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In this study, we present B isotopic data for cold-water corals at high spatial resolution determined by UV femtosecond laser ablation system coupled to a Nu Plasma II MC-ICP-MS. Boron isotope records in biogenic carbonates provide constraints on the evolution of seawater pH and insights into biomineralization processes. We investigated B isotopic ratios in recent cold-water corals (Desmophyllum dianthus) from a field experiment in the Comau Fjord (Chile) to resolve seasonal variations. Corals from the head and the mouth of the fjord and from different water depths were stained with calcein, replaced at the same location or cross-transplanted and observed for a whole year to investigate time-resolved environmental signatures and potential adaptation effects under new environmental conditions. B isotopic ratios were measured at the upper part of the calyx in transverse sections between septa at the skeletal increment. δ11B values range between 23.5 and 27.0‰ and varies with seasonality and location in the fjord. Isotope signature of cross-transplanted corals do not differ significantly from those of local corals showing that the recorded signature is directly influenced by environmental conditions. B isotopic signatures correspond to an internal pH up-regulation for calcification of 0.78 to 1.14 pH units, which is likely controlled by ambient seawater pH and nutrient availability. These results show the potential of in situ techniques to decipher marine archive to reconstruct environmental conditions.

Additional file 6: Dataset S5. Protocol of raw read processing using QIIME2 V2019.7.

Additional file 1. Supplementary materials.

Artificial upwelling (AU) is a novel geoengineering technology that brings seawater from the deep ocean to the surface. Within the context of global warming, AU techniques are proposed to reduce sea surface temperature at times of thermal stress around coral reefs. A computationally fast but coarse 3D Earth System model (3.6° longitude × 1.8° latitude) was used to investigate the environmental impacts of hypothetically implemented AU strategies in the Great Barrier Reef, South China Sea, and Hawaiian regions. While omitting the discussion on sub-grid hydrology, we simulated in our model a water translocation from either 130 or 550 m depth to sea surface at rates of 1 or 50 m3 s−1 as analogues to AU implementation. Under the Representative Concentration Pathway 8.5 emissions scenario from year 2020 on, the model predicted a prevention of coral bleaching until the year 2099 when AU was implemented, except under the least intense AU scenario (water from 130 m depth at 1 m3 s−1). Yet, intense AU implementation (water from 550 m depth at 50 m3 s-1) will likely have adverse effects on coral reefs by overcooling the surface water, altering salinity, decreasing calcium carbonate saturation, and considerably increasing nutrient levels. Our result suggests that if we utilize AU for mitigating coral bleaching during heat stress, AU implementation needs to be carefully designed with respect to AU’s location, depth, intensity and duration so that undesirable environmental effects are minimized. Following a proper installation and management procedure, however, AU has the potential to decelerate destructive bleaching events and buy corals more time to adjust to climate change.

Global warming is considered to be the most severe threat to coral reefs globally, which makes it important for scientists to develop novel strategies that mitigate the impact of warming on corals and associated habitats. Artificial upwelling of cooler deep water to the surface layer may be a possible mitigation/management tool. In this study, we investigated the effect of simulated artificial upwelling with deep water off Bermuda collected at 50 m (24°C) and 100 m (20°C) on coral symbiont biology of 3 coral species (Montastrea cavernosa, Porites astreoides, and Pseudodiploria strigosa) in a temperature stress experiment. The following treatments were applied over a period of 3 weeks: (i) control at 28°C (ii) heat at 31°C, (iii) heat at 31°C+ deep water from 50 m depth, and (iv) heat at 31°C+ deep water from 100 m depth. Artificial upwelling was simulated over a period of 25 min on a daily basis resulting in a reduction of temperature for 2 h per day and the following degree-heating-weeks: 5.7°C-weeks for ii, 4.6°C-weeks for iii and 4.2°C-weeks for iv. Comparative analysis of photosynthetic rate, chlorophyll-a concentration and zooxanthellae density revealed a reduction of heat stress responses in artificial upwelling treatments in 2 of the 3 investigated species, and a stronger positive effect of 100-m water than 50-m water. These results indicate that artificial upwelling could be an effective strategy to mitigate coral bleaching during heat stress events allowing corals to adjust to increasing temperatures more gradually. It will still be necessary to further explore the ecological benefits as well as potential ecosystem impacts associated with different artificial upwelling scenarios to carefully implement an effective in situ artificial upwelling strategy in coral reefs.

This study was supported through funds from ESF/CHECKREEF project (EI272/22-1 to AE; DU129/141-1 to W-CD; 20MA21-115944 and 200020-140618 to ES), by the collaborative research initiative CHARON (DFG Forschergruppe 1644 – Phase II) funded by the German Research Foundation, and by the GEOMAR Helmholtz Centre for Ocean Research Kiel. The Ph.D. program of RR was financially supported by the Tanzania Ministry of Education and Vocational Training (MoEVT) in collaboration with DAAD (Deutscher Akademischer Austauschdienst) fellowship. AR acknowledges support from Swiss National Science Foundation Project Number SNF 200021_149247. Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ18O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO3– precipitation rates. Overall, the Sr/Ca, U/Ca and δ18O trends are well correlated, but we observed a significant offset up to ± 7°C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as ΔSST. This approach shows that ΔSST records derived from different proxies agree with an amplitudinal variability of up to ± 2°C with respect to their Holocene mean value. Higher ΔSST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower ΔSST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10Be and 14C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene. Publisher PDF

Protecting the ocean has become a major goal of international policy as human activities increasingly endanger the integrity of the ocean ecosystem, often summarized as ‘‘ocean health.’’ By and large, efforts to protect the ocean have failed because, among other things, (1) the underlying socio-ecological pathways have not been properly considered, and (2) the concept of ocean health has been ill defined. Collectively, this prevents an adequate societal response as to how ocean ecosystems and their vital functions for human societies can be protected and restored. We review the confusion surrounding the term ‘‘ocean health’’ and suggest an operational ocean-health framework in line with the concept of strong sustainability. Given the accelerating degeneration of marine ecosystems, the restoration of regional ocean health will be ofincreasing importance. Our advocated transdisciplinary and multi-actor framework can help to advance the implementation of more active measures to restore ocean health and safeguard human health and well-being.

