Host pigments of hermatypic corals as indicators of environmental stress in reef communities

浅水珊瑚礁是海洋生物多样性最重要的中心之一，提供了宝贵的生态系统服务，因为数百万人在经济上依赖珊瑚礁及其生物。全球变暖、海洋酸化、日益严重的污染、过度捕捞、捕食者或病原体的袭击、盐度变化和沉积物增加等多种自然和人为应激源可能导致本世纪内珊瑚礁的急剧丧失。珊瑚礁的未来在很大程度上取决于能够确定受高度压力影响的珊瑚礁区域并预测白化事件的战略，以便加强对特别脆弱地区的管理努力。然而，目前可用的遥感技术的预测能力受到以下事实的限制：预测主要基于温度异常，而没有考虑上述多种共同作用的压力因素，这些因素增加了对漂白的敏感性。因此，需要分析珊瑚本身的累积反应，而不是单一的环境参数。最近，我们发现编码GFP(绿色荧光蛋白)样蛋白家族代表的基因在应激反应中受到差异调控。这些色素是造礁珊瑚呈现出强烈的紫蓝色、绿色或微红色的主要原因。这个蛋白质家族的荧光代表在适当波长的光刺激下发出从青色到红色的光谱范围的光子。我们可以发现，热应激导致FP转录水平显著降低，而在低温和氧化应激下，FP转录水平显著升高。珊瑚中FP基因对各种环境条件变化的强烈响应表明，这些色素可能是珊瑚健康急需的内在标志。对指示标本的荧光特征进行光谱分析，可以提供一种非侵入性、快速和低成本的方法来评估珊瑚礁群落的压力水平。事实上，激光荧光成像作为获取珊瑚礁大面积空间区域的珊瑚荧光提供的信息的潜在工具，在技术上是可行的。然而，需要详细了解珊瑚色素对环境变化的反应的特殊性和重要性，才能将珊瑚颜色确立为珊瑚健康的累积指标。目前，尚不清楚短期(数小时)实验报告的FP转录本下调是否也导致蛋白质水平的显著降低。这不是一个不言而喻的结果，因为我们之前发现，珊瑚组织中的蛋白质色素有一个缓慢的更替(半衰期~3周)，可能会掩盖转录本水平上的短期变化。这项拟议的研究将把重点放在温度压力对珊瑚礁的显著威胁上。此外，对这种压力因素对宿主色素组织含量的影响的研究最有希望证明新监测概念的原理证据，因为在短期实验中已经观察到编码GFP样蛋白的转录本下调。我们的结果将极大地促进未来的实验，这些实验将测试珊瑚对不同形式的压力组合的反应，并改进基于荧光蛋白的生物指示系统，以允许区分不同的生理反应。

Shallow water coral reefs are among the most important centres of marine biodiversity providing invaluable ecosystem services as millions of people are economically dependent on the reef and its creatures. A multiplicity of natural and anthropogenic stressors such as global warming, ocean acidification, rising pollution, overfishing, attacks by predators or pathogens, salinity variations and increased sedimentation might result in a dramatic loss of coral reefs within this century. The future of coral reefs is strongly dependent on strategies that enable identification of reef regions affected by high levels of stress and to forecast bleaching events in order to enhance management efforts on particularly vulnerable areas. However, the predictive capacity of currently available remote sensing techniques is limited by the fact that prediction is mainly based on temperature anomalies and does not consider the above mentioned multitude of stress factors that act together to increase susceptibility to bleaching. Consequently, approaches are required that analyse the cumulative response of corals themselves rather than single environmental parameters. Most recently, we showed that genes encoding representatives of the family of GFP (green fluorescent protein)-like proteins were differentially regulated in response to stress. These pigments are responsible for most of the intense purple-blue, green, or reddish hues of reef-building corals. Fluorescent representatives of this protein family emit photons in the spectral range from cyan to red upon stimulation with light of suitable wavelengths. We could show that FP transcript levels were strongly reduced due to heat stress, but increased under cold and oxidative stress. The strong response of FP genes in corals to various changes of the environmental conditions suggests that these pigments might serve as the urgently needed intrinsic markers of coral health. The spectral analysis of fluorescence signatures of indicator specimens could allow a non-invasive, fast and low cost method to assess stress levels in coral reef communities. Indeed, laser fluorescence imaging as potential tool to access the information provided by coral fluorescence in large spatial areas of coral reefs was shown to be technically feasible. However, detailed knowledge about the specificity and significance of the response of coral pigments to environmental changes is required to establish coral colour as a cumulative indicator of coral health. At present, it is not clear if the downregulation of FP transcripts reported from short term (hours) experiments results also in a measurable reduction of protein levels. This is not a self-evident consequence, as we found previously that the proteins pigments in coral tissue have a slow turnover (half-life ~3 weeks) that might mask short-term alterations at the transcript level. The proposed study will focus on temperature stress as an eminent threat for coral reefs. Moreover, examination of the effect of this stress factor on the tissue content of host pigments is most promising to demonstrate the proof of principle of the new monitoring concept as a downregulation of transcripts encoding GFP-like proteins was already observed in short term experiments. Our results will greatly facilitate future experiments that will test the reponse of corals to combinations of different forms of stress and refine the fluorescent protein-based bioindicator system to allow for the discrimination among different physiological responses.