Posts Tagged 'corals'

Full in vivo characterization of carbonate chemistry at the site of calcification in corals

Reef-building corals form their calcium carbonate skeletons within an extracellular calcifying medium (ECM). Despite the critical role of the ECM in coral calcification, ECM carbonate chemistry is poorly constrained in vivo, and full ECM carbonate chemistry has never been characterized based solely on direct in vivo measurements. Here, we measure pHECM in the growing edge of Stylophora pistillata by simultaneously using microsensors and the fluorescent dye SNARF-1, showing that, when measured at the same time and place, the results agree. We then conduct microscope-guided microsensor measurements of pH, [Ca2+], and [CO32−] in the ECM and, from this, determine [DIC]ECM and aragonite saturation state (Ωarag), showing that all parameters are elevated with respect to the surrounding seawater. Our study provides the most complete in vivo characterization of ECM carbonate chemistry parameters in a coral species to date, pointing to the key role of calcium- and carbon-concentrating mechanisms in coral calcification.

Continue reading ‘Full in vivo characterization of carbonate chemistry at the site of calcification in corals’

Carbonate dissolution by reef microbial borers: a biogeological process producing alkalinity under different pCO2 conditions

Rising atmospheric CO2 is acidifying the world’s oceans, affecting both calcification and dissolution processes in coral reefs. Among processes, carbonate dissolution by bioeroding microflora has been overlooked, and especially its impact on seawater alkalinity. To date, this biogeological process has only been studied using microscopy or buoyant weight techniques. To better understand its possible effect on seawater alkalinity, and thus on reef carbonate budget, an experiment was conducted under various seawater chemistry conditions (2 ≤ Ωarag ≤ 3.5 corresponding to 440 ≤ pCO2 (µatm) ≤ 940) at 25 °C under night and daylight (200 µmol photons m−2 s−1) with natural microboring communities colonizing dead coral blocks (New Caledonia). Both the alkalinity anomaly technique and microscopy methods were used to study the activity of those communities dominated by the chlorophyte Ostreobium sp. Results show that (1) the amount of alkalinity released in seawater by such communities is significant and varies between 12.8 ± 0.7 at ΩArag ~ 2 and 5.6 ± 0.4 mmol CaCO3 m−2 day−1 at ΩArag ~ 3–3.5 considering a 12:12 photoperiod; (2) although dissolution is higher at night (~ 80 vs. 20% during daylight), the process can occur under significant photosynthetic activity; and (3) the process is greatly stimulated when an acidity threshold is reached (pCO2 ≥ 920 µatm vs. current conditions at constant light intensity). We show that carbonate dissolution by microborers is a major biogeochemical process that could dissolve a large part of the carbonates deposited by calcifying organisms under ocean acidification.

Continue reading ‘Carbonate dissolution by reef microbial borers: a biogeological process producing alkalinity under different pCO2 conditions’

Diurnally fluctuating pCO2 modifies the physiological responses of coral recruits under ocean acidification

Diurnal pCO2 fluctuations have the potential to modulate the biological impact of ocean acidification (OA) on reef calcifiers, yet little is known about the physiological and biochemical responses of scleractinian corals to fluctuating carbonate chemistry under OA. Here, we exposed newly settled Pocillopora damicornis for 7 days to ambient pCO2, steady and elevated pCO2 (stable OA) and diurnally fluctuating pCO2 under future OA scenario (fluctuating OA). We measured the photo-physiology, growth (lateral growth, budding and calcification), oxidative stress and activities of carbonic anhydrase (CA), Ca-ATPase and Mg-ATPase. Results showed that while OA enhanced the photochemical performance of in hospite symbionts, it also increased catalase activity and lipid peroxidation. Furthermore, both OA treatments altered the activities of host and symbiont CA, suggesting functional changes in the uptake of dissolved inorganic carbon (DIC) for photosynthesis and calcification. Most importantly, only the fluctuating OA treatment resulted in a slight drop in calcification with concurrent up-regulation of Ca-ATPase and Mg-ATPase, implying increased energy expenditure on calcification. Consequently, asexual budding rates decreased by 50% under fluctuating OA. These results suggest that diel pCO2 oscillations could modify the physiological responses and potentially alter the energy budget of coral recruits under future OA, and that fluctuating OA is more energetically expensive for the maintenance of coral recruits than stable OA.

Continue reading ‘Diurnally fluctuating pCO2 modifies the physiological responses of coral recruits under ocean acidification’

Boron isotope composition of the cold-water coral Lophelia pertusa along the Norwegian margin: zooming into a potential pH-proxy by combining bulk and high-resolution approaches

