Ciliary vortex flows and oxygen dynamics in the coral boundary layer

The exchange of metabolites between environment and coral tissue depends on the flux across the diffusive boundary layer (DBL) surrounding the tissue. Cilia covering the coral tissue have been shown to create vortices that enhance mixing in the DBL in stagnant water. To study the role of cilia under simulated ambient currents, we designed a new light-sheet microscopy based flow chamber setup. Microparticle velocimetry was combined with high-resolution oxygen profiling in the coral Porites lutea under varying current and light conditions with natural and arrested cilia beating. Cilia-generated vortices in the lower DBL mitigated extreme oxygen concentrations close to the tissue surface. Under light and arrested cilia, oxygen surplus at the tissue surface increased to 350 µM above ambient, in contrast to 25 µM under ciliary beating. Oxygen shortage in darkness decreased from 120 µM (cilia arrested) to 86 µM (cilia active) below ambient. Ciliary redistribution of oxygen had no effect on the photosynthetic efficiency of the photosymbionts and overall oxygen flux across the DBL indicating that oxygen production and consumption was not affected. We found that corals actively change their environment and suggest that ciliary flows serve predominantly as a homeostatic control mechanism which may play a crucial role in coral stress response and resilience

Calcification, skeletal structure and composition of the cold-water coral Desmophyllum dianthus

In the naturally acidified Comau Fjord (Chile), high densities of the cosmopolitan cold-water coral (CWC) Desmophyllum dianthus are found at or below aragonite saturation (Ωar ≤ 1). However, it is not known so far if seasonal changes in Ωar lead to seasonal differences in calcification rates and the corals’ ability to up-regulate the pH in the calcifying fluid (pHcf). In the present study, corals were sampled along both horizontal and vertical pH gradients (pHT = 7.6-7.9, Ωar = 0.76-1.45) in Comau Fjord. We compared D. dianthus’ calcification rates (buoyant weight technique) with the physico-chemical conditions in the water column (T, Ωar) in austral summer 2016/2017 and winter 2017. In order to determine the biological pHcf up-regulation of D. dianthus, the skeletal boron isotopic composition (δ11B) was measured in the upper part of the calyx between the septa, using a UV femtosecond laser ablation system connected to a multicollector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS). Higher growth rates of D. dianthus were found in summer than in winter. Surprisingly, growth of D. dianthus was highest in undersaturated waters in both seasons (Ωar = 0.76 and 0.84) and cross-transplanted specimens were able to acclimatise to Ωar < 1. Therefore, the present study shows that Ωar alone is a poor predictor of D. dianthus growth. Skeletal analyses show a complex relationship between δ11B and the structure of the coral skeletons. δ11B measurements were highly variable, which may be attributed to the high calcification rates in the upper part of the coral calyx. Therefore, high resolution analyses of the skeletal composition and micro-structure will be conducted along the entire longitudinal section of D. dianthus skeletons using Raman microscopy and scanning electron microscopy (SEM). In addition, δ11B will be measured in different skeletal parts and compared to skeletal structure analyses for a reliable reconstruction of seawater pH at high temporal resolution using skeletons of D. dianthus grown under laboratory and field conditions (Comau Fjord, Chile)

Impacts of Warming and Acidification on Coral Calcification Linked to Photosymbiont Loss and Deregulation of Calcifying Fluid pH

