海洋生态与环境科学重点实验室知识库

人类排放的CO₂导致全球温室效应愈发加剧，由此引发的气温升高对全球气候造成了越来越多的不利影响。然而，虽然温度是公认的影响海洋生物基础代谢水平的最重要的压力源，但是温度并不是海洋生物所面临的唯一压力因素。因为随着海水温度的升高，自然伴随着溶解氧浓度的降低。高温导致的全球低氧区数量不断增加，并且这一现象在近海富营养化地区尤为明显。当溶解氧浓度开始降低，会迫使沉积环境从氧化状态转变为还原状态，在还原条件下，沉积物中的硫化物会转化为硫化氢，并从沉积环境中向水体扩散。因此高温伴随的溶解氧浓度降低时，通常会引起硫化氢浓度的升高。硫化氢会对好氧生物产生巨大的毒性作用，因此在近海富营养化地区和水产养殖区，更容易发生由高温引发的溶解氧降低和硫化氢升高等环境连锁反应，并对海洋生物造成难以想象的后果。然而迄今为止大部分的研究都是针对单一环境压力而进行的，很少有研究考虑多个压力源的综合作用对海洋生物的影响。不仅如此，海洋双壳贝类在环境压力条件下的行为特征变化和生理响应策略等重要问题仍亟待解决。

菲律宾蛤仔是我国重要的滩涂养殖贝类，因其经济价值高，适应能力强，目前已成为世界范围内的重要经济贝类，具有极高的商业价值和生态价值。菲律宾蛤仔单种产量在我国贝类养殖产量中仅次于牡蛎。然而，菲律宾蛤仔每年夏季几乎都会遭受大规模死亡的威胁，导致死亡的具体原因仍不清楚。并且，有关菲律宾蛤仔在高温条件下，应对其余环境压力时的耐受能力、行为特征、生理响应策略以及三者之间的相互联系还需进一步研究。

本研究中，我们对中国北方重要的菲律宾蛤仔自然栖息地进行了环境调查，厘清了菲律宾蛤仔栖息地夏季环境变化特征，分析了菲律宾蛤仔大面积死亡的原因，并对高温和低氧、高温和硫化氢条件下，菲律宾蛤仔的存活、行为特征、生理响应等问题进行了研究。

现场调查结果显示，2019年夏季菲律宾蛤仔出现大规模死亡现象，而2020年的调查过程中未发现菲律宾蛤仔的大面积死亡现象。菲律宾蛤仔的死亡率在2019年8月底层水温最高时开始升高，在底层水温高于28 ℃（28.74-30.03 ℃）的三个站位记录了大量死亡的开始 （>40 %）。在这些站位中，溶解氧浓度（2.95-4.09 mg/L）和沉积物氧化还原电位 （172.32-183.19 mv）明显低于其他站位的平均值8.21 mg/L和208.54 mv，而硫化氢浓度（20.32-24.72 μmol/L）则显著高于其他站位的平均浓度（7.89 μmol/L）。2019年从7月到9月，菲律宾蛤仔的条件指数（CI）和糖原含量的显著下降只呈现在大规模死亡区域。我们的结果表明，虽然高水温引发了菲律宾蛤仔的大量死亡，但沉积物环境的逐级恶化加强了菲律宾蛤仔的热应激。即使环境因子都处于亚致死条件，但是环境因子带来的综合影响已经远超人类的预计。

沉积物培养实验结果显示，底层水体溶解氧受温度、沉积物类型和时间的影响显著。在32 ℃条件下，溶解氧被消耗的最快，粉砂质砂沉积物对溶解氧的消耗速率要高于砂质沉积物。同样，沉积物中的硫化氢浓度也受温度、沉积物类型和时间的影响显著。温度越高，硫化氢的释放越强烈，而砂质沉积物中硫化氢的释放程度低于粉砂质砂沉积物。沉积物氧化还原电位的结果表明，粉砂质砂沉积物在高温条件下更容易出现还原状态。温度是导致沉积物中溶解氧消耗和硫化氢释放的重要因素，而沉积物类型也起着关键作用。沉积物中有机碳含量的差异以及氧化还原电位特征可能是导致不同类型沉积物耗氧率和硫化氢浓度差异的原因之一。在水产养殖过程中，当夏季温度持续偏高时，除了关注溶解氧浓度的变化外，还需警惕硫化氢的威胁。

