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Rapid increases in anthropogenic atmospheric CO2 partial pressure have led to a decrease in the pH of seawater. Calcifying organisms generally respond negatively to ocean acidification. Foraminifera are one of the major carbonate producers in the ocean; however, whether calcification reduction by ocean acidification affects either foraminiferal shell volume or density, or both, has yet to be investigated. In this study, we cultured asexually reproducing specimens of Amphisorus kudakajimensis, a dinoflagellate endosymbiont-bearing large benthic foraminifera (LBF), under different pH conditions (pH 7.7–8.3, NBS scale). The results suggest that changes in seawater pH would affect not only the quantity (i.e., shell volume) but also the quality (i.e., shell density) of foraminiferal calcification. We proposed that pH and temperature affect these growth parameters differently because (1) they have differences in the contribution to the calcification process (e.g., Ca2+-ATPase and Ω) and (2) pH mainly affects calcification and temperature mainly affects photosynthesis. Our findings also suggest that, under the IPCC RCP8.5 scenario, both ocean acidification and warming will have a significant impact on reef foraminiferal carbonate production by the end of this century, even in the tropics. [ABSTRACT FROM AUTHOR]

SUMMARY Both global and local environmental changes threaten coral reef ecosystems. To evaluate the effects of high seawater temperature and phosphate enrichment on reef-building crustose coralline algae, fragments of Porolithon onkodes were cultured for 1 month under laboratory conditions. The calcification rate of the coralline algae was not affected at 30° C, but it decreased to the negatives at 32° C in comparison to the control treatment of 27° C, indicating that the temperature threshold for maintaining positive production of calcium carbonate lies between 30 and 32° C. Phosphate enrichment of 1-2 μmol L−1 did not affect the calcification rate. The net oxygen production rate was enhanced by phosphate enrichment, suggesting that the photosynthetic rate was limited by the availability of phosphate. It was concluded that moderate phosphate enrichment does not directly deteriorate algal calcification but instead ameliorates the negative effects of high seawater temperature on algal photosynthesis. [ABSTRACT FROM AUTHOR]

In this study, we report the acidification impact mimicking the pre-industrial, the present, and near-future oceans on calcification of two coral species ( Porites australiensis , Isopora palifera ) by using precise p CO 2 control system which can produce acidified seawater under stable p CO 2 values with low variations. In the analyses, we performed Bayesian modeling approaches incorporating the variations of p CO 2 and compared the results between our modeling approach and classical statistical one. The results showed highest calcification rates in pre-industrial p CO 2 level and gradual decreases of calcification in the near-future ocean acidification level, which suggests that ongoing and near-future ocean acidification would negatively impact coral calcification. In addition, it was expected that the variations of parameters of carbon chemistry may affect the inference of the best model on calcification responses to these parameters between Bayesian modeling approach and classical statistical one even under stable p CO 2 values with low variations. [ABSTRACT FROM AUTHOR]

Abstract: Symbiotic and aposymbiotic juvenile corals, which were grown in the laboratory from the gametes of the scleractinian coral Acropora digitifera and had settled down onto plastic culture plates, were observed with a microscope under different nutrient conditions. The symbiotic corals successfully removed the surrounding benthic microalgae (BMA), whereas the aposymbiotic corals were in close physical contact with BMA. The areal growth rate of the symbiotic corals was significantly higher than that of the aposymbiotic corals. The addition of nutrients to the culture seawater increased the chlorophyll a content in the symbiotic coral polyps and enhanced the growth of some of the symbiotic corals, however the average growth rate was not significantly affected, most likely because of the competition with BMA. The comparison between the symbiotic and aposymbiotic juvenile corals showed that the establishment of a symbiotic association could be imperative for post-settlement juvenile corals to survive in high-nutrient seawater. [Copyright &y& Elsevier]

