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

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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.