抗氧化酶活性在低氧暴露期间保持稳定。夏季升温至栖息地温度以上7℃时，HCS的穆氏磷虾SOD活性及GSH水平升高，但未发生氧化损伤。冬季NCCS水体混合充分且OMZ更深时，升温4℃与低氧结合对太平洋磷虾构成致死条件。夏季OMZ向上扩展（至次表层100米）时，太平洋磷虾的抗氧化防御系统可抵消低氧及复氧效应，但仅在轻度升温（+2℃）条件下有效；该季节升温4℃会降低抗氧化活性，且与低氧结合再次导致暴露个体死亡。综上，升温与低氧结合的气候变化情景对太平洋磷虾及NCCS食物网构成严重威胁。