The type III inorganic phosphate (Pi) transporter Pit-1 was previously found to be preferentially expressed in developing long bones. Several studies also described a regulation of its expression in cultured bone cells by osteotropic factors, suggesting a role of this transporter in bone metabolism. In the present study, we investigated the effects of the transgenic overexpression of Pit-1 in Wistar male rats on calcium phosphate and bone metabolism. A threefold increase and doubling of Pi transport activity were recorded in primary cultured osteoblastic cells derived from calvaria of two transgenic (Tg) lines compared with wild-type littermates (WT), respectively. Skeletal development was not affected by the transgene, and bone mass, analyzed by DXA, was slightly decreased in Tg compared with WT. Enhanced Pi uptake in calvaria-derived osteoblasts from Pit-1 Tg was associated with a significantly decreased expression of alkaline phosphatase activity and a normal deposition and calcification of the collagenous matrix. In 4-month-old adult Tg rats, serum Pi and renal Pi transport were increased compared with WT. The decrease of serum Ca concentration was associated with increased serum parathyroid hormone levels. Variations in serum Pi in Pit-1 Tg rats were negatively correlated with serum fibroblast growth factor-23, whereas 1,25-dihydroxyvitamin D(3) was not affected by Pit-1 overexpression. In conclusion, transgenic Pit-1 overexpression in rats affected bone and calcium phosphate metabolism. It also decreased alkaline phosphatase activity in osteoblasts without influencing bone matrix mineralization as well as skeletal development. [ABSTRACT FROM AUTHOR]

Abstract: We investigated oxygen and carbon isotopes (δ 18O and δ 13C, respectively) along the growth axis of a Porites coral living near the northern limit of hermatypic corals, off Ushibuka, Japan, where winter temperatures fall below the minimum required by most hermatypic corals. The coral''s seasonal δ 18O cycle depended mainly on seawater temperature, and the slope of the regression line between δ 18O and sea-surface temperature for this coral was within reported values. The coral''s growth was inhibited in 1968, and at around this time the annual growth rate was reduced. This growth inhibition began in winter 1967/1968, a period of extraordinarily low seawater temperature. Moreover, the amplitude of the annual δ 18O fluctuation was small from winter 1967/1968 to winter 1969/1970. Although δ 18O and δ 13C fluctuations were out of phase most years, they were in phase some years. The in-phase fluctuations of δ 18O and δ 13C indicate that kinetic isotope effects may have been more important than metabolic isotope effects during those years. Sclerochronologic records thus reveal the coral response to low-temperature stress. [Copyright &y& Elsevier]

The effects of elevated CO 2 levels on growth and stable carbon and oxygen isotope compositions (δ 13 C and δ 18 O) of aragonitic shells were experimentally evaluated by rearing the bloody clam, Scapharca broughtonii , at two different temperatures and at six different p CO 2 levels (17 °C: 269, 382, 550, 757, 939, and 1114 µatm; 25 °C: 332, 463, 653, 872, 1137, and 1337 μatm). Ambient CO 2 levels did not much affect shell growth rates. Mass spectrometric analysis of the outer shell layer indicated a negative correlation between seawater pH and shell δ 18 O (equivalently, a positive correlation between seawater p CO 2 and shell δ 18 O) at both temperatures (regression slopes: −0.70‰ ± 0.14‰/pH unit at 17 °C; −0.36‰ ± 0.17‰/pH unit at 25 °C). A positive correlation was observed between seawater pH and shell δ 13 C at both temperatures, but the regression slopes were less steep than those between seawater pH and the δ 13 C of seawater DIC. As CO 2 levels rose (and pH dropped to about 7.6), shell δ 18 O and δ 13 C increased relative to ambient dissolved inorganic carbon (DIC). Both approached apparent isotopic equilibrium in acidified waters. Shell δ 18 O was below that of the ambient mix of bicarbonate and carbonate ions, and usually below apparent isotopic equilibrium with water. Kinetic discrimination against heavy isotopes during CO 2 hydration and hydroxylation reactions may contribute to this isotopic “enlightenment”, and elevated ambient CO 2 may “wash out” this effect. Our findings thus generally support models of calcification physiology and shell isotopic content, and have implications for isotopic paleontology. [ABSTRACT FROM AUTHOR]

