Percorrer por autor "Littke, Sophie"
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- Investigating the role of frequency-dependent pre-conditioning to low oxygen and pH in building coral stress tolerancePublication . Littke, Sophie; Schoepf, Verena; Engelen, AschwinSevere hypoxia, often coupled with low pH, is an emerging threat to coral reefs, yet their combined effects, and the potential for environmental priming to modulate coral responses under such stress, remain understudied. In particular, the frequency and timing of prior sublethal exposure to combined low oxygen and pH are largely unexplored as drivers of coral stress tolerance. We conducted a controlled laboratory experiment to test how the frequency of nightly low dissolved oxygen (DO) and pH exposure shapes physiological responses of two Caribbean coral species, Agaricia tenuifolia and Siderastrea siderea, during subsequent, acute combined stress. Corals received six (high frequency, HF), three (low frequency, LF), or no (control) priming pulses, each lasting two consecutive nights, over four weeks, mimicking natural diel DO and pH cycles in shallow reefs, followed by 4–7 days of acute stress or ambient conditions. Results revealed contrasting species-specific outcomes. In A. tenuifolia, HF priming reduced baseline photosynthetic efficiency by 32% by the end of the priming period, with no further change during acute stress. HF primed A. tenuifolia also exhibited a ~42% reduction in symbiont densities, and a nearly tenfold increased risk of tissue loss compared to controls, regardless of acute stress treatment, while biomass and calcification remained unchanged. In contrast, S. siderea showed physiological stability across treatments, with LF priming supporting tissue biomass maintenance under acute stress. By the experimental end, biomass declined 28% and 22% in unprimed and HF-primed corals under acute stress, respectively, whereas LF primed corals maintained biomass. These findings highlight stress frequency as a critical yet understudied dimension of environmental priming and a direct modulator of baseline coral physiology. HF priming impaired the hypoxia-sensitive A. tenuifolia, while LF priming was neutral. The more hypoxia-tolerant S. siderea showed subtle LF priming benefits and no HF effects. As climate change and coastal eutrophication intensify diel variability and acute low DO and pH events, understanding frequency-dependent stress responses will improve predictions of reef community trajectories and help identify resilient coral populations.
- Using reflectance to measure chlorophyll a in corals: calibration and implications of skeletal optical propertiesPublication . Watty, Kay; Schoepf, Verena; Johnson, Kelly W.; Littke, Sophie; Zande, Rene M. van derClimate change-driven coral bleaching and associated chlorophyll (Chl) loss increasingly threaten coral reefs, highlighting the need for non-invasive, cost-effective tools to assess corals’ physiological condition. Researchers have explored the reflectance-based Normalized Difference Vegetation Index (NDVI) as a proxy for Chl a. However, almost no calibration curves exist to date; thus it remains unknown if the relationship between Chl a and NDVI differs among coral species, and whether it is influenced by skeletal optical properties. Here, we correlated Chl a content to NDVI in three coral species with different skeletal morphologies and reflectance modes across a range of health states (i.e., bleaching states). We found a strong positive relationship between Chl a and NDVI in all species, validating NDVI as a proxy for Chl a in corals. However, this relationship is non-linear, showing deviations at the highest and lowest ends of the pigmentation spectrum. We propose that in healthy states, this non-linearity is due to pigment self-shading, whereas in bleaching states, it is caused by backscattering of light by the increasingly exposed coral skeleton, because pigment loss enhances absorption by the remaining Chl a pigments and lowers, rather than increases, reflectance. To avoid Chl a under- or overestimation, we recommend to log-transform Chl a and create species-specific calibration curves. Overall, the non-invasive, repeatable, and scalable nature of reflectance measurements and NDVI make them valuable tools for assessing coral health both ex- and in situ. They provide a comprehensive view of the holobiont’s physiological condition and hold potential for broader applications in reef-scale monitoring, remote sensing, coral restoration, and conservation.