Figures S1 and S2

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).

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledge on their capability of long-term acclimatization to ocean acidification (OA). We used a dual geochemical proxy approach to estimate the calcifying fluid pH (pH(cf)) and carbonate chemistry of a Mediterranean coral (Balanophyllia europaea) naturally growing along a pH gradient (range: pH(TS) 8.07-7.74). The pH(cf) derived from skeletal boron isotopic composition (delta B-11) was 0.3-0.6 units above seawater values and homogeneous along the gradient (mean +/- SEM: Site 1 = 8.39 +/- 0.03, Site 2 = 8.34 +/- 0.03, Site 3 = 8.34 +/- 0.02). Also carbonate ion concentration derived from B/Ca was homogeneous [mean +/- SEM (mu mol kg(-1)): Site 1 = 579 +/- 34, Site 2 = 541 +/- 27, Site 3 = 568 +/- 30] regardless of seawater pH. Furthermore, gross calcification rate (GCR, mass of CaCO 3 deposited on the skeletal unit area per unit of time), estimated by a "bio-inorganic model" (IpHRAC), was homogeneous with decreasing pH. The homogeneous GCR, internal pH and carbonate chemistry confirm that the features of the "building blocks" - the fundamental structural components - produced by the biomineralization process were substantially unaffected by increased acidification. Furthermore, the pH up-regulation observed in this study could potentially explain the previous hypothesis that less "building blocks" are produced with increasing acidification ultimately leading to increased skeletal porosity and to reduced net calcification rate computed by including the total volume of the pore space. In fact, assuming that the available energy at the three sites is the same, this energy at the low pH sites could be partitioned among fewer calicoblastic cells that consume more energy given the larger difference between external and internal pH compared to the control, leading to the production of less building blocks (i.e., formation of pores skeleton structure, determining increased porosity). However, we cannot exclude that also dissolution may play a role in increasing porosity. Thus, the ability of scleractinian corals to maintain elevated pH(cf) relative to ambient seawater might not always be sufficient to counteract declines in net calcification under OA scenarios.

El fiordo Comau en la Patagonia chilena norte se caracteriza por presentar un marcado gradiente de pH, de 7.4 a 8.1. Bajo estas condiciones, las cuales corresponden al pH pronosticado para los océanos en el año 2100, están prosperando bancos de mitílidos, bancos de braquiópodos, acumulaciones de picorocos, praderas de gorgonias y bancos de corales de aguas frías. Estos “bosques” de animales marinos forman la base estructural y funcional de un ecosistema bentónico marino muy diverso. La comunidad que domina principalmente en paredes rocosas desde los 80 m son los bancos de corales, con la especie matriz Desmophyllum dianthus. Interesantemente, D. dianthus crece en aguas de alto (sobresaturadas de aragonita) y bajo pH (insaturadas de aragonita), así como en aguas someras y profundas (desde aprox. 15 m hasta más de 400 m). Esto indica que el coral es capaz de regular y controlar su calcificación. Se incubó D. dianthus simulando futuros escenarios de acidificación oceánica y en dichas incubaciones se midió la composición isotópica del boro en su esqueleto (11B). D. dianthus presentó incrementos del pH interno de calcificación (pHcf) como respuesta reguladora frente a pH externos (pHsw) más bajos. Todavía falta una explicación fisiológica del pHcf en corales bajo diferentes pHsw. Utilizando microsensores para pH, calcio y oxígeno medimos el pHcf en D. dianthus en relación a la dinámica del calcio y la respiración a lo largo del pólipo del coral bajo diferentes pHsw. Encontramos que el pHcf y el pHsw están relacionados, pero no de forma directa debido a la gran heterogeneidad del pHcf. Esto sugiere una regulación del pHcf altamente compleja e inconsistente con los modelos anteriores, indicando que D. dianthus incrementa probablemente el pool interno de carbono y no el pH para facilitar la calcificación.

Cold-water corals are important habitat formers in deep-water ecosystems and at high latitudes. Ocean acidification and the resulting change in aragonite saturation are expected to affect these habitats and impact coral growth. Counter to expectations, the deep water coral Lophelia pertusa has been found to be able to sustain growth even in undersaturated conditions. However, it is important to know whether such undersaturation modifies the skeleton and thus its ecosystem functioning. Here we used Synchrotron X-Ray Tomography and Raman spectroscopy to examine changes in skeleton morphology and fibre orientation. We combined the morphological assessment with boron isotope analysis to determine if changes in growth are related to changes in control of calcification pH. We compared the isotopic composition and structure formed in their natural environment to material grown in culture at lower pH conditions. Skeletal morphology is highly variable but shows no distinctive differences between natural and low pH conditions. Raman investigations found no difference in macromorphological skeletal arrangement of early mineralization zones and secondary thickening between the treatments. The δ11B analyses show that L. pertusa up-regulates the internal calcifying fluid pH (pHcf) during calcification compared to ambient seawater pH and maintain a similar elevated pHcf at increased pCO2 conditions. We suggest that as long as the energy is available to sustain the up-regulation, i.e. individuals are well fed, there is no detrimental to the skeletal morphology.