Abstract: Ocean acidification has become recognized recently to be a major threat to calcifying organisms. Previous studies have reported that calcification rates of calcareous marine organisms (e.g., corals, foraminifers, coccolithophores, pteropods, mussels, and oysters) change in response to lowering pH levels even in waters oversaturated with respect to calcium carbonate. However, the impact of ocean acidification on large benthic foraminifers, which are major contributors to organic and inorganic carbon production in coral reefs, is still unclear. In this study, we cultured asexually-produced individuals of Marginopora kudakajimensis under four different pH conditions to examine the effects of lowering pH on their growth rates. Experimental results indicate that growth rate, measured by shell diameter, shell weight, and the number of chambers added, generally decreased with lowering pH after 10weeks of culture. Shell weight was most closely dependent upon pH, suggesting that fossil shell weight can be more useful for reconstruction of paleo-pH changes. The relationship between the shell weight and shell size also showed significant differences among the four pH conditions. Between pH 8.3 and 7.7 (NBS scale), the tendency of the growth rate of M. kudakajimensis to decrease with pH was consistent with that of most other calcifying organisms. However, the calcification/growth rates at pH 7.9 and ~8.2 (present seawater value, control) were not significantly different, and other organisms also display a nonlinear response to elevated pCO2 at around this pH range. These results suggest that 1) they already may have experienced a reduction in growth in natural environments since pre-industrial times and 2) although the seawater CO2 system of reef water shows great variation, the calcification rate of these large foraminifers should remain at the present level at pH 7.9–8.2. However, at around pH 7.7, their calcification rate would decline steeply, which would probably preclude their survival. [Copyright &y& Elsevier]

Light-enhanced calcification of reef-building corals, which eventually create vast coral reefs, is well known and based on coral-algal symbiosis. Several controversial hypotheses have been proposed as possible mechanisms for connecting symbiont photosynthesis and coral calcification, including pH rise in the internal pool, role of organic matrix secretion, and enzyme activities. Here, based on the skeletal chemical and isotopic compositions of symbiotic and asymbiotic primary polyps of Acropora digitifera corals, we show a simple pH increase in the calcification medium as the predominant contribution of symbionts to calcification of host corals. We used the symbiotic and asymbiotic primary polyps reared for 10 days at four temperatures (27, 29, 31, and 33 °C), five salinities (34, 32, 30, 28, and 26), and four pCO 2 levels (<300, 400, 800, and 1000 µatm). As a result of analyzing multiple geochemical tracers (U/Ca, Mg/Ca, Sr/Ca, δ 18 O, δ 13 C, and δ 44 Ca), a clear and systematic decrease in skeletal U/Ca ratio (used as a proxy for calcification fluid pH) was observed, indicating a higher pH of the fluid in symbiotic compared to asymbiotic polyps. In contrast, Mg/Ca ratios (used as a tentative proxy for organic matrix secretion) and δ 44 Ca (used as an indicator of Ca 2+ pathway to the fluid) did not differ between symbiotic and asymbiotic polyps. This suggests that organic matrix secretion related to coral calcification is controlled mainly by the coral host itself, and a transmembrane transport of Ca 2+ does not vary according to symbiosis relationship. Skeletal δ 18 O values of both symbiotic and asymbiotic polyps showed offsets between them with identical temperature dependence. Based on a newly proposed model, behavior of δ 18 O in the present study seems to reflect the rate of CO 2 hydration in the calcifying fluid. Since CO 2 hydration is promoted by enzyme carbonic anhydrase, the offset of δ 18 O values between symbiotic and asymbiotic polyps is attributed to the differences of enzyme activity, although the enzyme is functional even in the asymbiotic polyp. Symbiotic δ 13 C values in the temperature and salinity experiments were higher compared to those in the asymbiotic polyps due to photosynthesis, although photosynthetic δ 13 C signals in the pCO 2 experiment were masked by the dominant δ 13 C gradient in dissolved inorganic carbon in seawater caused by 13 C-depletd CO 2 gas addition in the higher pCO 2 treatments. Sr/Ca ratios showed a negligible relationship according to variation of temperature, salinity, and pCO 2 , although it might be attributed to relatively large deviations of replicates of Sr/Ca ratios in the present study. Overall, only the U/Ca ratio showed a significant difference between symbiotic and asymbiotic polyps throughout all experiments, indicating that the critical effect on coral calcification caused by symbiotic algae is the increase of pH of the calcifying fluid by photosynthesis. [ABSTRACT FROM AUTHOR]

