Effects of seawater chemistry (Mg2+/Ca2+ ratio) and diet on the skeletal Mg/Ca ratio in the common sea urchin Paracentrotus lividus

Abstract

It has been argued that concentration of major metallic ions such as Mg 2+ and Ca 2+ plays a role in determining the composition of the echinoderm skeleton. Consequently, in several studies Mg/Ca ratio from modern and fossil echinoderm ossicles was used as a reliable proxy of secular Mg 2+ /Ca 2+ changes of Phanerozoic seawater. However, although significant progress has been made in understanding biomineralization of echinoderms, it is still largely unknown what are the sources and physiological pathways of major ions that contribute to skeleton formation. Herein we tested the effects of modifications of ambient seawater Mg 2+ /Ca 2+ ratio (~5) and Mg-enrichment of the diet on the Mg/Ca ratio in regenerating spines of sea urchin Paracentrotus lividus under experimental conditions. We found that sea urchins cultured in seawater with decreased Mg 2+ /Ca 2+ ratio (~1.9) produced a skeleton with also decreased Mg/Ca ratio. However, the skeleton of specimens fed on a Mg-enriched diet showed significantly higher Mg/Ca ratio. This suggests that the seawater is an important but not the only source of ions that contributes to the Mg/Ca ratio of the skeleton. Consequently, the reliability of geochemical models that link directly seawater chemistry with the Mg/Ca ratio of the skeleton should be reevaluated.

Effects of seawater Mg2+/Ca2+ ratio and diet on the biomineralization and growth of sea urchins and the relevance of fossil echinoderms to paleoenvironmental reconstructions

“…Medians highlighted in bold; bars represent the 25% and 75% quartiles; whiskers represent the lowest and highest data points. Asterisks in low Mg 2+ /Ca 2+ seawater ratio represent single outliers brucite Mg(OH) 2 or magnesite MgCO 3 ; see Discussion in Kołbuk et al, 2019). Noteworthy, sea urchins can also actively graze and ingest different types of rock substrates that may be significantly enriched in Mg (Russell, Gibbs, & Duwan, 2018).…”

“…Experimental data to verify the applicability of fossil echinoderms to ancient seawater Mg 2+ /Ca 2+ ratio reconstructions are limited and involved only two species of echinoids (Eucidaris tribuloides in Ries, 2004; Paracentrotus lividus in Kołbuk et al, 2019).…”

“…Furthermore, in these experiments, the specimens from each treatment were either maintained in the same tank (Ries, 2004) inducing a problem of confounding effects or were incubated in artificial seawater in which only concentration of Ca 2+ was adjusted (Kołbuk et al, 2019), hindering interpretation of these results in the context of ancient marine conditions (where Ca 2+ concentration is believed to changed concomitantly with Mg 2+ level (Wilkinson & Algeo, 1989;Hardie, 1996)).…”

“…Interestingly, it has been recently shown that diet can also modify the skeletal Mg/Ca ratio in both juveniles and adults of the echinoid Paracentrotus lividus (Asnaghi et al, 2014; Kołbuk, Dubois, Stolarski, & Gorzelak, 2019). More specifically, these studies demonstrated that sea urchins fed on magnesium‐enriched artificial or natural diet displayed higher Mg/Ca ratios in their skeletons, which suggests that seawater is not the only source of Mg 2+ in echinoderm biomineralization.…”

Jurassic shift from abiotic to biotic control on marine ecological success

“…Among existing proxies, echinoderm skeletal remains have perhaps the highest potential to eventually provide a densely sampled continuous Phanerozoic record of a single proxy. However, the suitability of echinoderms skeletal remains for reconstructing seawater Mg/Ca ratios has been questioned as magnesium concentrations significantly vary within individual skeletons 83 , are dependent on the food source 84 , and do not always mirror expected secular Mg/Ca trends even in very well-preserved specimens 85 .…”