Corals are globally important calcifiers that exhibit complex responses to anthropogenic warming and acidification. Although coral calcification is supported by high seawater pH, photosynthesis by the algal symbionts of zooxanthellate corals can be promoted by elevated pCO2. To investigate the mechanisms underlying corals’ complex responses to global change, three species of tropical zooxanthellate corals (Stylophora pistillata, Pocillopora damicornis, and Seriatopora hystrix) and one species of asymbiotic cold-water coral (Desmophyllum pertusum, syn. Lophelia pertusa) were cultured under a range of ocean acidification and warming scenarios. Under control temperatures, all tropical species exhibited increased calcification rates in response to increasing pCO2. However, the tropical species’ response to increasing pCO2 flattened when they lost symbionts (i.e., bleached) under the high-temperature treatments—suggesting that the loss of symbionts neutralized the benefit of increased pCO2 on calcification rate. Notably, the cold-water species that lacks symbionts exhibited a negative calcification response to increasing pCO2, although this negative response was partially ameliorated under elevated temperature. All four species elevated their calcifying fluid pH relative to seawater pH under all pCO2 treatments, and the magnitude of this offset (Δ[H+]) increased with increasing pCO2. Furthermore, calcifying fluid pH decreased along with symbiont abundance under thermal stress for the one species in which calcifying fluid pH was measured under both temperature treatments. This observation suggests a mechanistic link between photosymbiont loss (‘bleaching’) and impairment of zooxanthellate corals’ ability to elevate calcifying fluid pH in support of calcification under heat stress. This study supports the assertion that thermally induced loss of photosymbionts impairs tropical zooxanthellate corals’ ability to cope with CO2-induced ocean acidification

Boron isotopes by femtosecond LA-ICP-MS with application to pH reconstruction in biogenic carbonates

In this study, we explore the capability of our customized UV femtosecond laser ablation system coupled to a Nu Plasma II MC-ICP-MS to determine B isotope composition by investigating standard materials of various matrices and foraminifera and coral samples. Boron isotope ratios were determined on ion counters using NIST SRM 610 as reference material. Multiple analysis of silicate and carbonate standard materials including NIST SRM 612, the MPI-DING series (komatiite to rhyolite glasses), IAEA-B-8 (clay) and JCp-1 (coral) reveal average δ11B values, which agree well with published data. The reproducibility is better than 0.8‰ (2 SD). Investigations of the benthic foraminifera species (C. wuellerstorfi) from the ODP core 1092 show little inter- and intra-shell variability in δ11B. Average δ11B values reveal small variation around 14‰ at the time of cooling of Antarctica ~14 Myr ago, which indicate a change in deep water pH of ca. 0.1 pH units. Furthermore, we studied recent cold-water corals (D. dianthus) from the Comau Fjord (Chile), a field site showing spatial and seasonal variation in seawater pH (7.59 to 7.86). δ11B values ranges between 23.5 and 27.0‰, which is controlled by ambient seawater pH but likely also by nutrient availability and precipitation rates. Our results demonstrate that fs-LA-ICP-MS provides a unique in situ technique to determine B isotope ratios at high spatial resolution for pH reconstruction

Ontogenetic differences in the response of the cold-water coral Caryophyllia huinayensis to ocean acidification, warming and food availability

Cold-water corals (CWC) are exposed to multiple environmental stressors in a changing ocean. Several laboratory experiments have shown that adult CWC can survive at low pH and elevated temperature, but the effects on early life stages are largely unknown, let alone the interactive effects of changing pH, temperature and food availability. We conducted a six-month aquarium experiment to investigate the physiological responses of early juveniles, late juveniles and adults of the CWC Caryophyllia huinayensis to multiple environmental stressors by measuring key coral traits (survival, growth and respiration). We examined the single and interactive effects of pH (7.5 and 8.0), temperature (11 and 15 °C) and food availability (low and high) on the three life stages. The treatment levels reflect current conditions in the natural habitat of C. huinayensis in Comau Fjord, Chile. All life stages of C. huinayensis were more affected by warming than by acidification. At elevated temperature and in the combined treatment of elevated temperature and reduced pH, growth rates of all three life stages decreased after three months. After six months, mortality was highest in early juveniles and adults in these treatments. High feeding did not compensate for the negative effect of elevated temperature on growth and respiration rates of all life stages, but increased the growth rates of early and late juveniles at ambient and low pH conditions compared to low feeding. We identified ontogenetic shifts in resilience to future environmental conditions and highlight the importance of increased food availability for CWC resistance to ocean acidification. Our findings underscore the need to consider potential ontogenetic differences when addressing CWC responses to climate change