Coral skeletons preserve useful archival indicators from which to reconstruct past environments. We conducted temperature-controlled culture experiments (20, 22, 27, and 31 °C) with primary polyps of Acropora digitifera to accurately assess the impact of a wide range of temperatures on the calcification rate and to methodically evaluate the skeletal trace elements (Sr/Ca, Mg/Ca, and Na/Ca ratios). Water temperatures positively affected the calcification rate up to 31 °C, which exceeds the temperature threshold for this species. The calcification rates also varied widely (> 20%) during each of the four temperature treatments. The skeletal Sr/Ca ratio was most strongly correlated with water temperature ( R 2 = 0.68, p < 0.001), whereas the Mg/Ca and Na/Ca ratios showed lower correlations ( R 2 = 0.53, p < 0.001; R 2 = 0.34, p = 0.011, respectively). The variations in the skeletal Sr/Ca, Mg/Ca, and Na/Ca ratios at the four different temperatures were 0.36%–1.20%, 0.83%–3.76%, and 0.52%–3.24%, respectively. Thus, the Sr/Ca ratio showed the least variation, despite the wide variations in the calcification rate. We confirm that the Sr/Ca ratio of A. digitifera juveniles is a robust proxy of temperature, regardless of variations in the calcification rate and the wide range of ambient temperatures. In addition, our results regarding skeletal Mg/Ca and Na/Ca ratios further our understanding of coral calcification processes. [ABSTRACT FROM AUTHOR]

Coral skeletal P/Ca ratio has been developed as an indicator of temporal seawater dissolved inorganic phosphorus (DIP). The use of coral P/Ca proxy helps to assess oceanographic and climatic impacts such as upwelling, circulation, and continent runoffs on marine biogeochemical cycles. However, factors controlling skeletal P incorporation and elemental partitioning between seawater and coral skeletons remain elusive. We conducted temperature-controlled (∼21 to 29 °C) aquaria culture experiments using two colonies of Porites australiensis corals (here refer to B and C) with the only difference in zooxanthellae density (B > C). The coral growth rate ranges from 9.4 to 19.4 mg/day (B) and 0.7 to 14.1 mg/day (C). Only the growth rate of colony C significantly correlates to temperature, potentially reflecting physiological controls on the two colonies given the difference in the zooxanthellae density. We measured coral P/Ca ratios by Inductively coupled plasma mass spectrometry and determined skeletal P speciation through a synchrotron-based spectroscopic approach. Coral P/Ca ratio ranges from 6.5 to 18.6 μmol/mol (B) and 7.2 to 19.8 μmol/mol (C). The dominance of organic-P is confirmed, and the presence of inorganic-P cannot be excluded. Only colony C has a strong P/Ca dependence on temperature and both colonies show strong correlations between P/Ca and growth rate. Although growth rate and temperature are intercorrelated, the growth rate is more likely the direct controlling factor on coral P/Ca in our experiments. Combined laboratory data with field observations, we suggest that the validity of the Porites P/Ca proxy may be influenced by seawater DIP, coral species, and growth rate. • Cultured coral P/Ca ratio can be variable with relatively constant seawater chemistry. • Organic phosphorus is widely present in coral skeletons despite oxidative cleaning. • The validity of the P/Ca proxy could be influenced by seawater DIP, coral species, and growth rate. [ABSTRACT FROM AUTHOR]

