Posts Tagged 'calcification'

The requirement for calcification differs between ecologically important coccolithophore species

Summary

  • Coccolithophores are globally distributed unicellular marine algae that are characterized by their covering of calcite coccoliths. Calcification by coccolithophores contributes significantly to global biogeochemical cycles. However, the physiological requirement for calcification remains poorly understood as non‐calcifying strains of some commonly used model species, such as Emiliania huxleyi, grow normally in laboratory culture.
  • To determine whether the requirement for calcification differs between coccolithophore species, we utilized multiple independent methodologies to disrupt calcification in two important species of coccolithophore: E. huxleyi and Coccolithus braarudii. We investigated their physiological response and used time‐lapse imaging to visualize the processes of calcification and cell division in individual cells.
  • Disruption of calcification resulted in major growth defects in C. braarudii, but not in E. huxleyi. We found no evidence that calcification supports photosynthesis in C. braarudii, but showed that an inability to maintain an intact coccosphere results in cell cycle arrest.
  • We found that C. braarudii is very different from E. huxleyi as it exhibits an obligate requirement for calcification. The identification of a growth defect in C. braarudii resulting from disruption of the coccosphere may be important in considering their response to future changes in ocean carbonate chemistry.

 

Continue reading ‘The requirement for calcification differs between ecologically important coccolithophore species’

Water chemistry reveals a significant decline in coral calcification rates in the southern Red Sea

Experimental and field evidence support the assumption that global warming and ocean acidification is decreasing rates of calcification in the oceans. Local measurements of coral growth rates in reefs from various locations have suggested a decline of ~6–10% per decade since the late 1990’s. Here, by measuring open water strontium-to-alkalinity ratios along the Red Sea, we show that the net contribution of hermatypic corals to the CaCO3 budget of the southern and central Red Sea declined by ~100% between 1998 and 2015 and remained low between 2015 and 2018. Measured differences in total alkalinity of the Red Sea surface water indicate a 26 ± 16% decline in total CaCO3 deposition rates along the basin. These findings suggest that coral reefs of the southern Red Sea are under severe stress and demonstrate the strength of geochemical measurements as cost-effective indicators for calcification trends on regional scales.

Continue reading ‘Water chemistry reveals a significant decline in coral calcification rates in the southern Red Sea’

The effect of ocean acidification on tropical coral calcification: insights from calcification fluid DIC chemistry

Highlights

• Calcification fluid pH and [co-precipitating DIC] are positively correlated in all corals.
• [Precipitating DIC] and coral calcification rate are positively correlated in all but one outlier coral.
• Corals cultured at high seawater pCO2 usually have low fluid pH and [precipitating DIC]. Reduced DIC substrate at the calcification site is the likely cause of decreased coral calcification rates under ocean acidification scenarios.
• The outlier coral maintained a high calcification fluid pH and [co-precipitating DIC] at high seawater pCO2 but exhibited a low calcification rate suggesting that corals have a limited energy budget for calcification which is apportioned between proton extrusion from the calcification site and other processes e.g. synthesis of the skeletal organic matrix.

Abstract

Ocean acidification typically reduces calcification in tropical marine corals but the mechanism for this process is not understood. We use skeletal boron geochemistry (B/Ca and δ11B) to reconstruct the calcification fluid DIC of corals cultured over both high and low seawater pCO2 (180, 400 and 750 μatm). We observe strong positive correlations between calcification fluid pH and concentrations of the DIC species potentially implicated in aragonite precipitation (be they CO32−, HCO3 or HCO3 + CO32−). Similarly, with the exception of one outlier, the fluid concentrations of precipitating DIC species are strongly positively correlated with coral calcification rate. Corals cultured at high seawater pCO2 usually have low calcification fluid pH and low concentrations of precipitating DIC, suggesting that a reduction in DIC substrate at the calcification site is responsible for decreased calcification. The outlier coral maintained high pHCF and DICCF at high seawater pCO2 but exhibited a reduced calcification rate indicating that the coral has a limited energy budget to support proton extrusion from the calcification fluid and meet other calcification demands. We find no evidence that increasing seawater pCO2 enhances diffusion of CO2 into the calcification site. Instead the overlying [CO2] available to diffuse into the calcification site appears broadly comparable between seawater pCO2 treatments, implying that metabolic activity (respiration and photosynthesis) generates a similar [CO2] in the vicinity of the calcification site regardless of seawater pCO2.

Continue reading ‘The effect of ocean acidification on tropical coral calcification: insights from calcification fluid DIC chemistry’

The effects of nutrient addition and ocean acidification on tropical crustose coralline algae

