Posts Tagged 'corals'

In vivo pH measurement at the site of calcification in an octocoral

Calcareous octocorals are ecologically important calcifiers, but little is known about their biomineralization physiology, relative to scleractinian corals. Many marine calcifiers promote calcification by up-regulating pH at calcification sites against the surrounding seawater. Here, we investigated pH in the red octocoral Corallium rubrum which forms sclerites and an axial skeleton. To achieve this, we cultured microcolonies on coverslips facilitating microscopy of calcification sites of sclerites and axial skeleton. Initially we conducted extensive characterisation of the structural arrangement of biominerals and calcifying cells in context with other tissues, and then measured pH by live tissue imaging. Our results reveal that developing sclerites are enveloped by two scleroblasts and an extracellular calcifying medium of pH 7.97 ± 0.15. Similarly, axial skeleton crystals are surrounded by cells and a calcifying medium of pH 7.89 ± 0.09. In both cases, calcifying media are more alkaline compared to calcifying cells and fluids in gastrovascular canals, but importantly they are not pH up-regulated with respect to the surrounding seawater, contrary to what is observed in scleractinians. This points to a potential vulnerability of this species to decrease in seawater pH and is consistent with reports that red coral calcification is sensitive to ocean acidification.

Continue reading ‘In vivo pH measurement at the site of calcification in an octocoral’

Microbiome dynamics in early life stages of the scleractinian coral Acropora gemmifera in response to elevated pCO2

Reef-building corals are complex holobionts, harbouring diverse microorganisms that play essential roles in maintaining coral health. However, microbiome development in early life stages of corals remains poorly understood. Here, microbiomes of Acropora gemmifera were analysed during spawning and early developmental stages, and also under different seawater partial pressure of CO2 (pCO2) conditions, using amplicon sequencing of 16S rRNA gene for bacteria and archaea and of ITS2 for Symbiodinium. No remarkable microbiome shift was observed in adults before and after spawning. Moreover, microbiomes in eggs were highly similar to those in spawned adults, possibly suggesting a vertical transmission from parents to offspring. However, significant stage-specific changes were found in coral microbiome during development, indicating that host development played a dominant role in shaping coral microbiome. Specifically, Cyanobacteria were particularly abundant in 6-day-old juveniles, but decreased largely in 31-day-old juveniles with a possible subclade shift in Symbiodinium dominance from C2r to D17. Larval microbiome showed changes in elevated pCO2, while juvenile microbiomes remained rather stable in response to higher pCO2. This study provides novel insights into the microbiome development during the critical life stages of coral.

Continue reading ‘Microbiome dynamics in early life stages of the scleractinian coral Acropora gemmifera in response to elevated pCO2’

Low recruitment due to altered settlement substrata as primary constraint for coral communities under ocean acidification

The future of coral reefs under increasing CO2 depends on their capacity to recover from disturbances. To predict the recovery potential of coral communities that are fully acclimatized to elevated CO2, we compared the relative success of coral recruitment and later life stages at two volcanic CO2 seeps and adjacent control sites in Papua New Guinea. Our field experiments showed that the effects of ocean acidification (OA) on coral recruitment rates were up to an order of magnitude greater than the effects on the survival and growth of established corals. Settlement rates, recruit and juvenile densities were best predicted by the presence of crustose coralline algae, as opposed to the direct effects of seawater CO2. Offspring from high CO2 acclimatized parents had similarly impaired settlement rates as offspring from control parents. For most coral taxa, field data showed no evidence of cumulative and compounding detrimental effects of high CO2 on successive life stages, and three taxa showed improved adult performance at high CO2 that compensated for their low recruitment rates. Our data suggest that severely declining capacity for reefs to recover, due to altered settlement substrata and reduced coral recruitment, is likely to become a dominant mechanism of how OA will alter coral reefs.

Continue reading ‘Low recruitment due to altered settlement substrata as primary constraint for coral communities under ocean acidification’

Measuring coral calcification under ocean acidification: methodological considerations for the 45Ca-uptake and total alkalinity anomaly technique

As the oceans become less alkaline due to rising CO2 levels, deleterious consequences are expected for calcifying corals. Predicting how coral calcification will be affected by on-going ocean acidification (OA) requires an accurate assessment of CaCO3 deposition and an understanding of the relative importance that decreasing calcification and/or increasing dissolution play for the overall calcification budget of individual corals. Here, we assessed the compatibility of the 45Ca-uptake and total alkalinity (TA) anomaly techniques as measures of gross and net calcification (GC, NC), respectively, to determine coral calcification at pHT 8.1 and 7.5. Considering the differing buffering capacity of seawater at both pH values, we were also interested in how strongly coral calcification alters the seawater carbonate chemistry under prolonged incubation in sealed chambers, potentially interfering with physiological functioning. Our data indicate that NC estimates by TA are erroneously ∼5% and ∼21% higher than GC estimates from 45Ca for ambient and reduced pH, respectively. Considering also previous data, we show that the consistent discrepancy between both techniques across studies is not constant, but largely depends on the absolute value of CaCO3 deposition. Deriving rates of coral dissolution from the difference between NC and GC was not possible and we advocate a more direct approach for the future by simultaneously measuring skeletal calcium influx and efflux. Substantial changes in carbonate system parameters for incubation times beyond two hours in our experiment demonstrate the necessity to test and optimize experimental incubation setups when measuring coral calcification in closed systems, especially under OA conditions.

