Live microbial cells adsorb Mg²⁺ more effectively than lifeless organic matter

Abstract

The Mg²⁺ content is essential in determining different Mg-CaCO₃ minerals. It has been demonstrated that both microbes and the organic matter secreted by microbes are capable of allocating Mg²⁺ and Ca²⁺ during the formation of Mg-CaCO₃, yet detailed scenarios remain unclear. To investigate the mechanism that microbes and microbial organic matter potentially use to mediate the allocation of Mg²⁺ and Ca²⁺ in inoculating systems, microbial mats and four marine bacterial strains (Synechococcus elongatus, Staphylococcus sp., Bacillus sp., and Desulfovibrio vulgaris) were incubated in artificial seawater media with Mg/Ca ratios ranging from 0.5 to 10.0. At the end of the incubation, the morphology of the microbial mats and the elements adsorbed on them were analyzed using scanning electronic microscopy (SEM) and energy diffraction spectra (EDS), respectively. The content of Mg²⁺ and Ca²⁺ adsorbed by the extracellular polysaccharide substances (EPS) and cells of the bacterial strains were analyzed with atomic adsorption spectroscopy (AAS). The functional groups on the surface of the cells and EPS of S. elongatus were estimated using automatic potentiometric titration combined with a chemical equilibrium model. The results show that live microbial mats generally adsorb larger amounts of Mg²⁺ than Ca²⁺, while this rarely is the case for autoclaved microbial mats. A similar phenomenon was also observed for the bacterial strains. The living cells adsorb more Mg²⁺ than Ca²⁺, yet a reversed trend was observed for EPS. The functional group analysis indicates that the cell surface of S. elongatus contains more basic functional groups (87.24%), while the EPS has more acidic and neutral functional groups (83.08%). These features may be responsible for the different adsorption behavior of Mg²⁺ and Ca²⁺ by microbial cells and EPS. Our work confirms the differential Mg²⁺ and Ca²⁺ mediation by microbial cells and EPS, which may provide insight into the processes that microbes use to induce Mg-carbonate formation.

Keywords

microbe / adsorption / magnesium / calcium

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Acknowledgements

We greatly thank Man Lu for her help with the titration work. This study was jointly supported by the National Natural Science Foundation of China (Grant Nos. 41130207 and 41502317), the Special Funds for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan) (CUG120103), and the open research program from the State Key Laboratory of Biogeology and Environmental Biology (GBL21503).

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