Temperature tolerance of different larval stages of the spider crab Hyas araneus exposed to elevated seawater PCO2

Exposure to elevated seawater PCO2 limits the thermal tolerance of crustaceans but the underlying mechanisms have not been comprehensively explored. Larval stages of crustaceans are even more sensitive to environmental hypercapnia and possess narrower thermal windows than adults. In a mechanistic approach, we analysed the impact of high seawater CO2 on parameters at different levels of biological organization, from the molecular to the whole animal level. At the whole animal level we measured oxygen consumption, heart rate and activity during acute warming in zoea and megalopa larvae of the spider crab Hyas araneus exposed to different levels of seawater PCO2. Furthermore, the expression of genes responsible for acid-base regulation and mitochondrial energy metabolism, and cellular responses to thermal stress (e.g. the heat shock response) was analysed before and after larvae were heat shocked byrapidly raising the seawater temperature from 10°C rearing temperature to 20°C. Zoea larvae showed a high heat tolerance, which decreased at elevated seawater PCO2, while the already low heat tolerance of megalopa larvae was not limited further by hypercapnic exposure. There was a combined effect of elevated seawater CO2 and heat shock in zoea larvae causing elevated transcript levels of heat shock proteins. In all three larval stages, hypercapnic exposure elicited an up-regulation of genes involved in oxidative phosphorylation, which was, however, not accompanied by increased energetic demands. The combined effect of seawater CO2 and heat shock on the gene expression of heat shock proteins reflects the downward shift in thermal limits seen on the whole animal level and indicates an associated capacity to elicit passive thermal tolerance. The up-regulation of genes involved in oxidative phosphorylation might compensate for enzyme activities being lowered through bicarbonate inhibition and maintain larval standard metabolic rates at high seawater CO2 levels. The present study underlines the necessity to align transcriptomic data with physiological responses when addressing mechanisms affected by an interaction of elevated seawater PCO2 and temperature extremes.

海水二氧化碳分压（seawater PCO₂）升高暴露会限制甲壳类动物的热耐受能力，但其潜在调控机制尚未得到全面阐明。甲壳类动物的幼体阶段对环境高碳酸血症（environmental hypercapnia）更为敏感，且相较于成体，其热安全窗（thermal windows）更为狭窄。本研究采用机制性研究方法，分析了高浓度海水CO₂对不同生物组织层级（从分子层面至整体动物层面）相关参数的影响。在整体动物层面，我们针对暴露于不同浓度海水PCO₂的蛛形蟹（Hyas araneus）的溞状幼体（zoea）与大眼幼体（megalopa），测定了其在急性升温过程中的耗氧量、心率及活动能力。此外，我们还分析了幼体在经快速升温（将养殖温度从10℃升至20℃）实施热激处理前后，参与酸碱调控、线粒体能量代谢的基因表达情况，以及细胞对热应激的响应（如热休克反应（heat shock response））。结果显示，溞状幼体展现出较高的热耐受能力，该能力会随海水PCO₂升高而下降；而大眼幼体本就较低的热耐受能力，并不会因高碳酸暴露而进一步受限。海水CO₂升高与热激处理在溞状幼体中存在联合效应，可使热休克蛋白（heat shock proteins）的转录水平上调。在所有三个幼体阶段中，高碳酸暴露均可诱导参与氧化磷酸化（oxidative phosphorylation）的基因表达上调，但这并未伴随能量需求的提升。海水CO₂与热激处理对热休克蛋白基因表达的联合效应，呼应了整体动物层面观测到的热耐受阈值下移现象，同时表明机体可通过该过程获得被动热耐受能力。参与氧化磷酸化的基因表达上调，或可弥补因碳酸氢盐抑制作用降低的酶活性，从而在高海水CO₂浓度下维持幼体的基础代谢速率。本研究强调，在探究海水PCO₂升高与极端高温交互作用所影响的调控机制时，需将转录组数据与生理响应相结合，这一要求具备重要的研究指导意义。