MICROBIAL BIOMASS: A CATALYST OF CACO₃ PRECIPITATION IN ADVECTIVELY DOMINATED FLOW REGIMES

Microorganisms have long been thought to impact CaCO₃ precipitation, yet interpreting the extent of their influence on sediment formation has been hampered by our inability to obtain direct experimental evidence about mineral formation processes in natural environments. We address this problem by conducting kinetic experiments within a modern terrestrial carbonate spring to determine aragonite precipitation rates and to quantify the relative influences of aragonite saturation state (Ω_a), biomass concentration and microbial colonization on CaCO₃ mineralization in advectively dominated flow regimes. At an Ω_a value consistent with modern seawater (3.63 ± 0.09), our results show that steady state precipitation rates measured in the presence of natural hot spring microbial communities are more than twice those determined when the microbial communities are depleted by 0.2 μm filtration. In addition, rates from UV experiments that prevent microbial colonization are statistically inseparable from those of the natural control treatments, suggesting that biologically induced precipitation dominates in these hot spring systems. Modeling that uses the empirical growth rate expression R=k_emp*A*(Ω-1)ⁿ and additional in situ kinetic measurements to test the significance of these experimental results across a broad range of n, reveals that reducing biomass concentrations by forty five percent can decrease the empirical rate constant (k_emp) by more than an order of magnitude. These findings strongly suggest that microorganisms catalyze the mineralization process in natural systems and imply that changes in calcium carbonate precipitation rates are intimately linked with changes in local microbial biomass concentration throughout geologic history.