As the global population increases, the occurrence of multiple anthropogenic
impacts on valuable coastal ecosystems, such as coral reefs, also increases. These
stressors can be global and long-term, like ocean acidification (OA), or local and short term, like nutrient runoff in some areas. The combination of these stressors can  potentially have additive or interactive effects on the organisms in coral reef
communities. Among the most important groups of organisms on coral reefs are crustose coralline algae (CCA), calcifying algae that cement the reef together and contribute to the global carbon cycle. This thesis studied the effects of nutrient addition and OA on Lithophyllum kotschyanum, a common species of CCA on the fringing reefs of Mo’orea, French Polynesia. Two mesocosm experiments tested the individual and interactive effects of OA and short-term nitrate and phosphate addition on L. kotschyanum. These experiments showed that nitrate and phosphate addition together increased photosynthesis, OA had interactive effects with nutrient addition, and after nutrient addition ended, calcification and photosynthetic rates changed in unpredictable ways in different OA and nutrient treatments. Because the results of the first two experiments showed impacts of nutrients even after addition stopped, two more mesocosm experiments were conducted to study the changes in photosynthesis and calcification over hourly time scales more relevant to a single nutrient pulse event. These two experiments revealed the existence of diurnal variation in light-saturated photosynthetic rate, but not calcification rate, under ambient and elevated pCO2. This pattern of increased maximum photosynthesis in the middle of the day can have important implications for how the time of nutrient runoff events during the day impacts CCA physiology. Finally, a field experiment was conducted to determine the effects of short- and long-term nutrient addition on L. kotschyanum. The results showed that a series of short-term nutrient additions did not increase photosynthesis or calcification rates above those in ambient nutrient conditions, but continual nutrient enrichment for 6 weeks increased photosynthetic rates. This increase in photosynthesis under only long-term enrichment shows the need for consideration of specific nutrient addition scenarios on coral reefs when predicting how the community will be affected.

Continue reading ‘The effects of nutrient addition and ocean acidification on tropical crustose coralline algae’

The potential role of internal pH manipulation by active proton pumping in foraminifera during biomineralization  

Calcifying foraminifera are one of the major marine calcifiers in open oceans and they are affected by ocean acidification. However, their response to acidified seawater is not according to inorganic precipitation but a biological regulation takes place during biomineralization. Two widely accepted models exist to explain the biological control during calcification, both supporting internal pH regulation. To predict how these calcifying organisms will be affected by increased pCO2 levels and therefore, a lowered seawater pH, culture experiments – investigating calcification rates are necessary. Here we present results from a culture experiment investigating the response of two benthic symbiont-bearing calcifying foraminifera under a range of four pCO2 concentrations (400, 700, 1000 and 2200 ppm) projected for the future. The greatest change in total alkalinity caused by calcification was observed at a pCO2 concentration of 700 ppm, while it was significantly lower at 1000 and 2200 ppm, indicating less successful biomineralization under very high pCO2 concentrations. Calcification rates suggest species specific responses to ocean acidification with H. depressa performing worse under the 2200 ppm treatment than A. lessonii. The different responses of the foraminifera between varying pCO2 treatments and between the two different species suggest that at least some foraminifera will be able to cope with increasing atmospheric CO2 concentrations.

Continue reading ‘The potential role of internal pH manipulation by active proton pumping in foraminifera during biomineralization  ‘

Thresholds and drivers of coral calcification responses to climate change

Increased temperature and CO2‐levels are considered key drivers of coral reef degradation. However, individual assessments of ecological responses (calcification) to these stressors are often contradicting. To detect underlying drivers of heterogeneity in coral calcification responses, we developed a procedure for the inclusion of stress‐effect relationships in ecological meta‐analyses. We applied this technique to a dataset of 294 empirical observations from 62 peer‐reviewed publications testing individual and combined effects of elevated temperature and pCO2 on coral calcification. Our results show an additive interaction between warming and acidification, which reduces coral calcification by 20% when pCO2 levels exceed 700 ppm and temperature increases by 3°C. However, stress levels varied among studies and significantly affected outcomes, with unaffected calcification rates under moderate stresses (pCO2 ≤ 700 ppm, ΔT < 3°C). Future coral reef carbon budgets will therefore depend on the magnitude of pCO2 and temperature elevations and, thus, anthropogenic CO2 emissions. Accounting for stress‐effect relationships enabled us to identify additional drivers of heterogeneity including coral taxa, life stage, habitat, food availability, climate, and season. These differences can aid reef management identifying refuges and conservation priorities, but without a global effort to reduce CO2 emissions, coral capacity to build reefs will be at risk.

Continue reading ‘Thresholds and drivers of coral calcification responses to climate change’

Resistance of corals and coralline algae to ocean acidification: physiological control of calcification under natural pH variability

Ocean acidification is a threat to the continued accretion of coral reefs, though some undergo daily fluctuations in pH exceeding declines predicted by 2100. We test whether exposure to greater pH variability enhances resistance to ocean acidification for the coral Goniopora sp. and coralline alga Hydrolithon reinboldii from two sites: one with low pH variability (less than 0.15 units daily; Shell Island) and a site with high pH variability (up to 1.4 pH units daily; Tallon Island). We grew populations of both species for more than 100 days under a combination of differing pH variability (high/low) and means (ambient pH 8.05/ocean acidification pH 7.65). Calcification rates of Goniopora sp. were unaffected by the examined variables. Calcification rates of H. reinboldii were significantly faster in Tallon than in Shell Island individuals, and Tallon Island individuals calcified faster in the high variability pH 8.05 treatment compared with all others. Geochemical proxies for carbonate chemistry within the calcifying fluid (cf) of both species indicated that only mean seawater pH influenced pHcf. pH treatments had no effect on proxies for Ωcf. These limited responses to extreme pH treatments demonstrate that some calcifying taxa may be capable of maintaining constant rates of calcification under ocean acidification by actively modifying Ωcf.

Continue reading ‘Resistance of corals and coralline algae to ocean acidification: physiological control of calcification under natural pH variability’


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Ocean acidification in the IPCC AR5 WG II

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