Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.
Laboratoire de Biologie Marine, Faculté des Sciences, Université Libre de Bruxelles, Bruxelles, Belgium.
Geobiology. 2020 Nov;18(6):710-724. doi: 10.1111/gbi.12409. Epub 2020 Aug 9.
It has been argued that skeletal Mg/Ca ratio in echinoderms is mostly governed by Mg and Ca concentrations in the ambient seawater. Accordingly, well-preserved fossil echinoderms were used to reconstruct Phanerozoic seawater Mg /Ca 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 Mg /Ca 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 Mg /Ca conditions to simulate fluctuations in the Mg /Ca 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 Mg /Ca 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 Mg /Ca 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 Mg /Ca 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).
有人认为棘皮动物骨骼的 Mg/Ca 比值主要受周围海水中 Mg 和 Ca 浓度的控制。因此，保存完好的化石棘皮动物被用来重建显生宙海水 Mg/Ca 比值。然而，棘皮动物骨骼中的 Mg/Ca 比值可能受到许多环境和生理因素的影响，这些因素的影响仍知之甚少。值得注意的是，支持棘皮动物在古环境重建中应用的实验数据仍然有限。在这里，我们研究了环境 Mg/Ca 海水比和饮食对两种海胆（Psammechinus miliaris 和 Prionocidaris baculosa）骨骼 Mg/Ca 比值和生长速率的影响。海胆被贴上锰标签，然后在不同的 Mg/Ca 条件下培养，以模拟显生宙海水 Mg/Ca 比值的波动。同时，它们以含有不同镁量的饮食为食。我们的结果表明，两种物种的骨骼 Mg/Ca 比值不仅在不同类型的小骨之间有所不同，而且在同一小骨内的不同类型的立体骨之间也有所不同。重要的是，两种物种的骨骼 Mg/Ca 比值都随海水 Mg/Ca 比值的降低而呈比例降低。然而，摄食富含镁饮食的海胆产生的骨骼 Mg/Ca 比值较高。我们还发现，尽管在较低的环境 Mg/Ca 比下孵育不会影响棘皮动物的呼吸率，但它会导致它们的生长减少或受到抑制。总的来说，这些结果表明，尽管棘皮动物骨骼的 Mg/Ca 比值主要受海水化学的影响，但饮食类型也可能影响骨骼地球化学，这对将化石棘皮动物作为可靠替代物的应用构成了限制。古海水 Mg/Ca 计算的准确性还受到以下事实的限制：Mg 分配系数在不同尺度上（物种之间、摄食不同类型食物的标本、不同小骨类型、同一小骨内的不同立体骨类型）差异很大。
