Abstract
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; pHT = 8.02 ± 0.03 1 SD; Ωcalcite = 3.3 ± 0.2 1 SD). However, when reared under elevated pCO2 (900 μatm; pHT = 7.73 ± 0.03; Ωcalcite = 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.
Highlights
Rising levels of atmospheric carbon dioxide (CO2) have resulted in increased dissolution of CO2 in seawater and reduced pH of the upper ocean (Caldeira and Wickett, 2003; Byrne et al, 2010)
We found that the Mg contents of adult S. purpuratus spines and tests are within a factor of two of those found for previously studied sea urchin species
The adult and 5.5-month post-settlement juvenile S. purpuratus tests are composed of high-Mg calcite (6–7 % MgCO3), while adult spines are composed of low-Mg calcite (Scoffin, 1987)
Summary
Rising levels of atmospheric carbon dioxide (CO2) have resulted in increased dissolution of CO2 in seawater and reduced pH of the upper ocean (Caldeira and Wickett, 2003; Byrne et al, 2010). The effects of environmental variation on echinoderm skeletal Mg or Sr substitution rates can be large and variable across taxa, with recent work by Ries (2011) showing that coralline red algae exposed to elevated pCO2 incorporate ∼ 20 % less Mg into their skeleton than under control conditions, whereas serpulid worms incorporate ∼ 26 % more Mg into their tubes. Portions of these differences may stem from variation in the sensitivity of species-specific mineralization pathways to pCO2 (Ries, 2011)
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