Anthropogenic ocean acidification (OA) has compromised the ability of marine organisms to calcify. However, many coastal environments naturally exhibit high variability in seawater pH (pHsw) and the impact of OA on these environments is unclear. For instance, sub-tropical corals can modify the pH of the calcifying fluid (pHcf) from which they precipitate their skeleton. This study examines the influence of OA on pHcf upregulation of Porites lutea microatolls inhabiting reef flat environments. Environmental measurements including pHsw and temperature were performed on reef flats and adjacent fore-reefs on Kiritimati Island (Kiribati), Arno Atoll (Marshall Islands), and Rarotonga (Cook Islands) to quantify the temporal and spatial variability of these parameters. Slabs were removed from microatolls to construct multi-decadal (1938 – 2018) records of their boron isotopic (δ11B) and geochemical composition. The sensitivity of microatoll pHcf upregulation to ambient pHsw was evaluated by comparing annual band δ11B with synchronously recorded pHsw and temperature, and microatoll records were compared to a fore-reef record of similar age. Although daily means in pHsw on reef flats and fore-reefs were relatively similar, large diurnal cycles in pHsw (ΔpHsw = 0.28) and temperature (ΔT = 2.0°C) were found on reef flats exceeding that on fore-reefs by far (ΔpHsw = 0.07, ΔT = 0.7°C). Furthermore, spatial variations in pHsw and temperature were observed that were linked to reef flat hydrodynamics. Microatoll pHcf revealed a higher correlation to ambient seawater temperatures than to pHsw and only the fore-reef core showed a long-term trend in pHcf (-0.0003±0.0009 year-1) that is indicative of OA, while microatoll records revealed variable long-term trends unlikely reflecting ocean conditions (-0.0030±0.0005 to +0.0007±0.0003 year-1). Corals from the three sites revealed similar mean pHcf ≈ 8.44 although the difference in pHsw between the locations (ΔpHsw = 0.17) noticeably exceeded the decline in pHsw due to OA (ΔpHsw = 0.10). In conclusion, Porites lutea microatoll pHcf appeared to be relatively insensitive to OA. This is likely a result of the large variability in seawater conditions on reef flats that supersede OA, and the strong modification of coral pHcf by physiological processes.
Continue reading ‘Boron isotope records from Pacific microatolls: modifications in Porites lutea calcifying fluid composition under anthropogenic ocean acidification and natural pH variability’Posts Tagged 'corals'
Boron isotope records from Pacific microatolls: modifications in Porites lutea calcifying fluid composition under anthropogenic ocean acidification and natural pH variability
Published 2 December 2020 Science ClosedTags: biogeochemistry, chemistry, corals, field, paleo, South Pacific
Simulated climate change scenarios impact the reproduction and early life stages of a soft coral
Published 25 November 2020 Science ClosedTags: biological response, corals, laboratory, mortality, multiple factors, photosynthesis, protists, Red Sea, reproduction, temperature
Highlights
- A change in the timing of onset of the soft coral breeding event occurred under elevated temperature and reduced pH seawater conditions.
- A disruption of the synchronicity of the breeding event occurred under elevated temperature and reduced pH seawater conditions.
- End-of-the-century seawater conditions are expected to affect the reproduction of the soft coral Rhytisma fulvum.
- Planula survival and polyp metamorphosis rates were significantly reduced under both end-of-the-century seawater conditions compared to propagules reared under ambient conditions.
- The photosynthetic capacity of the parent soft coral colonies was reduced under the end-of-the-century seawater conditions in comparison to those under the ambient conditions.
