Compositional Analysis of Cultured Biominerals

Compositional Analysis of Cultured Biominerals

Mineral composition is influenced by a host of environmental, chemical, and biological factors during growth. To accurately  interpret these signals we need to separate the impact of each parameter on tracer behavior. These data can then be used to build a chemical-scale understanding of mineral growth. Biomineral culture and inorganic precipitation experiments, where growth parameters are isolated and manipulated independently, are uniquely suited to address these questions.

Schematic of an experimentally grown mineral sample comprised of two components mixed in unknown proportions: (o) the pre-experiment “seed” material, and (x) new growth corresponding to experimental conditions. Using the mixed-spike overgrowth method it is possible to determine the composition of the newly grown material through bulk isotope analysis of the entire sample.

Culture and precipitation experiments often involve overgrowth on an initial material. For example, seed crystals are used to control mineralogy, avoid nucleation, and access low growth rates during inorganic precipitation. Similarly, biomineral culture experiments typically start from wild specimens with preexisting and poorly characterized skeletons. In both classes of experiments, new growth corresponding to experimental conditions must be separated from initial material.

We developed a new method of compositional analysis during mineral growth.  The method relies on growth from a solution enriched in multiple stable isotopes and is an adaptation of the isotope-dilution technique, however in this case isotope dilution occurs within a growing mineral. The new technique has several advantageous characteristics: (1) it requires neither the amount nor the composition of the initial material to be known, (2) it harnesses the precision, sensitivity, and accessibility typically associated with bulk analysis, and (3) it works even when it is impossible to physically identify and separate newly grown material. Furthermore, the method allows new modalities. For example, it can be used to isolate different events through time.  Amongst other applications, this feature can be used to answer an important question in biomineralization and paleoceanography: the effect of light and photosymbionts on skeletal composition.  Finally, the method can be extended to resolve stable isotope fractionation during mineral growth for select systems of non-traditional stable isotopes.

Read more: Gagnon, Adkins, and DePaolo,. (2012) Precise Overgrowth Composition During Biomineral Culture and Inorganic Precipitation.  Chemical Geology.

Schematic of the overgrowth fractionation method – a technique that can resolve isotope fractionation for certain non-traditional sytems like calcium isotopes during mineral and biomineral growth. In practice this method only requires one set of isotope ratio measurements per sample once the growth solution has been characterized and the double-spike is calibrated.