Ocean acidification (OA) threatens calcifying marine organisms including reef-building corals. In this study, we examined the OA responses of individual colonies of the branching scleractinian coral Montipora digitata . We exposed nubbins of unique colonies (n = 15) to ambient or elevated p CO 2 under natural light and temperature regimes for 110 days. Although elevated p CO 2 exposure on average reduced calcification, individual colonies showed unique responses ranging from declines in positive calcification to negative calcification (decalcification) to no change. Similarly, mortality was greater on average in elevated p CO 2 , but also showed colony-specific patterns. High variation in colony responses suggests the possibility that ongoing OA may lead to natural selection of OA-tolerant colonies within a coral population. [ABSTRACT FROM AUTHOR]

• Large variations in coral K phase and isotope composition depending on temperature. • Corals show both higher and lower δ 41 K values compared to the seawater δ 41 K value. • Increasing temperature causes increases in organic-K but decreases in carbonate-K contribution. • The isotopic composition in coral skeletons corresponds to skeletal K phase partitioning. Potassium (K) participates in coral biological activities and accumulates in their skeletons, driving the fractionation of stable K isotopes (41K/39K). Constraining the influences of biotic and abiotic controls on K isotope fractionation is important for interpreting coral records. However, the processes and mechanisms regulating K incorporation into coral skeletons and K isotope fractionation between seawater and coral skeletons remain unknown. Here, we combined isotopic and synchrotron-based spectroscopic analyses to evaluate the phase distribution and corresponding isotope variation of K in the skeleton of scleractinian (Porites australiensis) corals at a seawater temperature range from 20 to 29 °C in aquaria culture experiments. Potassium in coral skeletons exists mainly as organic-K (K hosted in soluble and insoluble organic matrices) and carbonate-K (K incorporated into K 2 CO 3 and aragonite) phases of various proportions. Coral δ 41 K values vary substantially in both direction and magnitude from the modern seawater δ 41 K composition (∼0.12‰), showing marked deviations (Δ 41 K Coral-Sea) from −2.00 to 0.67‰. As seawater temperature increases, the organic-K fraction increases, whereas δ 41 K Coral decreases. The variation in δ 41 K Coral reflects the relative proportions of organic-bound K to carbonate-associated K. In most cases, coral intracrystalline organic matrices preferentially sequester isotopically lighter K whereas carbonate phases prefer heavier K. Distinguishable inter-colony difference in skeletal δ 41 K of corals growing under the same culturing conditions reveals the influence of physiological controls on K partitioning and isotope fractionation. Although calcification rate correlates with temperature to different degrees in the studied corals, likely reflecting control of the difference in zooxanthellae density, we infer that calcification rate is not a major controlling factor on skeletal δ 41 K. Rather, skeletal δ 41 K correlates closely with K phase partitioning, which is ascribed to temperature-sensitive physiological modulation. [ABSTRACT FROM AUTHOR]

Abstract: We evaluated acidification effects on two crustose coralline algal species common to Pacific coral reefs, Lithophyllum kotschyanum and Hydrolithon samoense. We used genetically homogeneous samples of both species to eliminate misidentification of species. The growth rates and percent calcification of the walls of the epithallial cells (thallus surface cells) of both species decreased with increasing pCO2. However, elevated pCO2 more strongly inhibited the growth of L. kotschyanum versus H. samoense. The trend of decreasing percent calcification of the cell wall did not differ between these species, although intercellular calcification of the epithallial cells in L. kotschyanum was apparently reduced at elevated pCO2, a result that might indicate that there are differences in the solubility or density of the calcite skeletons of these two species. These results can provide knowledge fundamental to future studies of the physiological and genetic mechanisms that underlie the response of crustose coralline algae to environmental stresses. [Copyright &y& Elsevier]