Abstract
Coral reefs are threatened worldwide by global climate change, manifested in anthropogenic ocean warming and acidification. Despite the importance of coral sexual reproduction for the continuity of coral reefs, our understanding of the extent of the impact of climate change on coral sexual reproduction, particularly on coral reproductive phenology and early life stages, is limited. Here, we experimentally examined the effects of predicted end-of-the-century seawater conditions on the sexual reproduction and photosynthetic capacity of a Red-Sea zooxanthellate octocoral, Rhytisma fulvum. Sexually mature colonies were exposed to ambient temperature and pH conditions and to Representative Concentration Pathway (RCP) conditions (4.5 and 8.5), five weeks prior to their expected surface-brooding event. The reproductive phenology of the colonies under the simulated seawater conditions was compared to that on the natural reef. In addition, subsequent planulae development and their metamorphosis into primary polyps under the same RCP conditions as their parent colonies were monitored in a running seawater system. The results reveal that both RCP conditions led to a change in the timing of onset of the surface-brooding event and its synchronicity. In contrast, the surface-brooding event under ambient conditions co-occurred with that of the in-situ reef colonies and maintained its synchrony. Similarly, planula survival and polyp metamorphosis rate were significantly reduced under both RCP conditions compared to propagules reared under ambient conditions. In addition, the photosynthetic capacity of the parent colonies under both RCPs showed a reduction relative to that under the ambient conditions in the experiment, suggesting a reduction in carbon fixation during the late stages of gametogenesis. While our findings indicate that octocoral reproductive phenology is affected by environmental changes, further work is required in order to elucidate the long-term implications for the R. fulvum population in the northern Red Sea.
Continue reading ‘Simulated climate change scenarios impact the reproduction and early life stages of a soft coral’Assessing coral reef condition indicators for local and global stressors using Bayesian networks
Published 23 November 2020 Science ClosedTags: biological response, communitymodeling, corals, globalmodeling, modeling
Coral reefs are highly valued ecosystems currently threatened by both local and global stressors. Given the importance of coral reef ecosystems, a Bayesian network approach can benefit an evaluation of threats to reef condition. To this end, we used data to evaluate the overlap between local stressors (overfishing and destructive fishing, watershed‐based pollution, marine‐based pollution, and coastal development threats), global stressors (acidification and thermal stress) and management effectiveness with indicators of coral reef health (live coral index, live coral cover, population bleaching, colony bleaching and recently killed corals). Each of the coral health indicators had Bayesian networks constructed globally and for Pacific, Atlantic, Australia, Middle East, Indian Ocean, and Southeast Asia coral reef locations. Sensitivity analysis helped evaluate the strength of the relationships between different stressors and reef condition indicators. The relationships between indicators and stressors were also evaluated with conditional analyses of linear and nonlinear interactions. In this process, a standardized direct effects analysis was emphasized with a target mean analysis to predict changes in the mean value of the reef indicator from individual changes to the distribution of the predictor variables. The standardized direct effects analysis identified higher risks in the Middle East for watershed‐based pollution with population bleaching and Australia for overfishing and destructive fishing with living coral. For thermal stress, colony bleaching and recently killed coral in the Indian Ocean were found to have the strongest direct associations. For acidification threat, Australia had a relatively strong association with colony bleaching and the Middle East had the strongest overall association with recently killed coral although extrapolated spatial data were used for the acidification estimates. The Bayesian network approach helped to explore the relationships among existing databases used for policy development in coral reef management by examining the sensitivity of multiple indicators of reef condition to spatially‐distributed stress.
Continue reading ‘Assessing coral reef condition indicators for local and global stressors using Bayesian networks’Resilience of the temperate coral Oculina arbuscula to ocean acidification extends to the physiological level
Published 20 November 2020 Science ClosedTags: abundance, adaptation, biological response, calcification, corals, laboratory, North Atlantic, otherprocess, photosynthesis, physiology, protists
Both juvenile and adult life stages of the temperate scleractinian coral Oculina arbuscula are resilient to the effects of moderate ocean acidification (OA) in contrast to many tropical corals in which growth and calcification rates are suppressed. Here, potential mechanisms of resilience to OA related to photosynthetic physiology and inorganic carbon processing were studied in adult O. arbuscula colonies. After exposing colonies to ambient and elevated carbon dioxide (CO2) treatments for 7 weeks, photosynthetic performance was characterized using photosynthesis versus irradiance experiments, chlorophyll fluorescence kinetics, and algal pigment content. Inorganic carbon-processing capabilities were assessed by measurement of internal and external carbonic anhydrase activity of the coral host, internal carbonic anhydrase activity of symbiotic algae, and the reliance of photosynthesis on external carbonic anhydrase. Photosynthetic physiology was unaffected by OA ruling out the possibility that resilience was mediated by increased photosynthetic energy supply. Carbonic anhydrase activities were maintained at elevated CO2 suggesting no major rearrangements of the inorganic carbon-processing machinery, but this could be a sign of resilience since tropical corals often down-regulate carbonic anhydrases at high CO2. The general lack of effect of ocean acidification on these physiological traits suggests other characteristics, such as maintenance of calcifying fluid pH and ability to acquire energy from heterotrophy, may be more important for the resilience of O. arbuscula to OA.
