Biologically driven isotope fractionation in ultrastructurally different shell portions of freshwater pearl mussels (Margaritifera margaritifera): Implications for stream water δ18O reconstructions

Oxygen isotopes in stream water can serve as natural tracers of watershed dynamics. Freshwater pearl mussels provide δ¹⁸O_water estimates that overcome temporal and spatial limitations of instrumental records. The reliability of shell-based δ¹⁸O_water reconstructions depends on understanding which shell layer biomineralizes closer to oxygen isotopic equilibrium with ambient water. To determine this, both the (outer) prismatic and (inner) nacreous sublayers of the outer shell layer were sampled. Over 2500 isotope values were obtained from shells collected from the Our River (Luxembourg) and from mussels cultured in tanks at constant temperature and monitored δ¹⁸O_water. Calculated δ¹⁸O_water from the prismatic portion was in excellent agreement with monitored δ¹⁸O_water, while δ¹⁸O_shell of the nacreous portion was systematically offset by +0.43‰, overestimating δ¹⁸O_water by +0.53‰. Although shell portions were formed simultaneously from the same extrapallial fluid, they underwent different fractionation mechanisms, presumably due to differences in carbonic anhydrase activity catalyzing mineralization processes.