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It has been argued that skeletal Mg/Ca ratio in echinoderms is mostly governed by Mg2+ and Ca2+ concentrations in the ambient seawater. Accordingly, well-preserved fossil echinoderms were used to reconstruct Phanerozoic seawater Mg2+ /Ca2+ ratio. However, Mg/Ca ratio in echinoderm skeleton can be affected by a number of environmental and physiological factors, the effects of which are still poorly understood. Notably, experimental data supporting the applicability of echinoderms in paleoenvironmental reconstructions remain limited. Here, we investigated the effect of ambient Mg2+ /Ca2+ seawater ratio and diet on skeletal Mg/Ca ratio and growth rate in two echinoid species (Psammechinus miliaris and Prionocidaris baculosa). Sea urchins were tagged with manganese and then cultured in different Mg2+ /Ca2+ conditions to simulate fluctuations in the Mg2+ /Ca2+ seawater ratios in the Phanerozoic. Simultaneously, they were fed on a diet containing different amounts of magnesium. Our results show that the skeletal Mg/Ca ratio in both species varied not only between ossicle types but also between different types of stereom within a single ossicle. Importantly, the skeletal Mg/Ca ratio in both species decreased proportionally with decreasing seawater Mg2+ /Ca2+ ratio. However, sea urchins feeding on Mg-enriched diet produced a skeleton with a higher Mg/Ca ratio. We also found that although incubation in lower ambient Mg2+ /Ca2+ ratio did not affect echinoid respiration rates, it led to a decrease or inhibition of their growth. Overall, these results demonstrate that although skeletal Mg/Ca ratios in echinoderms can be largely determined by seawater chemistry, the type of diet may also influence skeletal geochemistry, which imposes constraints on the application of fossil echinoderms as a reliable proxy. The accuracy of paleoseawater Mg2+ /Ca2+ calculations is further limited by the fact that Mg partition coefficients vary significantly at different scales (between species, specimens feeding on different types of food, different ossicle types, and stereom types within a single ossicle).

Molluscs defend themselves against predation and environmental stressors through the possession of mineralized shells. Mussels are widely used to predict the effects of abiotic factors such as salinity and pH on marine calcifiers in the context of changing ocean conditions. Shell matrix proteins are part of the molecular control regulating the biomineralization processes underpinning shell production. Under changing environmental conditions, differential expression of these proteins leads to the phenotypic plasticity of shells seen in many mollusc species. Low salinity decreases the availability of calcium and inorganic carbon in seawater and consequently energetic constraints often lead to thin, small and fragile shells in Mytilid mussels inhabiting Baltic Sea. To understand how the modulation of shell matrix proteins alters biomineralization, we compared the shell proteomes of mussels living under full marine conditions in the North Sea to those living in the low saline Baltic Sea. Modulation of proteins comprising the Mytilus biomineralization tool kit is observed. These data showed a relative increase in chitin related proteins, decrease in SD-rich, GA-rich shell matrix proteins indicating that altered protein scaffolding and mineral nucleation lead to impaired shell microstructures influencing shell resistance in Baltic Mytilid mussels. Interestingly, proteins with immunity domains in the shell matrix are also found to be modulated. Shell traits such as periostracum thickness, organic content and fracture resistance qualitatively correlates with the modulation of SMPs in Mytilid mussels providing key insights into control of biomineralization at molecular level in the context of changing marine conditions., SCOPUS: ar.j, info:eu-repo/semantics/published

It has been argued that skeletal Mg/Ca ratio in echinoderms is mostly governed by Mg

International audience; Ocean acidification (OA) is a major global driver that leads to substantial changes in seawater carbonate chemistry, with potentially serious consequences for calcifying organisms. Marine shelled molluscs are ecologically and economically important species, providing essential ecosystem services and food sources for other species. Due to their physiological characteristics and their use of calcium carbonate (CaCO 3) to build their shells, molluscs are among the most vulnerable invertebrates with regard to OA, with early developmental stages being particularly sensitive to pH changes. This study investigated the effects of CO 2-induced OA on juveniles of the European abalone Haliotis tuberculata, a commercially important gastropod species. Six-month-old juvenile abalones were cultured for 3 months at four pH levels (8.1, 7.8, 7.7, 7.6) representing current and predicted near-future conditions. Survival, growth, shell microstructure, thickness, and strength were compared across the four pH treatments. After 3 months of exposure, significant reductions in juvenile shell length, weight, and strength were revealed in the pH 7.6 treatment. Scanning electron microscopy observations also revealed modified texture and porosity of the shell mineral layers as well as alterations of the periostracum at pH 7.6 which was the only treatment with an aragonite saturation state below 1. It is concluded that low pH induces both general effects on growth mechanisms and corrosion of deposited shell in H. tuberculata. This will impact both the ecological role of this species and the costs of its aquaculture.

SCOPUS: ar.j, info:eu-repo/semantics/published

Antarctic surface waters are expected to be the first to experience severe ocean acidification (OA) with carbonate undersaturation and large decreases in pH forecasted before the end of this century. Due to the long stability in environmental conditions and the relatively low daily and seasonal variations to which they are exposed, Antarctic marine organisms, especially those with a supposedly poor machinery to eliminate CO2 and protons and with a heavily calcified skeleton like echinoderms, are hypothesized as highly vulnerable to these environmental shifts. The opportunities offered by the natural pH gradient generated by vent activities in Deception Island caldera, Western Antarctic Peninsula, were used to investigate for the first time the acid-base physiologies, the impact of OA on the skeleton and the impact of pH on metal accumulation in the Antarctic sea star Odontaster validus and sea urchin Sterechinus neumayeri. The two species were sampled in four stations within the caldera, two at pH (total scale) 8.0–8.1 and two at reduced pH 7.8. Measured variables were pH, alkalinity, and dissolved inorganic carbon of the coelomic fluid; characteristic fracture force, stress and Young's modulus using Weibull statistics and Cd, Cu, Fe, Pb and Zn concentrations in the integument, gonads and digestive system. Recorded acid-base characteristics of both studied species fit in the general picture deduced from temperate and tropical sea stars and sea urchins but conditions and possibly confounding factors, principally food availability and quality, in the studied stations prevented definitive conclusions. Reduced seawater pH 7.8 and metals had almost no impact on the skeleton mechanical properties of the two investigated species despite very high Cd concentrations in O. validus integument. Reduced pH was correlated to increased contamination by most metals but this relation was weak. Translocation and caging experiments taking into account food parameters are proposed to better understand future processes linked to ocean acidification and metal contamination in Antarctic echinoderms., SCOPUS: ar.j, info:eu-repo/semantics/published