Continue reading ‘Resilience of the temperate coral Oculina arbuscula to ocean acidification extends to the physiological level’From particle attachment to space-filling coral skeletons
Published 16 November 2020 Science ClosedTags: calcification, corals, methods
Reef-building corals and their aragonite (CaCO3) skeletons support entire reef ecosystems, yet their formation mechanism is poorly understood. Here we used synchrotron spectromicroscopy to observe the nanoscale mineralogy of fresh, forming skeletons from six species spanning all reef-forming coral morphologies: Branching, encrusting, massive, and table. In all species, hydrated and anhydrous amorphous calcium carbonate nanoparticles were precursors for skeletal growth, as previously observed in a single species. The amorphous precursors here were observed in tissue, between tissue and skeleton, and at growth fronts of the skeleton, within a low-density nano- or microporous layer varying in thickness from 7 to 20 µm. Brunauer-Emmett-Teller measurements, however, indicated that the mature skeletons at the microscale were space-filling, comparable to single crystals of geologic aragonite. Nanoparticles alone can never fill space completely, thus ion-by-ion filling must be invoked to fill interstitial pores. Such ion-by-ion diffusion and attachment may occur from the supersaturated calcifying fluid known to exist in corals, or from a dense liquid precursor, observed in synthetic systems but never in biogenic ones. Concomitant particle attachment and ion-by-ion filling was previously observed in synthetic calcite rhombohedra, but never in aragonite pseudohexagonal prisms, synthetic or biogenic, as observed here. Models for biomineral growth, isotope incorporation, and coral skeletons’ resilience to ocean warming and acidification must take into account the dual formation mechanism, including particle attachment and ion-by-ion space filling.
Continue reading ‘From particle attachment to space-filling coral skeletons’Intracellular pH regulation: characterization and functional investigation of H+ transporters in Stylophora pistillata
Published 10 November 2020 Science ClosedTags: biological response, corals, laboratory, molecular biology, physiology, protists
Background: Reef-building corals regularly experience changes in intra and extracellular H+ concentration ([H+]) due to physiological and environmental processes. Stringent control of [H+] is required for the maintenance of homeostatic acid-base balance in coral cells and is achieved through the regulation of intracellular pH (pHi). This task is especially challenging for reef-building corals that share an endosymbiotic relationship with photosynthetic dinoflagellates (family Symbiodinaceae), which exert a significant effect on the pHi of coral cells. Despite their importance, the pH regulatory proteins involved in the homeostatic acid-base balance have been scarcely investigated in corals. Here, we reported the full characterisation in terms of genomic structure, domain topology and phylogeny of three majors H+ transporter families implicated in pHi regulation; we investigated their tissue-specific expression and we assessed the effect of seawater acidification on their level of expression.
Results: We identified members of the Na+/H+ exchanger (SLC9), vacuolar-type electrogenic H+-ATP hydrolases (V-ATPase) and voltage-gated proton channels (HvCN) families in the genome and transcriptome of S. pistillata. In addition, we identified a novel member of the HvCN gene family in the cnidarian subclass Hexacorallia, which has never been described in any species to date. We also reported key residues that participate to the H+ transporters substrate specificity, protein function and regulation. Lastly, we demonstrated that some of these have different tissue expression patterns and are mostly unaffected by exposure to seawater acidification.
Conclusions: In this study, we provide the first characterization of the H+ transporters genes that contribute to homeostatic acid-base balance in coral cells. This work will enrich knowledge about basic aspects of coral biology, bearing important implications for our understanding of how corals regulate their intracellular environment.
Continue reading ‘Intracellular pH regulation: characterization and functional investigation of H+ transporters in Stylophora pistillata’Coral-macroalgal competition under ocean warming and acidification
Published 9 November 2020 Science ClosedTags: algae, biological response, BRcommunity, calcification, corals, laboratory, light, morphology, multiple factors, performance, photosynthesis, physiology, protists, respiration, South Pacific, temperature
Highlights
- Study investigates a common coral-macroalgal interaction under a low end emission scenario.
- Light calcification is negatively influenced by an interaction of macroalgal contact and scenario.
- Protein content, zooxanthellae density and Chlorophyll a were enhanced under scenario conditions.
