Ocean acidification is predicted to impact ecosystems reliant on calcifying organisms, potentially reducing the socioeconomic benefits these habitats provide. Here we investigate the acclimation potential of stony corals living along a pH gradient caused by a Mediterranean CO₂ vent that serves as a natural long-term experimental setting. We show that in response to reduced skeletal mineralization at lower pH, corals increase their skeletal macroporosity (features >10 μm) in order to maintain constant linear extension rate, an important criterion for reproductive output. At the nanoscale, the coral skeleton’s structural features are not altered. However, higher skeletal porosity, and reduced bulk density and stiffness may contribute to reduce population density and increase damage susceptibility under low pH conditions. Based on these observations, the almost universally employed measure of coral biomineralization, the rate of linear extension, might not be a reliable metric for assessing coral health and resilience in a warming and acidifying ocean.

Organisms that accumulate calcium carbonate structures are particularly vulnerable to ocean warming (OW) and ocean acidification (OA), potentially reducing the socioeconomic benefits of ecosystems reliant on these taxa. Since rising atmospheric CO₂is responsible for global warming and increasing ocean acidity, to correctly predict how OW and OA will affect marine organisms, their possible interactive effects must be assessed. Here we investigate, in the field, the combined temperature (range: 16–26 °C) and acidification (range: pH_TS8.1–7.4) effects on mortality and growth of Mediterranean coral species transplanted, in different seasonal periods, along a natural pH gradient generated by a CO₂vent. We show a synergistic adverse effect on mortality rates (up to 60%), for solitary and colonial, symbiotic and asymbiotic corals, suggesting that high seawater temperatures may have increased their metabolic rates which, in conjunction with decreasing pH, could have led to rapid deterioration of cellular processes and performance. The net calcification rate of the symbiotic species was not affected by decreasing pH, regardless of temperature, while in the two asymbiotic species it was negatively affected by increasing acidification and temperature, suggesting that symbiotic corals may be more tolerant to increasing warming and acidifying conditions compared to asymbiotic ones.

Porous media containing fluids and subject to the field of an NMR instrument usually have incremental local magnetic fields due to susceptibility differences ${\mathrm{\chi }}_{\mathit{d}}$, such as between a fluid and a solid matrix. If ‖${\mathrm{\chi }}_{\mathit{d}}$‖≪1 the effective variation ω of the local precession angular frequency is limited to ±1/2${\mathrm{\chi }}_{\mathit{d}}$ ${\mathrm{\omega }}_0$, where ${\mathrm{\omega }}_0$ is the mean precession angular frequency. Diffusion of fluid molecules through these local fields leads to a τ-dependent increase ${\mathit{R}}_{\mathit{d}}$ in the value of 1/${\mathit{T}}_2$ obtained from Carr-Purcell-Meiboom-Gill (CPMG) measurements. Many porous media appear likely to have significant ω variation over a substantial range of diffusion time scales, or correlation times. For a sample with a single correlation time ${\mathrm{\tau }}_{\mathit{c}}$ the logarithm of the additional decay of the nth echo amplitude due to diffusion through regions of different ω is ${\mathrm{\Omega }}^2$ ${\mathrm{\tau }}_{\mathit{c}}${2nτf(τ/${\mathrm{\tau }}_{\mathit{c}}$)-${\mathrm{\tau }}_{\mathit{c}}$[(1-x${)}^4$/(1+${\mathit{x}}^2$ ${)}^2$] [1-(-${)}^{\mathit{n}}$ ${\mathit{x}}^{2\mathit{n}}$]}, where f(t)=1-(tanht)/t, ${\mathrm{\Omega }}^2$=〈${\mathrm{\omega }}^2$〉, and x=exp(-τ/${\mathrm{\tau }}_{\mathit{c}}$). The term without n causes only a shift in the relaxation curve, and the terms in ${\mathit{x}}^{2\mathit{n}}$ are small, so ${\mathit{R}}_{\mathit{d}}$≊${\mathrm{\Omega }}^2$ ${\mathrm{\tau }}_{\mathit{c}}$f(τ/${\mathrm{\tau }}_{\mathit{c}}$). The function f(t) starts quadratically at small t, has a nearly linear portion, and then approaches 1-1/t at large t. However, the superposition of terms of the form f(τ/${\mathrm{\tau }}_{\mathit{c}\mathit{i}}$) tends to give a nearly linear portion of the ${\mathit{R}}_{\mathit{d}}$ vs τ curve extending from small values of ${\mathit{R}}_{\mathit{d}}$ to about a third of the asymptotic value if there is a significant range of ${\mathrm{\tau }}_{\mathit{c}\mathit{i}}$.