Experimental investigations of organic and microbially mediated reactions of aluminosilicates and phosphates.

Abstract

Microbial respiration is a major process at Earth's surface. Either directly, or indirectly (via metabolic by-products such as organic acids), microbial activity controls the rate and path of mineral weathering reactions as well as overall biogeochemical cycling of elements such as carbon and phosphorus. The impacts of microbial activity and metabolic by-products were examined experimentally for two important near-surface chemical processes: (1) dissolution of aluminosilicate minerals and formation of secondary minerals and, (2) processing of dissolved phosphatic compounds and potential for exchange of oxygen isotopes between phosphates and environmental water. The effects of two organic acids (oxalate and citrate) on the dissolution rate and solubility of feldspars, Al-hydroxide, and quartz were evaluated in simple and complex ionic media at near neutral pH values. This pH regime is common in natural settings and one where Al-mobility is at a minimum. Both organic acids enhanced the solubility and dissolution rates of feldspars and Al-hydroxides, but had only minor effects on quartz. The net effect was to promote congruent dissolution of feldspars and reduce back-precipitation of secondary clay minerals. The ligand-promoted dissolution mechanism involves specific interactions between organic ligands and Al-sites and is most effective in solutions of low ionic strength where the fraction of free oxalate or citrate is greatest. Microbial metabolism of organic and inorganic phosphate compounds was investigated in controlled culture experiments in solutions of known chemical and oxygen isotope composition. Results establish unequivocally that bacteria facilitate oxygen isotope exchange with ambient water during processing of phosphatic compounds. Equilibrium oxygen isotope exchange between phosphate and water accompanying metabolism of organophosphorus compounds may be masked by variable inheritance of oxygen from the organic substrate. In contrast, phosphate processed by purified cell-free enzymes (inorganic pyrophosphatase) exhibited complete, equilibrium isotope exchange with water and provided, for the first time, a low-temperature calibration of the phosphate-water temperature equation. These results suggest that processing of organic matter and inorganic phosphates in geological systems could reset original phosphate oxygen isotope compositions of biogenic and sedimentary phosphate minerals during diagenetic alteration in sediment pore waters.