- Negative impacts of macroalgae on corals were observed, but not enhanced by scenario conditions.
- More research on the impacts of climate change on the dynamics of coral-algal interactions is needed.
Abstract
Competition between corals and macroalgae is frequently observed on reefs with the outcome of these interactions affecting the relative abundance of reef organisms and therefore reef health. Anthropogenic activities have resulted in increased atmospheric CO2 levels and a subsequent rise in ocean temperatures. In addition to increasing water temperature, elevated CO2 levels are leading to a decrease in oceanic pH (ocean acidification). These two changes have the potential to alter ecological processes within the oceans, including the outcome of competitive coral-macroalgal interactions. In our study, we explored the combined effect of temperature increase and ocean acidification on the competition between the coral Porites lobata and on the Great Barrier Reef abundant macroalga Chlorodesmis fastigiata. A temperature increase of +1 °C above present temperatures and CO2 increase of +85 ppm were used to simulate a low end emission scenario for the mid- to late 21st century, according to the Representative Concentration Pathway 2.6 (RCP2.6). Our results revealed that the net photosynthesis of P. lobata decreased when it was in contact with C. fastigiata under ambient conditions, and that dark respiration increased under RCP2.6 conditions. The Photosynthesis to Respiration (P:R) ratios of corals as they interacted with macroalgal competitors were not significantly different between scenarios. Dark calcification rates of corals under RCP2.6 conditions, however, were negative and significantly decreased compared to ambient conditions. Light calcification rates were negatively affected by the interaction of macroalgal contact in the RCP2.6 scenario, compared to algal mimics and to coral under ambient conditions. Chlorophyll a, and protein content increased in the RCP2.6 scenario, but were not influenced by contact with the macroalga. We conclude that the coral host was negatively affected by RCP2.6 conditions, whereas the productivity of its symbionts (zooxanthellae) was enhanced. While a negative effect of the macroalga (C. fastigiata) on the coral (P. lobata) was observed for the P:R ratio under control conditions, it was not enhanced under RCP2.6 conditions.
Continue reading ‘Coral-macroalgal competition under ocean warming and acidification’Experimental techniques to assess coral physiology in situ: current approaches and novel insights
Published 6 November 2020 Science ClosedTags: biological response, calcification, corals, methods, physiology, respiration, review
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.
Continue reading ‘Experimental techniques to assess coral physiology in situ: current approaches and novel insights’High light alongside elevated PCO2 alleviates thermal depression of photosynthesis in a hard coral (Pocillopora acuta)
Published 30 October 2020 Science ClosedTags: abundance, biological response, corals, laboratory, light, multiple factors, North Pacific, otherprocess, photosynthesis, protists, respiration, temperature
The absorbtion of human-emitted CO2 by the oceans (elevated PCO2) is projected to alter the physiological performance of coral reef organisms by perturbing seawater chemistry (i.e. ocean acidification). Simultaneously, greenhouse gas emissions are driving ocean warming and changes in irradiance (through turbidity and cloud cover), which have the potential to influence the effects of ocean acidification on coral reefs. Here, we explored whether physiological impacts of elevated PCO2 on a coral–algal symbiosis (Pocillopora acuta–Symbiodiniaceae) are mediated by light and/or temperature levels. In a 39 day experiment, elevated PCO2 (962 versus 431 µatm PCO2) had an interactive effect with midday light availability (400 versus 800 µmol photons m−2 s−1) and temperature (25 versus 29°C) on areal gross and net photosynthesis, for which a decline at 29°C was ameliorated under simultaneous high-PCO2 and high-light conditions. Light-enhanced dark respiration increased under elevated PCO2 and/or elevated temperature. Symbiont to host cell ratio and chlorophyll a per symbiont increased at elevated temperature, whilst symbiont areal density decreased. The ability of moderately strong light in the presence of elevated PCO2 to alleviate the temperature-induced decrease in photosynthesis suggests that higher substrate availability facilitates a greater ability for photochemical quenching, partially offsetting the impacts of high temperature on the photosynthetic apparatus. Future environmental changes that result in moderate increases in light levels could therefore assist the P. acuta holobiont to cope with the ‘one–two punch’ of rising temperatures in the presence of an acidifying ocean.