Continue reading ‘Measuring coral calcification under ocean acidification: methodological considerations for the 45Ca-uptake and total alkalinity anomaly technique’

Diel CO2 cycles reduce severity of behavioural abnormalities in coral reef fish under ocean acidification

Elevated CO2 levels associated with ocean acidification (OA) have been shown to alter behavioural responses in coral reef fishes. However, all studies to date have used stable pCO2 treatments, not considering the substantial diel pCO2 variation that occurs in shallow reef habitats. Here, we reared juvenile damselfish, Acanthochromis polyacanthus, and clownfish, Amphiprion percula, at stable and diel cycling pCO2 treatments in two experiments. As expected, absolute lateralization of A. polyacanthus and response to predator cue of Am. percula were negatively affected in fish reared at stable, elevated pCO2 in both experiments. However, diel pCO2 fluctuations reduced the negative effects of OA on behaviour. Importantly, in experiment two, behavioural abnormalities that were present in fish reared at stable 750 µatm CO2 were largely absent in fish reared at 750 ± 300 µatm CO2. Overall, we show that diel pCO2 cycles can substantially reduce the severity of behavioural abnormalities caused by elevated CO2. Thus, past studies may have over-estimated the impacts of OA on the behavioural performance of coral reef fishes. Furthermore, our results suggest that diel pCO2 cycles will delay the onset of behavioural abnormalities in natural populations.

Continue reading ‘Diel CO2 cycles reduce severity of behavioural abnormalities in coral reef fish under ocean acidification’

Amorphous calcium carbonate particles form coral skeletons

Whether coral skeleton crystals grow by attachment of ions from solution or particles from tissue determines (i) corals’ growth rate, (ii) how they survive acidifying oceans, and (iii) the isotopes in the crystals used for reconstructing ancient temperatures. Our data show that two amorphous precursors exist, one hydrated and one dehydrated amorphous calcium carbonate; that these are formed in the tissue as ∼400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally crystallize into aragonite. Since these particles are formed inside tissue, coral skeleton growth may be less susceptible to ocean acidification than previously assumed. Coral bleaching and postmortem dissolution of the skeleton will occur, but a calcification crisis may not.

Do corals form their skeletons by precipitation from solution or by attachment of amorphous precursor particles as observed in other minerals and biominerals? The classical model assumes precipitation in contrast with observed “vital effects,” that is, deviations from elemental and isotopic compositions at thermodynamic equilibrium. Here, we show direct spectromicroscopy evidence in Stylophora pistillata corals that two amorphous precursors exist, one hydrated and one anhydrous amorphous calcium carbonate (ACC); that these are formed in the tissue as 400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally, crystallize into aragonite (CaCO3). We show in both coral and synthetic aragonite spherulites that crystal growth by attachment of ACC particles is more than 100 times faster than ion-by-ion growth from solution. Fast growth provides a distinct physiological advantage to corals in the rigors of the reef, a crowded and fiercely competitive ecosystem. Corals are affected by warming-induced bleaching and postmortem dissolution, but the finding here that ACC particles are formed inside tissue may make coral skeleton formation less susceptible to ocean acidification than previously assumed. If this is how other corals form their skeletons, perhaps this is how a few corals survived past CO2 increases, such as the Paleocene–Eocene Thermal Maximum that occurred 56 Mya.

Continue reading ‘Amorphous calcium carbonate particles form coral skeletons’

Effects of ocean acidification and contact with the brown alga Stypopodium zonale on the settlement and early survival of the coral Porites astreoides

To evaluate the effects of ocean acidification (OA) and algal presence on the early life-history stages of corals, we conducted an aquarium study that examined the isolated and combined effects of reduced pH (pH 8.10 vs. 7.85) and contact with the alga Stypopodium zonale on the survival, settlement, and post-settlement growth of larvae from the brooding coral Porites astreoides. Two settlement substrates, biofilmed tiles and the crustose coralline alga (CCA) Hydrolithon boergesenii, were initially incubated for 12 d in separate tanks under a factorial combination of low pH and S. zonale contact, and then subjected to a series of settlement assays. Across both substrate types, S. zonale presence significantly reduced coral settlement. Low pH imposed relatively minor effects; however, there was a significant interaction between pH and S. zonale presence for settlement on the CCA substrate, such that low pH exacerbated the negative effects of S. zonale. Post-settlement growth for 2 wk was unaffected by either S. zonale presence or low pH on either substrate. While our results demonstrate that S. zonale contact likely remains a dominant threat to larval survival and settlement, in certain cases, OA may amplify the negative effects of S. zonale presence, highlighting the need to consider multiple factors in studies aimed at assessing the future health of coral reef ecosystems.

Continue reading ‘Effects of ocean acidification and contact with the brown alga Stypopodium zonale on the settlement and early survival of the coral Porites astreoides’


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