Experimental Techniques to Assess Coral Physiology In Situ: Current Approaches and Novel Insights

Coral reefs are declining worldwide due to global changes in the marine environment. The increasing frequency and severity of massive bleaching events in the tropics are highlighting the need to better understand the stages of coral physiological responses to extreme conditions. Moreover, like many other coastal regions, coral reef ecosystems are facing additional localized anthropogenic issues such as nutrient loading, increased turbidity, and coastal development. The changes in coral metabolism under local or global stress conditions is studied largely through laboratory manipulation and field observations. Different strategies have been developed to measure the health status of a damaged reef, ranging from the resolution of individual polyps to an entire coral community, but techniques for measuring coral physiology in situ are not yet widely implemented. For instance, while there are many studies of the coral holobiont response in single or limited-number multiple stressor experiments, they provide only partial insights to metabolic performance under more complex temporally and spatially variable natural conditions. Here, we discuss the current status of coral reefs and their global and local stressors in the context of current experimental techniques that measure core processes in coral metabolism (respiration, photosynthesis, and biocalcification) and their role in indicating the health status of colonies and communities. The state of the art of in situ techniques for experimental and monitoring purposes is explored. We highlight the need to improve the capability of in situ studies in order to better understand the resilience and stress response of corals under multiple global and local scale stressors.

maintain optimal chemical conditions at the tissue/calcium carbonate interface. Hard corals build their skeleton (Wa) [23], which can have a strong effect on net community calcification (NCC), along with the balance of organic 178 matter production and respiration, or net community production (NCP) [81]. Moreover, the projected decreases 179 in pH in the future ocean eventually will affect the stability of existing reef ecosystems, because the CaCO3 180 formed by corals (aragonite) is more susceptible to dissolution than that formed by other bio-calcifying 181 organisms [82]. These effects overlay natural seasonal variations in the carbonate chemistry in reef systems, and 182 the question remains of what extent does globally and locally derived ocean acidification have on the seasonal 183 balance of NCP and NCC [83]. However, these estimates remain imprecise, owing to the lack of information 184 about seawater residence time and volume, which would modulate reef chemistry [84]. The relationship 185 between corals and their symbiotic dinoflagellates is exacerbated under the combined effects global drivers of 186 stress, leading to lower energy reserves for growth [6,20,85,86], and local impacts of eutrophic conditions of 187 seawater surrounding the coral reefs, causing reduced calcification, oxidative stress, and eventually leading to 188 bleaching events [46,78].

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Even with these impacts though, it is possible for coral communities to recover after destructive events. For

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We conducted a literature review based on case studies of in situ observations of coral metabolism from 207 published peer-reviewed scientific literature. We included cases involved in original research on direct (e.g.,

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using SCUBA divers, benthic chambers, optical sensors) underwater measurements of metabolic rates on both 209 coral individuals and coral communities. We excluded sample collections (e.g., coral fragmentation), laboratory 210 experiments, and indirect measurements of metabolic fluxes (e.g., ex situ from sample incubations).

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We used the advanced search on Google Scholar to identify studies with the keywords "coral in situ 212 metabolism" and "underwater", excluding the keyword "collection", in articles published between 1991 and 213 2020. The search yield (n = 2090) was scrutinized, and the literature was manually reviewed to fulfil our selection 214 criteria described above. A final list of 54 studies was included for the analysis in this review.

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The literature screening was categorized in eight separate categories based on the methodology used (Table   216 1), including: 1) SCUBA FRR fluorometry; 2) SCUBA PAM fluorometry; 3) Clark-type O2 sensors; 4) boundary 217 layer; 5) SCUBA imaging; 6) benthic chambers; 7) submersible chambers; and 8) automated sensors. The identified the most commonly used techniques for studying in situ coral metabolism. Finally, we analyzed the 223 objectives of the studies included in this review (Fig. 2) in order to give an overview on the topics mostly studied 224 between 1991 and 2020.

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The focus of instrumentation development for in situ studies of coral metabolism has shifted over the last 253 three decades from the host-symbiosis relationship using fluorometric techniques to measure the 254 photosynthetic efficiency of endosymbionts to a more comprehensive investigation of individuals and 255 communities (Table 2). Indeed, through the use of benthic chambers it is possible to run experimental 256 manipulations of carbon fluxes or nutrients and monitor long-term ecosystem responses via automated sensors.

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With this shift, the biogeochemical processes involved in corals and coral reefs have received advanced

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The estimation of the potential quantum yield of PSII as a metric for photosynthetic rates was first applied   Laboratory-based studies have provided a strong foundation for understanding coral metabolism and the 481 responses to stress, and they will continue to serve as a primary means of research under controlled conditions.

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However, the expanding role of in situ-based studies of coral systems is essential for extrapolating and modulating these laboratory-based findings to the temporal and spatial complexity of natural reef and