High-latitude cold-water coral reefs are particularly vulnerable to climate change due to enhanced CO2 uptake in these regions. To evaluate their physiological functioning and potential application as pH archives, we retrieved both recent and fossil samples of Lophelia pertusa along the Norwegian margin from Oslofjord (59°N), over to Trondheimsfjord, Sula and Lopphavet (70.6°N). Boron isotope analyses (δ11B) were undertaken using solution-based and laser ablation multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS; LA-ICP-MS), and secondary ion mass spectrometry (SIMS). Epi-fluorescence microscopy was employed to provide a rapid pre-screening routine for structure-specific subsampling in the coral skeleton. This integrated approach enabled us to assess heterogeneities within single specimens, as well as to investigate the role of local environmental influences including recent and past variations. All three mass spectrometry methods show substantial differences in the δ11B of the theca wall (TW) and the centres of calcification (COC’s). Micro-bulk subsamples milled from the theca wall of modern specimens originating from different habitats but with comparable seawater pH (8–8.16) gave consistent δ11B values averaging 26.7 (±0.2‰, 2σ, n = 4), while COC subsamples systematically deviated towards lower B/Ca (by ~40%) and depleted δ11B values (minimum 22.7 ± 0.3‰, 2σ), implying a difference of at least 4‰ between TW and COC. SIMS and LA-ICP-MS measurements identified much larger internal heterogeneities with maximum variation of ~10‰ between the distinct skeletal structures; minimal SIMS δ11B values of ~17.3 ± 1.2‰ (2σ) were associated with the pure COC material. Our findings may be interpreted in terms of the occurrence of two main, but likely different, biomineralisation mechanisms in L. pertusa, with the COC’s generally exhibiting minimal pH up-regulation, potentially supporting the use of bicarbonate in the early stages of biomineralisation. Furthermore, we highlight the potential utility of L. pertusa for palaeo-proxy studies if targeting the compositionally homogenous TW zones devoid of COC admixtures, which appear to provide highly reproducible measurements.

Continue reading ‘Boron isotope composition of the cold-water coral Lophelia pertusa along the Norwegian margin: zooming into a potential pH-proxy by combining bulk and high-resolution approaches’

Ecosystem calcification and production in two Great Barrier Reef coral reefs: methodological challenges and environmental drivers

This thesis investigates the drivers of coral reef ecosystem metabolism and the abilities of the different methodologies and analytical approaches to accurately represent reef dynamics. It encompassed tracing natural nutrient additions through bird guano into a coral cay. Developing a new, automated system for measuring carbonate chemistry for coral reef metabolism and the effects of mass coral bleaching on ecosystem functioning were quantified. Overall, it showed that natural nutrient additions and bleaching differentially affect coral reef metabolism and that subtle differences in analytical methods, sampling approaches, and data interpretation techniques can cause significant variation in metabolic estimates.

Continue reading ‘Ecosystem calcification and production in two Great Barrier Reef coral reefs: methodological challenges and environmental drivers’

Gene expression profiles of two coral species with varied resistance to ocean acidification

Recent studies have indicated that various corals might have different degrees of resistance to elevated CO2 levels. However, the underlying molecular mechanism accounting for these differences is still poorly understood. In this study, RNA-seq data were analyzed to identify differentially expressed genes in two coral species (Acropora austera and Acropora cerealis) in response to high CO2 levels. The calcification rates were higher in high CO2 treatment than the control in A. austera, but was not significantly different in A. cerealis. A KEGG database search revealed that in both coral species, most Ca2+ transporters were present in the calcium signaling pathway, which could be important in the CO2 regulation of coral calcification. The gene expression levels of many CO2 and HCO3 transporters were not affected by elevated CO2. Nevertheless, high CO2 levels did have an effect on the expression of certain Ca2+ transporters. The upregulation of Ca2+ transporters likely explained the higher resistance of A. austera to high CO2 than A. cerealis.

Continue reading ‘Gene expression profiles of two coral species with varied resistance to ocean acidification’

Role of host genetics and heat tolerant algal symbionts in sustaining populations of the endangered coral Orbicella faveolata in the Florida Keys with ocean warming

Identifying which factors lead to coral bleaching resistance is a priority given the global decline of coral reefs with ocean warming. During the second year of back‐to‐back bleaching events in the Florida Keys in 2014 and 2015, we characterized key environmental and biological factors associated with bleaching resilience in the threatened reef‐building coral Orbicella faveolata. Ten reefs (five inshore, five offshore, 179 corals total) were sampled during bleaching (September 2015) and recovery (May 2016). Corals were genotyped with 2bRAD and profiled for algal symbiont abundance and type. O. faveolata at the inshore sites, despite higher temperatures, demonstrated significantly higher bleaching resistance and better recovery compared to offshore. The thermotolerant Durusdinium trenchii (formerly Symbiondinium trenchii) was the dominant endosymbiont type region‐wide during initial (78.0% of corals sampled) and final (77.2%) sampling; > 90% of the non‐bleached corals were dominated by D. trenchii. 2bRAD host genotyping found no genetic structure among reefs, but inshore sites showed a high level of clonality. While none of the measured environmental parameters were correlated with bleaching, 71% of variation in bleaching resistance and 73% of variation in the proportion of D. trenchii was attributable to differences between genets, highlighting the leading role of genetics in shaping natural bleaching patterns. Notably, D. trenchii was rarely dominant in O. faveolata from the Florida Keys in previous studies, even during bleaching. The region‐wide high abundance of D. trenchii was likely driven by repeated bleaching associated with the two warmest years on record for the Florida Keys (2014 and 2015). On inshore reefs in the upper Florida Keys, O. faveolata was most abundant, had the highest bleaching resistance, and contained the most corals dominated by D. trenchii, illustrating a causal link between heat tolerance and ecosystem resilience with global change.

Continue reading ‘Role of host genetics and heat tolerant algal symbionts in sustaining populations of the endangered coral Orbicella faveolata in the Florida Keys with ocean warming’


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