Uranium in Larval Shells As a Barometer of Molluscan Ocean Acidification Exposure

Abstract

As the ocean undergoes acidification, marine organisms will become increasingly exposed to reduced pH, yet variability in many coastal settings complicates our ability to accurately estimate pH exposure for those organisms that are difficult to track. Here we present shell-based geochemical proxies that reflect pH exposure from laboratory and field settings in larvae of the mussels Mytilus californianus and M. galloprovincialis. Laboratory-based proxies were generated from shells precipitated at pH 7.51 to 8.04. U/Ca, Sr/Ca, and multielemental signatures represented as principal components varied with pH for both species. Of these, U/Ca was the best predictor of pH and did not vary with larval size, with semidiurnal pH fluctuations, or with oxygen concentration. Field applications of U/Ca were tested with mussel larvae reared in situ at both known and unknown pH conditions. Larval shells precipitated in a region of greater upwelling had higher U/Ca, and these U/Ca values corresponded well with the laboratory-derived U/Ca-pH proxy. Retention of the larval shell after settlement in molluscs allows use of this geochemical proxy to assess ocean acidification effects on marine populations.

Ocean acidification can interact with ontogeny to determine the trace element composition of bivalve shell

“…If using TE:Ca ratios in CaCO 3 structures to reconstruct environmental conditions it is important that the elements used have a direct relationship with pH and are not affected by biological factors. Although it has previously been suggested that U:Ca ratios in bivalve shell will serve as useful proxies of seawater pH (Frieder et al 2014, Levin et al 2015, this study showed biological changes may modify the relationship between pH and U:Ca ratios. In this study Co:Ca, Ni: Ca, and Ti:Ca ratios were shown to clearly and consistently differ across the shell between the two CO 2 treatments to which they were exposed.…”

Environmental pH, O2 and Capsular Effects on the Geochemical Composition of Statoliths of Embryonic Squid Doryteuthis opalescens

“…B:Ca and U:Ca were investigated as pH proxies; however, they did not exhibit the direct relationship with environmental pH that has been found with foraminifera shells [60][61][62], coral skeletons [63] and mollusk shells [64]. These B:Ca results suggest that, when environmental pH T is low (7.55-7.56) and [O 2 ] is high ), squid embryos can regulate the endolymph pH within the statocyst, where the statolith crystal grows.…”

“…Clear geochemical proxies for exposure to low pH have been established for foraminifera shells [60][61][62], coral skeletons [63] and mollusk shells [64] using either δ 11 boron or uranium:calcium ratios. In addition, δ…”

Element composition of shark vertebrae shows promise as a natural tag

“…However, such attempts will be key to confirming the suitability of using these structures as natural tags and tracers of environmental acidification in changing oceans. Uranium concentrations have been shown to increase with the acidity of the environment in marine molluscan shells ) and cephalopod statoliths (Frieder et al 2014), and we hypothesised a similar trend for U : Ca in shark vertebrae. However, that was not the case.…”