Continue reading ‘High light alongside elevated PCO2 alleviates thermal depression of photosynthesis in a hard coral (Pocillopora acuta)’Progressive seawater acidification on the Great Barrier Reef continental shelf
Published 29 October 2020 Science ClosedTags: chemistry, corals, field, South Pacific
Coral reefs are highly sensitive to ocean acidification due to rising atmospheric CO2 concentrations. We present 10 years of data (2009–2019) on the long-term trends and sources of variation in the carbon chemistry from two fixed stations in the Australian Great Barrier Reef. Data from the subtropical mid-shelf GBRWIS comprised 3-h instrument records, and those from the tropical coastal NRSYON were monthly seawater samples. Both stations recorded significant variation in seawater CO2 fugacity (fCO2), attributable to seasonal, daytime, temperature and salinity fluctuations. Superimposed over this variation, fCO2 progressively increased by > 2.0 ± 0.3 µatm year−1 at both stations. Seawater temperature and salinity also increased throughout the decade, whereas seawater pH and the saturation state of aragonite declined. The decadal upward fCO2 trend remained significant in temperature- and salinity-normalised data. Indeed, annual fCO2 minima are now higher than estimated fCO2 maxima in the early 1960s, with mean fCO2 now ~ 28% higher than 60 years ago. Our data indicate that carbonate dissolution from the seafloor is currently unable to buffer the Great Barrier Reef against ocean acidification. This is of great concern for the thousands of coral reefs and other diverse marine ecosystems located in this vast continental shelf system.
Continue reading ‘Progressive seawater acidification on the Great Barrier Reef continental shelf’Impact of ocean warming and acidification on symbiosis establishment and gene expression profiles in recruits of reef coral Acropora intermedia
Published 28 October 2020 Science ClosedTags: biological response, corals, laboratory, molecular biology, mortality, multiple factors, North Pacific, otherprocess, physiology, protists, reproduction, temperature
The onset of symbiosis and the early development of most broadcast spawning corals play pivotal roles in recruitment success, yet these critical early stages are threatened by multiple stressors. However, molecular mechanisms governing these critical processes under ocean warming and acidification are still poorly understood. The present study investigated the interactive impact of elevated temperature (∼28.0°C and ∼30.5°C) and partial pressure of carbon dioxide (pCO2) (∼600 and ∼1,200 μatm) on early development and the gene expression patterns in juvenile Acropora intermedia over 33 days. The results showed that coral survival was >89% and was unaffected by high temperature, pCO2, or the combined treatment. Notably, high temperature completely arrested successful symbiosis establishment and the budding process, whereas acidification had a negligible effect. Moreover, there was a positive exponential relationship between symbiosis establishment and budding rates (y = 0.0004e6.43x, R = 0.72, P < 0.0001), which indicated the importance of symbiosis in fueling asexual budding. Compared with corals at the control temperature (28°C), those under elevated temperature preferentially harbored Durusdinium spp., despite unsuccessful symbiosis establishment. In addition, compared to the control, 351 and 153 differentially expressed genes were detected in the symbiont and coral host in response to experimental conditions, respectively. In coral host, some genes involved in nutrient transportation and tissue fluorescence were affected by high temperature. In the symbionts, a suite of genes related to cell growth, ribosomal proteins, photosynthesis, and energy production was downregulated under high temperatures, which may have severely hampered successful cell proliferation of the endosymbionts and explains the failure of symbiosis establishment. Therefore, our results suggest that the responses of symbionts to future ocean conditions could play a vital role in shaping successful symbiosis in juvenile coral.
