Ordered low-temperature dolomite mediated by carboxyl-group density of microbial cell walls

Abstract

Abundant in the ancient rock record, early dolomite remains scarce in modern systems at low temperatures (50C), even those systems supersaturated with respect to dolomite. This scarcity is attributed to kinetic inhibition including complexation of Mg2+ by water and sulfate, carbonate activity, and Mg:Ca ratio. Recent investigations point to a function for microbial metabolisms and surfaces, in which disordered phases are formed. Here, we report the precipitation of primary ordered dolomite at 30C, facilitated solely by the cell walls of two nonmetabolizing archaea from saline solutions with an Mg:Ca ratio of 1:1, 5:1, and 10:1, and slightly saturated with respect to dolomite. Control experiments using bacteria and functionalized microspheres did not precipitate dolomite. Archaeal cell wall functional groups were approximately one order of magnitude higher than the bacteria and spheres used in this study. From these results, we propose a mechanistic model in which carboxyl groups associated with cell wall biomass and exopolymeric substances dehydrate Mg ions, further promoting carbonation and leading to dolomite nucleation. These data explain reports of low-temperature dolomite formation associated with numerous microbial metabolic guilds, including bacteria and archaea, and those reported in association with exopolymeric substances or cell wall surfaces, and identify a key and widespread mechanism in the formation of disordered dolomite and ordered primary phases of dolomite at low temperature. Importantly, the functionalized dead and nonmetabolizing biomass is the key in low-temperature dolomite precipitation, not active microbial metabolism. These observations may lead to new predictive models for the distribution of dolomite.