Physiological exclusion of magnesium from Mytilus edulis calcite

Abstract

CALCITE produced by many marine organisms (including molluscs¹, brachiopods², coccolithophorids³, and planktonic foraminifera^4,5) contains only about 5% as much magnesium as is predicted by laboratory studies of distribution coefficients^6,7 (ratio of the Mg : Ca ratio in the solid against that in the solution). Understanding the cause of this low magnesium content is important to our comprehension of the calcification process. It is also of considerable interest to the field of low temperature geochemistry for several reasons. First, calcite is the major authigenic component of marine sediments, and its magnesium content is an important feature of its bulk composition. Second, the controls on the magnesium content of invertebrate skeleta must be understood if Mg : Ca ratios of calcitic fossils are to be used as indicators of sea salt palaeo-chemistry² or sea surface palaeotemperature⁸. Third, the mechanism of magnesium inclusion may determine the role of magnesium in regulating selective dissolution of the tests from planktonic foraminifera⁴. Fourth, since calcite solubility depends on the magnesium content of the calcite, the mode of magnesium inclusion is a factor to be considered in calculating the degee of carbonate saturation in the oceans. Finally, factors governing the mode of magnesium incorporation into skeletal calcite are important to sedimentary petrologists using magnesium as a diagenetic indicator. We report here that the mussel Mytilus edulis, which normally secretes low magnesium calcite, incorporates anomalously high amounts of magnesium into its calcitic shell layer when it is grown in solutions of higher than normal magnesium content. Thus in normal conditions Mytilus physiologically excludes magnesium from its shell-forming fluid, and at higher magnesium concentrations ion regulatory systems breakdown, causing a substantial increase in the amount of magnesium coprecipitated into Mytilus shell calcite.