Continue reading ‘Impact of ocean warming and acidification on symbiosis establishment and gene expression profiles in recruits of reef coral Acropora intermedia’Seasonal controls of the carbon biogeochemistry of a fringing coral reef in the Gulf of California, Mexico
Published 28 October 2020 Science ClosedTags: biogeochemistry, chemistry, corals, field, North Pacific
The surface of the ocean has absorbed one-third of the CO2gas that has been released by anthropogenic activities, which has resulted in a reduction in pH and the aragonite saturation state (Ωara) with potential negative impacts in calcifying organisms, such as corals. To evaluate these effects, the natural variability present must first be understood, including that of processes that operate at diurnal, seasonal, and interannual frequencies. The objective of this study was to determine the influence of physical and biogeochemical processes on the seasonal variability of the CO2-system in a fringe coral reef of the Eastern Tropical Pacific (ETP). To achieve this, a SeapHOx sensor was installed to measure temperature, salinity, dissolved oxygen, and pHTot at 30-min intervals from November 2013 (early winter) to July 2014 (early summer). The recorded temperature and salinity data fed a mixing model to identify the water masses present in the reef. We show how physical and biogeochemical oceanic processes influence and control the variability of the carbonate system. The presence of water masses with different carbon chemistries responded to two scenarios: (1) seasonal circulation on the order of months and (2) an intermittence between water masses related to mesoscale structures (eddies) on the order of weeks. A low-pH and Ωara condition was detected during summer, which was related to the presence of warm and respired Tropical Surface Water. The broadest changes in Ωara were the result of physical processes (winter ΔΩara = 0.14 and summer ΔΩara = 0.34 units) and corresponded to the transition between water masses with different carbon-biogeochemistry signals. Our results suggest that the Cabo Pulmo coral community develops in an environment with a wide range of pH and Ωara conditions and that seasonal changes are controlled by open ocean carbon biogeochemistry.
Continue reading ‘Seasonal controls of the carbon biogeochemistry of a fringing coral reef in the Gulf of California, Mexico’Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events
Published 23 October 2020 Science ClosedTags: chemistry, corals, paleo, sediment
The Southern Ocean plays a crucial role in regulating atmospheric CO2 on centennial to millennial time scales. However, observations of sufficient resolution to explore this have been lacking. Here, we report high-resolution, multiproxy records based on precisely dated deep-sea corals from the Southern Ocean. Paired deep (∆14C and δ11B) and surface (δ15N) proxy data point to enhanced upwelling coupled with reduced efficiency of the biological pump at 14.6 and 11.7 thousand years (ka) ago, which would have facilitated rapid carbon release to the atmosphere. Transient periods of unusually well-ventilated waters in the deep Southern Ocean occurred at 16.3 and 12.8 ka ago. Contemporaneous atmospheric carbon records indicate that these Southern Ocean ventilation events are also important in releasing respired carbon from the deep ocean to the atmosphere. Our results thus highlight two distinct modes of Southern Ocean circulation and biogeochemistry associated with centennial-scale atmospheric CO2 jumps during the last deglaciation.
A new “business as usual” climate scenario and the stress response of the Caribbean coral Montastraea cavernosa
Published 16 October 2020 Science ClosedTags: abundance, biological response, community composition, corals, laboratory, molecular biology, multiple factors, North Atlantic, otherprocess, performance, photosynthesis, prokaryotes, protists, temperature
The climate change related decline of shallow (<30 m) coral reef ecosystems has been driven by the mortality of scleractinian corals caused primarily by the phenomenon known as “coral bleaching.” But despite pervasive phase shifts and macroalgal dominance on many coral reefs, some coral species have persisted. One of those species is Montastraea cavernosa which has been categorized as resilient to a range of biotic and abiotic stressors. In order to understand the mechanism(s) of resistance in this coral, we present the results of a thermal stress and ocean acidification (OA) experiment on M. cavernosa, both its brown and orange color morphs, representing conditions predicted by the Representative Concentration Pathway (RCP) 6.0 scenario in the year 2100. We assessed the community response of the prokaryotic microbiome, the photophysiological response of the endosymbiotic Symbiodiniaceae and the molecular responses of critical pathways in the host by quantifying transcript abundances of genes encoding fluorescent proteins, heat shock proteins, antioxidant enzymes and regulators of apoptosis. After a 12 d acclimatization experiment, no visible bleaching was observed in any treatment, and the excitation pressure on photosystem II of the symbiotic Symbiodiniaceae showed no effects of the independent or interactive effects of thermal stress and OA, while only minor, but significant, changes in the prokaryotic microbiome were observed when exposed to RCP 6.0 predicted OA conditions. At the end of the experiment, the host heat shock protein 90 showed an increase in transcript abundance under the combined effects of thermal stress and OA compared to high temperatures alone, but these treatment groups were not significantly different from treatments under normal temperatures. While Bax, an activator of apoptosis, was significantly higher under thermal stress alone compared to control samples. Taken together, M. cavernosa, exhibits ecological stability over time and this may be based on its physiological persistence, resistance and resilience when experimentally exposed to the ecologically realistic RCP 6.0 climate model predictions.