Seasonal growth and metabolism of the cold-water coral Desmophyllum dianthus in a plankton-rich naturally-acidified fjord

Cold-water corals (CWC) face an uncertain future under climate change. They seem to grow successfully under low pH conditions but physiological mechanisms and the role of energy efficiency in sustaining metabolic rates are largely unknown. The solitary, pseudo-colonial CWC Desmophyllum dianthus thrives in Comau Fjord (Northern Patagonia, Chile) despite low levels of aragonite saturation (Ωarag). To examine the seasonal growth and metabolism in relation with food availability and the physico-chemical environment of the fjord, we carried out an in situ reciprocal transplantation-experiment between (1) fjord mouth (20 m: Ωarag > 1, high seasonality) and fjord head (20 m: Ωarag > 1 in winter, Ωarag 1, high seasonality; 300 m: Ωarag 1) and plankton supply during summer combined with a well-functioning redistribution of energy reserves within D. dianthus in winter can explain this picture. At the fjord head growth changed seasonally in control and transplanted corals with higher rates during summer indicating good growth conditions despite Ωarag < 1 due to sufficient food availability but a less effective energy allocation during low food supply in winter. The present results emphasize the effect of seasonal environmental changes on CWC to conceive both the extent of natural variability where these corals live in and their acclimation potential to deal with it

Seawater carbonate chemistry and biomass, calcification and respiration rates of cold-water coral Desmophyllum dianthus in Comau Fjord

The stratified Chilean Comau Fjord sustains a dense population of the cold-water coral (CWC) Desmophyllum dianthus in aragonite supersaturated shallow and aragonite undersaturated deep water. This provides a rare opportunity to evaluate CWC fitness trade-offs in response to physico-chemical drivers and their variability. Here, we combined year-long reciprocal transplantation experiments along natural oceanographic gradients with an in situ assessment of CWC fitness. Following transplantation, corals acclimated fast to the novel environment with no discernible difference between native and novel (i.e. cross-transplanted) corals, demonstrating high phenotypic plasticity. Surprisingly, corals exposed to lowest aragonite saturation (Omega arag < 1) and temperature (T < 12.0 °C), but stable environmental conditions, at the deep station grew fastest and expressed the fittest phenotype. We found an inverse relationship between CWC fitness and environmental variability and propose to consider the high frequency fluctuations of abiotic and biotic factors to better predict the future of CWCs in a changing ocean

Environmental stability and phenotypic plasticity benefit the cold-water coral Desmophyllum dianthus in an acidified fjord

The stratified Chilean Comau Fjord sustains a dense population of the cold-water coral (CWC) Desmophyllum dianthus in aragonite supersaturated shallow and aragonite undersaturated deep water. This provides a rare opportunity to evaluate CWC fitness trade-offs in response to physico-chemical drivers and their variability. Here, we combined year-long reciprocal transplantation experiments along natural oceanographic gradients with an in situ assessment of CWC fitness. Following transplantation, corals acclimated fast to the novel environment with no discernible difference between native and novel (i.e. cross-transplanted) corals, demonstrating high phenotypic plasticity. Surprisingly, corals exposed to lowest aragonite saturation (omega(arag) < 1) and temperature (T < 12.0 degrees C), but stable environmental conditions, at the deep station grew fastest and expressed the fittest phenotype. We found an inverse relationship between CWC fitness and environmental variability and propose to consider the high frequency fluctuations of abiotic and biotic factors to better predict the future of CWCs in a changing ocean. The cold-water coral Desmophyllum dianthus benefits from stable environmental conditions in deep waters of Comau Fjord (Chile) and is able to acclimatise quickly to new environmental conditions after transplantation

Health status in experiment with different life stages of the cold-water coral Caryophyllia huinayensis

We conducted a long-term (6 months) aquarium experiment with the cold-water coral Caryophyllia huinayensis to investigate the response of three different life stages to a combination of aragonite saturation, temperature and food availability. We determined the health status of the corals after one, three and six months