室内实验的结果显示，高温和低氧条件下，温度越高，菲律宾蛤仔耐受低氧的能力越差，不同沉积物类型对菲律宾蛤仔的生存影响差异显著。相较于溶解氧，温度是显著影响菲律宾蛤仔贝壳开闭合行为的重要因素，温度越高菲律宾蛤仔越倾向于紧闭贝壳。菲律宾蛤仔在低氧条件下会通过改变埋栖深度来获取更多的溶解氧，但是高温延缓了菲律宾蛤仔向沉积物水界面移动的速度。菲律宾蛤仔对低氧具有强大的耐受能力，可以通过调节新陈代谢速率和糖酵解的异化等过程来应对低氧压力。在适宜温度条件下，即使溶解氧浓度在1 mg/L时，也不会影响菲律宾蛤仔正常的生理响应策略，但是温度升高影响了菲律宾蛤仔的生理响应，当温度高于28℃，溶解氧浓度低于2 mg/L时，菲律宾蛤仔的生理响应策略即发生改变，并且随着压力暴露时间的延长，高温会导致菲律宾蛤仔的生理响应过程出现紊乱。

在高温和高浓度硫化氢条件下，温度升高严重降低了菲律宾蛤仔可耐受的硫化氢范围。不同温度也影响了菲律宾蛤仔应对硫化氢的响应策略。在24℃条件下，菲律宾蛤仔会通过提高呼吸代谢频率，获取更多的氧气用于硫化氢的解毒过程，同时激活体内氧化防御系统保护细胞组织不受损伤，当温度超过28℃后，菲律宾蛤仔反而会降低呼吸代谢频率，减少代谢过程的耗氧，并利用剩余氧气进行解毒，但是温度越高，越容易造成菲律宾蛤仔生理响应的紊乱。

我们的研究结果发现多重环境压力对海洋生物的破坏性影响，即温度升高可能严重缩窄了海洋生物对其他环境压力的耐受范围。多重环境压力对海洋生物的共同作用和影响可以很好的解释环境亚致死条件下大面积死亡现象发生的原因。而行为特征和生理响应策略的改变是海洋生物应对环境压力耐受能力发生变化的主要原因。另外，我们的研究也发现，即使游泳能力较差的双壳贝类，其应对环境压力的行为响应也是迅速的。

The human emissions of CO₂ have resulted in intensifying the global greenhouse effect, and the increase in temperature has caused additional adverse effects on the global climate. However, although the temperature is recognized as the most important stressor affecting the basal metabolic rate of marine organisms, it is not the only problem facing marine organisms, as seawater temperature increases, the concentration of dissolved oxygen decreases. The number of hypoxic zones caused by high temperature is increasing globally, and this is particularly pronounced in the eutrophic regions offshore. When the dissolved oxygen concentration begins to decrease, it forces the depositional environment to change from an oxidizing state to a reducing state. Under reducing conditions, sulfide in the sediment is converted to hydrogen sulfide and is diffused from the depositional environment into the surrounding water. As sulfides are widely distributed in marine sediments, a decrease in the concentration of dissolved oxygen accompanied by high temperatures usually leads to an increase in hydrogen sulfide concentration. Hydrogen sulfide is highly toxic to aerobic organisms. Therefore, in offshore eutrophic and aquaculture areas, the environmental chain reactions, such as the reduction in dissolved oxygen and the increase in hydrogen sulfide, are more likely to occur due to high temperature, resulting in consequences to marine organisms. However, most studies have examined a single environmental stressor, and few studies have considered the combined effects of multiple stressors on marine organisms. Such important issues need to be resolved, as the behavior and physiological responses of marine bivalve mollusks change under environmental stress.

The Manila clam (Ruditapes philippinarum) is an important cultured shellfish in China. Because of its high economic value and strong adaptability, the Manila clam has become an important commercial shellfish worldwide. Additionally, Manila clams are a key link in the food web and the carbon cycle with significant ecological value. The production of Manila clam is second only to oysters in shellfish aquaculture production. However, Manila clams are threatened by mass mortality almost every summer, but the exact cause of death remains unclear. Further research is needed on the tolerance, behavioral characteristics, physiological responses, and the interrelationships among the three in response to environmental stressors under high-temperature conditions.