Coral reef sediment dissolution in a changing ocean: insights from a temporal field study
Published 7 October 2020 Science ClosedTags: biological response, BRcommunity, calcification, corals, dissolution, field, photosynthesis, physiology, primary production, respiration, sediment, South Pacific
Calcium carbonate sediments form an essential part of coral reefs yet have often been overlooked when studying the effects of future ocean acidification (OA). This original field-based research aims to assess the temporal variability of organic and inorganic sediment metabolism under ambient and elevated pCO2. OA caused a shift from net precipitation to net dissolution, but the sensitivity to OA varied seasonally, depending on interactions with temperature and benthic productivity. A slack-water approach of net ecosystem calcification revealed that sediments can play an important role in carbonate budgets, particularly at night, and become increasingly important as the oceans continue acidifying.
Continue reading ‘Coral reef sediment dissolution in a changing ocean: insights from a temporal field study’Ocean acidification causes variable trait shifts in a coral species
Published 5 October 2020 Science ClosedTags: adaptation, biological response, calcification, chemistry, corals, field, growth, Mediterranean, molecular biology, morphology, otherprocess, vents
High pCO2 habitats and their populations provide an unparalleled opportunity to assess how species may survive under future ocean acidification conditions, and help to reveal the traits that confer tolerance. Here we utilize a unique CO2 vent system to study the effects of exposure to elevated pCO2 on trait‐shifts observed throughout natural populations of Astroides calycularis, an azooxanthellate scleractinian coral endemic to the Mediterranean. Unexpected shifts in skeletal and growth patterns were found. Colonies shifted to a skeletal phenotype characterized by encrusting morphology, smaller size, reduced coenosarc tissue, fewer polyps, and less porous and denser skeletons at low pH. Interestingly, while individual polyps calcified more and extended faster at low pH, whole colonies found at low pH site calcified and extended their skeleton at the same rate as did those at ambient pH sites. Transcriptomic data revealed strong genetic differentiation among local populations of this warm water species whose distribution range is currently expanding northward. We found excess differentiation in the CO2 vent population for genes central to calcification, including genes for calcium management (calmodulin, calcium‐binding proteins), pH regulation (V‐type proton ATPase), and inorganic carbon regulation (carbonic anhydrase). Combined, our results demonstrate how coral populations can persist in high pCO2 environments, making this system a powerful candidate for investigating acclimatization and local adaptation of organisms to global environmental change.
Continue reading ‘Ocean acidification causes variable trait shifts in a coral species’
Effects of nearshore processes on carbonate chemistry dynamics and ocean acidification
Published 22 September 2020 Science ClosedTags: biogeochemistry, BRcommunity, calcification, chemistry, corals, field, modeling, North Atlantic, primary production, regionalmodeling, respiration
Time series from open ocean fixed stations have robustly documented secular changes in carbonate chemistry and long-term ocean acidification (OA) trends as a direct response to increases in atmospheric carbon dioxide (CO2). However, few high-frequency coastal carbon time series are available in reef systems, where most affected tropical marine organisms reside. Seasonal variations in carbonate chemistry at Cheeca Rocks (CR), Florida, and La Parguera (LP), Puerto Rico, are presented based on 8 and 10 years of continuous, high-quality measurements, respectively. This dissertation synthesizes autonomous and bottle observations to model carbonate chemistry and to understand how physical and biological processes affect seasonal carbonate chemistry at both locations. The autonomous carbonate chemistry and oxygen observations are used to examine a mass balance approach using a 1-D model to determine net rates of ecosystem calcification and production (NEC and NEP) from communities close (<5km) to the buoys. The results provide evidence to suggest that seasonal response between benthic metabolism and seawater chemistry at LP is attenuated relative to that at CR because their differences in benthic cover and how benthic metabolism modifies the water chemistry. Simple linear trends cannot explain the feedback between metabolism and reef water chemistry using long-term observations over natural variations. The effects of community production on partial pressure of CO2 (pCO2sw) make these interactions complex at short- and long-term scales. Careful consideration should be taken when inferring local biogeochemical processes, given that pCO2sw (and presumably pH) respond on much shorter time and local scales than dissolved inorganic carbon (DIC) and total alkalinity (TA). The observations highlight the need for more comprehensive observing systems that can reliably measure both the fast-response (pCO2sw, pH) and slow-response (DIC) carbon pools.
