Microsc. Microanal. 23, 1189–1196, 2017 doi:10.1017/S1431927617012715


© MICROSCOPY SOCIETY OF AMERICA 2017


Nucleation and Growth of Mg-Calcite Spherulites Induced by the Bacterium Curvibacter lanceolatus Strain HJ-1


Chonghong Zhang, Jiejie Lv, Fuchun Li,* and Xuelin Li College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, 210095, China


Abstract: Calcite spherulites have been observed in many laboratory experiments with different bacteria, and spherulitic growth has received much interest in mineralogy research. However, the nucleation and growth mechanism, as well as geological significance of calcite spherulites in solution with bacteria is still unclear. Herein, spherulites composed of an amorphous core, a Mg-calcite body and an organic film were precipitated by the Curvibacter lanceolatus HJ-1 bacterial strain in a solution with a molar Mg/Ca ratio of 3. Based on the results, we provide a possible mechanism for the biomineralization of Mg-calcite spherulites. First, amorphous calcium carbonate particles are deposited and aggregated into a stable sphere-like core in combination with organic molecules. The core then acts as the nucleus of spherulitic radial growth. Finally, the organic film grows on the surface of Mg-calcite spherulites as a result of bacterial metabolism and calcification. These findings provide insight into the growth mode and crystallization of biogenic spherulites during biomineralization, and are of significance in the application of novel biological materials.


Key words: bacteria, Mg-calcite, ACC, spherulite, growth


INTRODUCTION Carbonate precipitation, and particularly calcite biominer- alization has been studied widely in recent decades (Ercole et al., 2007; Yoshimura et al., 2017). Some bacteria from various natural habitats are able to precipitate calcite under both natural and laboratory conditions (Benzerara et al., 2006; Sánchez-Navas et al., 2009; Lors et al., 2017). Microbial metabolism can lead to an increase in pH and CO3


2− content


in solution. Calcite nucleation frequently takes place on cell walls due to ion exchange through the cell membrane (Castanier et al., 2000) and/or due to promotion by negatively charged cell wall functional groups that adsorb Ca2+ ions (Rivadeneyra et al., 1998). In addition, extracellular polymeric substance (EPS) can trap Ca2+ ions and serve as a nucleation template or growth modifier (Braissant et al., 2003; Lian et al., 2006). However, the precise roles of bacteria in the process of calcite crystallization remain largely elusive. Calcite crystals grown in the presence of bacteria or biomolecules express excellent mechanical properties, biocompatibility, and biodegradability, and their geochemistry and application to various industrial fields have received considerable attention (Le Metayer-Levrel et al., 1999; Rodriguez-Navarro et al., 2003; Yang et al., 2010; Della Porta, 2015; Declet et al., 2016).


dependent on various strictly defined parameters, such as crystal size, particle composition, polymorphism, and mor- phology (Kitamura, 2001; Kontrec et al., 2013). In most cases, particle morphology is regarded as one of the most


The physicochemical properties of calcite crystals are


*Corresponding author. fchli@njau.edu.cn Received August 17, 2017; accepted October 31, 2017


important parameters. Biogenic calcite grown under laboratory conditions in the presence of additives or foreign ions (such as Mg2+ or Ba2+) display a wide variety of morphologies, and can have dumbbell-like (Wright & Wacey, 2005), star-like (Zhu et al., 2006), spindle-like (Xue et al., 2011), or spherulitic structures (Ercole et al., 2007). These biogenic calcite monocrystals and polycrystals appear to include extensive local disorder, and differ in their crys- tallographic parameters and mineral growth orientation due to ion substitution and the involvement of organic molecules (Bischoff et al., 1983; Li et al., 2017). Such particles have been widely studied owing to their significance in biomineraliza- tion processes and their ability to produce nanoscale or micron-sized organic/inorganic materials with unique morphologies. Among them, the spherulitic precipitate is the most common. Biogenic calcite spherulites are found in association with many biomolecules, and have been linked to several human diseases. Indeed, many scientists consider particles with this spherulitic morphology as biomarkers (Khan, 1997; Magill, 2001; Mercedes-Martín et al., 2016). The nucleation and growth of calcite spherulites is


closely associated with the ambient microenvironment, such as the activities and types of bacteria present, alkalinity, ion species, additives, and impurities (Beck & Andreassen, 2010a). The influence of various factors on the precipita- tion of calcite spherulites has been investigated by many authors (Zhang et al., 2015; Xu et al., 2017), and spherulitic precipitation appears to be extremely complex since virtually any parameter may trigger morphological change. Despite nearly a century of research on the growth features of such crystals (Goldenfeld, 1987; Gránásy et al., 2005; Beck & Andreassen 2010a, 2010b), the formation of spherulites


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