In this study, we conducted an environmental survey of the natural habitat of Manila clam in northern China, clarified the characteristics of environmental change in the Manila clam habitat during summer, analyzed the reasons for the large-scale death of Manila clams, and assessed the high temperature, hypoxic, and hydrogen sulfide conditions. The survival, behavior, and physiological responses of Manila clam were studied. In addition, the environmental chain reactions induced by temperature were verified.

The results showed that no large-scale mortality was observed in Manila clams during the 2020 survey, which included the summers of 2019 and 2020. Summer mortality of the Manila clam and the associated environmental factors were investigated monthly in the mariculture estuary of Laizhou Bay during 2019 and 2020. Clam mortality was highest during August 2019 and the highest bottom water temperature and mass mortality (>40%) were recorded at three stations with bottom water temperatures > 28℃ (28.74-30.03℃). At these stations, the dissolved oxygen concentrations (2.95-4.09 mg/L) and Eh (172.32-183.19 mv) were significantly lower than at other stations, which averaged 8.21 mg/L and 208.54 mv, whereas hydrogen sulfide concentrations (20.32-24.72 μmol/L) were higher than the average of 7.89 μmol/L. Significant decreases in the condition index and glycogen content occurred only in the mass mortality regions from July to September 2019. Our results indicate that high water temperature triggered mass mortality, but the cascading deterioration of the sedimentary environment reinforced the thermal stress on the Manila clam.

Dissolved oxygen and hydrogen sulfide were significantly affected by temperature, substrate, and time. Dissolved oxygen consumption was the fastest at 32℃, and the consumption rate of dissolved oxygen in silty sand sediments was higher than that in sandy sediments. The higher the temperature, the more intense the release of hydrogen sulfide. The degree of hydrogen sulfide release from sandy sediments was lower than that from silty sand sediments. The Eh results showed that the silty sand sediments were more prone to a reduced state under high temperatures. The temperature was an important factor leading to the consumption of dissolved oxygen and the release of hydrogen sulfide from the sediments, and the sediment type also played a key role. The difference in organic carbon content in the sediment and the Eh may be reasons for the differences in oxygen consumption and hydrogen sulfide concentration in the different types of sediments. When the temperature is high during the summer, changes in dissolved oxygen concentration should be carefully monitored, and the aquaculturist should be alerted to the threat of hydrogen sulfide.

The higher the temperature, the poorer the ability of Manila clam to tolerate hypoxia. Different types of substrates were key to the significantly different survival rates of Manila clam. Compared with dissolved oxygen, temperature is an important factor that significantly affects shell opening and closing in Manila clams. The higher the temperature, the more inclined the shell is to close. Manila clams acquire more dissolved oxygen by changing their burial depth under low oxygen conditions, but a high temperature slows the rate that Manila clams move towards the sediment-water interface. The Manila clam is highly tolerant to hypoxia and copes with hypoxic stress by regulating metabolic rate and the dissimilation of glycolysis. If the dissolved oxygen concentration is 1 mg/L under suitable temperature conditions, the normal physiological response of Manila clam will not be affected, but increasing the temperature affects the physiological response. When the temperature was higher than 28℃, and the dissolved oxygen concentration was < 2 mg/L, the physiological response of Manila clam changed. If exposure to high pressure is extended, the high temperature caused a disrupted physiological response in the Manila clam.

Under high temperature and high concentration hydrogen sulfide conditions, the temperature increase severely reduces the range of the hydrogen sulfide that can be tolerated by the Manila clam. Different temperatures also affected the response strategies of Manila clam to hydrogen sulfide. Under normal temperature conditions, the Manila clam obtains more oxygen to detoxify the hydrogen sulfide by increasing their respiration rate and metabolism and activates the oxidative defense system to protect cells from damage. When the temperature rises, the Manila clam reduces its respiration rate and metabolism, which reduces the oxygen consumption needed for metabolism that can be used as the remaining oxygen for detoxification.

Our findings validate the damaging effects of multiple environmental stressors on marine organisms; namely, increasing temperatures may severely narrow the tolerance range of marine organisms to other environmental stressors. The joint action and impact of multiple environmental stressors on marine organisms explain the large-scale mortality under environmentally sublethal conditions. Changes in behavior and physiological response are the main reasons for the changes in the tolerance of marine organisms to environmental stress. Our study shows that bivalve mollusks respond rapidly to environmental stress.