The elemental composition of purple sea urchin (Strongylocentrotus purpuratus) calcite and potential effects of pCO₂ during early life stages

Abstract. Ocean acidification will likely have negative impacts on invertebrates producing skeletons composed of calcium carbonate. Skeletal solubility is partly controlled by the incorporation of "foreign" ions (e.g. magnesium) into the crystal lattice of these skeletal structures, a process that is sensitive to a variety of biological and environmental factors. Here we explore effects of life stage, oceanographic region of origin, and changes in the partial pressure of carbon dioxide in seawater (pCO₂) on trace elemental composition in the purple sea urchin (Strongylocentrotus purpuratus). We show that, similar to other urchin taxa, adult purple sea urchins have the ability to precipitate skeleton composed of a range of biominerals spanning low- to high-Mg calcites. Mg / Ca and Sr / Ca ratios were substantially lower in adult spines compared to adult tests. On the other hand, trace elemental composition was invariant among adults collected from four oceanographically distinct regions spanning a range of carbonate chemistry conditions (Oregon, Northern California, Central California, and Southern California). Skeletons of newly settled juvenile urchins that originated from adults from the four regions exhibited intermediate Mg / Ca and Sr / Ca between adult spine and test endmembers, indicating that skeleton precipitated during early life stages is more soluble than adult spines and less soluble than adult tests. Mean skeletal Mg / Ca or Sr / Ca of juvenile skeleton did not vary with source region when larvae were reared under present-day, global-average seawater carbonate conditions (400 μatm; pH_T = 8.02 ± 0.03 1 SD; Ω_calcite = 3.3 ± 0.2 1 SD). However, when reared under elevated pCO₂ (900 μatm; pH_T = 7.73 ± 0.03; Ω_calcite = 1.8 ± 0.1), skeletal Sr / Ca in juveniles exhibited increased variance across the four regions. Although larvae from the northern populations (Oregon, Northern California, Central California) did not exhibit differences in Mg or Sr incorporation under elevated pCO₂ (Sr / Ca = 2.10 ± 0.06 mmol mol⁻¹; Mg / Ca = 67.4 ± 3.9 mmol mol⁻¹), juveniles of Southern California origin partitioned ~8% more Sr into their skeletons when exposed to higher pCO₂ (Sr / Ca = 2.26 ± 0.08 vs. 2.09 ± 0.005 mmol mol⁻¹ 1 SD). Together these results suggest that the diversity of carbonate minerologies present across different skeletal structures and life stages in purple sea urchins does not translate into an equivalent geochemical plasticity of response associated with geographic variation or temporal shifts in seawater properties. Rather, composition of S. purpuratus skeleton precipitated during both early and adult life history stages appears relatively robust to spatial gradients and predicted future changes in carbonate chemistry. An exception to this trend may arise during early life stages, where certain populations of purple sea urchins may alter skeletal mineral precipitation rates and composition beyond a given pCO₂ threshold. This potential for geochemical plasticity during early development in contrast to adult stage geochemical resilience adds to the growing body of evidence that ocean acidification can have differing effects across organismal life stages.