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.

为阐明磷虾类（euphausiid，即磷虾）对低氧区（oxygen minimum zones, OMZ）的适应机制，本研究开展了低氧/复氧暴露联合增温的呼吸响应与胁迫实验。实验所用磷虾样本分别采自南极（南乔治亚海域、洪堡洋流系统（Humboldt Current system, HCS，智利沿岸）以及北加州洋流系统（Northern California Current system, NCCS，俄勒冈州海域）。来自洪堡洋流系统的尖额磷虾（Euphausia mucronata）在空气饱和度低于80%（18 kPa）时，表现出氧顺应型（oxyconforming）呼吸模式，其呼吸速率随氧分压（partial pressure of oxygen, pO2）变化。冬季常态含氧的次表层氧合环境对该物种构成了“高氧胁迫”。北加州洋流系统的太平洋磷虾（Euphausia pacifica）与南极磷虾（Euphausia superba）可维持稳定的呼吸速率，直至分别低至6 kPa（30%空气饱和度）与11 kPa（55%空气饱和度）的临界氧分压。南极磷虾的抗氧化酶活性最低，但其分子抗氧化剂谷胱甘肽（glutathione, GSH）浓度最高，且经6小时中度低氧暴露后未受显著影响。温带磷虾物种冬季的超氧化物歧化酶（superoxide dismutase, SOD）活性高于夏季，这与其冬季更高的代谢速率相关（以太平洋磷虾为例）。在所有受试物种中，在栖息地温度下进行低氧暴露时，其抗氧化酶活性均保持稳定。夏季时，在栖息地温度基础上增温7℃，可提升尖额磷虾（洪堡洋流系统种群）的SOD活性与GSH水平，但未引发氧化损伤。冬季时，北加州洋流系统海域混合充分且低氧区更深，此时增温4℃联合低氧暴露对太平洋磷虾构成致死胁迫。夏季时，低氧区向上扩张至100米次表层，此时太平洋磷虾的抗氧化防御系统可抵御低氧与复氧带来的影响，但仅在轻度增温（+2℃）条件下。本季中，若增温4℃会降低其抗氧化酶活性，且联合低氧暴露会导致受试个体死亡。本研究结论为：气候变暖与低氧共存的气候变化情景，对太平洋磷虾乃至北加州洋流系统食物网构成严重威胁。