Crumbling reefs and cold-water coral habitat loss in a future ocean: evidence of “coralporosis” as an indicator of habitat integrity
Published 21 September 2020 Science ClosedTags: biological response, calcification, chemistry, corals, dissolution, field, laboratory, morphology, North Pacific
Ocean acidification is a threat to the net growth of tropical and deep-sea coral reefs, due to gradual changes in the balance between reef growth and loss processes. Here we go beyond identification of coral dissolution induced by ocean acidification and identify a mechanism that will lead to a loss of habitat in cold-water coral reef habitats on an ecosystem-scale. To quantify this, we present in situ and year-long laboratory evidence detailing the type of habitat shift that can be expected (in situ evidence), the mechanisms underlying this (in situ and laboratory evidence), and the timescale within which the process begins (laboratory evidence). Through application of engineering principals, we detail how increased porosity in structurally critical sections of coral framework will lead to crumbling of load-bearing material, and a potential collapse and loss of complexity of the larger habitat. Importantly, in situ evidence highlights that cold-water corals can survive beneath the aragonite saturation horizon, but in a fundamentally different way to what is currently considered a biogenic cold-water coral reef, with a loss of the majority of reef habitat. The shift from a habitat with high 3-dimensional complexity provided by both live and dead coral framework, to a habitat restricted primarily to live coral colonies with lower 3-dimensional complexity represents the main threat to cold-water coral reefs of the future and the biodiversity they support. Ocean acidification can cause ecosystem-scale habitat loss for the majority of cold-water coral reefs.
The Northeast Atlantic is running out of excess carbonate in the horizon of cold-water corals communities
Published 18 September 2020 Science ClosedTags: biogeochemistry, chemistry, corals, field, North Atlantic
The oceanic uptake of atmospheric carbon dioxide (CO2) emitted by human activities alters the seawater carbonate system. Here, the chemical status of the Northeast Atlantic is examined by means of a high-quality database of carbon variables based on the GO-SHIP A25 section (1997–2018). The increase of atmospheric CO2 leads to an increase in ocean anthropogenic carbon (Cant) and a decrease in carbonate that is unequivocal in the upper and mid-layers (0–2,500 m depth). In the mid-layer, the carbonate content in the Northeast Atlantic is maintained by the interplay between the northward spreading of recently conveyed Mediterranean Water with excess of carbonate and the arrival of subpolar-origin waters close to carbonate undersaturation. In this study we show a progression to undersaturation with respect to aragonite that could compromise the conservation of the habitats and ecosystem services developed by benthic marine calcifiers inhabiting that depth-range, such as the cold-water corals (CWC) communities. For each additional ppm in atmospheric pCO2 the waters surrounding CWC communities lose carbonate at a rate of − 0.17 ± 0.02 μmol kg−1 ppm−1. The accomplishment of global climate policies to limit global warming below 1.5–2 ℃ will avoid the exhaustion of excess carbonate in the Northeast Atlantic.
Acclimatization drives differences in reef-building coral calcification rates
Published 17 September 2020 Science ClosedTags: adaptation, biological response, calcification, chemistry, corals, field, growth, morphology, North Pacific, otherprocess
Coral reefs are susceptible to climate change, anthropogenic influence, and environmental stressors. However, corals in Kāneʻohe Bay, Hawaiʻi have repeatedly shown resilience and acclimatization to anthropogenically-induced rising temperatures and increased frequencies of bleaching events. Variations in coral and algae cover at two sites—just 600 m apart—at Malaukaʻa fringing reef suggest genetic or environmental differences in coral resilience between sites. A reciprocal transplant experiment was conducted to determine if calcification (linear extension and dry skeletal weight) for dominant reef-building species, Montipora capitata and Porites compressa, varied between the two sites and whether or not parent colony or environmental factors were responsible for the differences. Despite the two sites representing distinct environmental conditions with significant differences between temperature, salinity, and aragonite saturation, M. capitata growth rates remained the same between sites and treatments. However, dry skeletal weight increases in P. compressa were significantly different between sites, but not across treatments, with linear mixed effects model results suggesting heterogeneity driven by environmental differences between sites and the parent colonies. These results provide evidence of resilience and acclimatization for M. capitata and P. compressa. Variability of resilience may be driven by local adaptations at a small, reef-level scale for P. compressa in Kāneʻohe Bay.
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