Oxygen respiration and metabolic enzyme malate dehydrogenase of Euphasia superba in the South Atlantic Ocean@en

Melatonin, the chief secretory product of the vertebrate pineal gland is suspected to be a ubiquitous molecule principally involved in the transduction of photoperiodic information. Besides vertebrates, melatonin has been detected throughout phylogeny in numerous non-vertebrate taxa. In the present study, the occurrence of melatonin in Antarctic krill Euphausia superba and its possible role in mediating seasonal metabolic changes was evaluated. Melatonin was quantified by enzyme linked immunosorbent assay (ELISA) in high performance liquid chromatography (HPLC) purified extracts of eyestalks and hemolymph of krill sampled in the Lazarev Sea during the Antarctic winter and summer. In addition, oxygen uptake rates and the activities of the metabolic enzyme malate dehydrogenase (MDH) were recorded to assess the metabolic status of krill. Validation of melatonin measurements was carried out on the basis of three different extraction methods with parallel determination of melatonin by ELISA in crude extracts and in HPLC purified extracts, and after derivatization of melatonin under alkaline conditions in the presence of hydrogen peroxide. A significantly higher respiration rate and MDH activity was found in summer krill than in winter krill indicating that krill was in a state of reduced metabolic activity during winter. However, neither during winter nor during summer there were detectable melatonin concentrations in the visual system or hemolymph of krill. Based on these results, we question a mediating role of melatonin in the control of seasonal metabolic changes in Antarctic krill in particular and its physiological significance in krill in general.褪黑素（Melatonin）是脊椎动物松果体的主要分泌产物，被视为一类广泛分布的遍在分子，主要参与光周期信息的转导过程。除脊椎动物外，研究人员已在众多无脊椎动物类群中检测到褪黑素的存在。本研究针对南极磷虾（Euphausia superba）体内褪黑素的分布情况，及其在介导季节代谢变化中的潜在作用进行了评估。研究人员采用酶联免疫吸附试验（ELISA），对南极冬季与夏季在拉扎列夫海采集的磷虾眼柄及血淋巴的高效液相色谱（HPLC）纯化提取物进行了褪黑素定量分析。此外，还通过记录摄氧速率与代谢酶苹果酸脱氢酶（MDH）的活性，评估了磷虾的代谢状态。本研究采用三种不同的提取方法开展褪黑素检测的验证工作：分别对粗提取物及HPLC纯化提取物采用ELISA进行平行测定，并在碱性条件下结合过氧化氢对褪黑素进行衍生化处理后开展检测。研究发现夏季磷虾的呼吸速率与MDH活性显著高于冬季磷虾，表明冬季磷虾的代谢活动处于减弱状态。然而，无论冬季还是夏季，均未在磷虾的视觉系统或血淋巴中检测到可检出浓度的褪黑素。基于上述结果，我们尤其对褪黑素介导南极磷虾季节代谢变化的调控作用提出质疑，同时也对褪黑素在磷虾体内的普遍生理意义提出了探讨。