The simplest organisms, such as unicellular marine algae, are capable of controlling the inorganic process of crystal nucleation and growth to a degree that is unparalleled by any man-made process. These organisms are experts in funneling the mineralization reaction through a series of intermediate phases, resulting in a genetically prescribed product. The cells control the precipitation process by producing functional organic macromolecules that specifically interact with the inorganic phase. Usually the interactions between the organic macromolecules and the inorganic phase are considered to occur in aqueous environment that resembles bulk solution. However, biological mineral formation is often confined in membrane-bound vesicles where the chemical environment is markedly different from an aqueous solution. Our research focuses on the formation of coccoliths, intricately shaped calcite crystal arrays that form intracellularly by a group of marine single-celled organisms, coccolithophores. The coccolith calcite crystals nucleate and grow with the utmost control over their crystallography and morphology on an organic template. Using electron tomography of vitrified cells, and<i> in vitro</i> mineralization assays, we showed that electrostatic interactions, mediated by calcium ions, between the organic template and charged soluble macromolecules create a dense local environment at the sites of crystal nucleation. This calcium-rich dense phase is a preferred site for calcite nucleation and growth if carbonate ions are introduced <i>in vitro</i>. The crystallization process is limited to this nanoscale interphase and the resulting crystals show very different characteristics from solution-mediated crystallization. These observations suggest that part of the biological strategy to control the crystallization process is by altering the local chemistry at the crystallization site by the activity of macromolecules. It is possible that the function of these macromolecules is very different from simple soluble additives in solution and that the key to their function lies in alteration of the chemical environment in ways that are yet to be characterized.

Elucidating the intracellular chemical environment that regulates biological calcite crystallization in marine plankton

255th National Meeting (2018)

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