Determining Threshold Values of Phosphates for Methanogenic Growth and Biomineralization in Anaerobic Environments

Sammanfattning

Phosphate is a crucial element in biochemistry, but phosphate cycling during the early evolution of life remains poorly understood. Methanogens are a group of Archaea of particular interest as, through the catalysis of CO2 reduction, they can cause pH changes which in turn influence the solubility and bioavailability of phosphate. Methanobacterium AMB_03 is a new species of hydrogenotrophic methanogen which, like the closely related M. oryzae, was found to be precipitating calcium phosphates. In this experiment, we set up inoculated and blank growth cultures of M. AMB_03 across a range of phosphate concentrations, between 0 and 6 mmol/l (mM), to identify phosphate concentration thresholds in methane production and hydroxyapatite precipitation. We found that whilst inorganic phosphate availability played a role, it was not a major limiting factor in methanogenic growth when energy and nutrients are widely available. The minimum threshold for the precipitation of calcium phosphate, in the form of hydroxyapatite, occurs between 1 and 3 mM. We also identified calcite at the lowest phosphate concentrations, with phosphate inhibiting the precipitation of calcium carbonates from 0.5 mM. The methanogens were found to produce extracellular polymeric substances (EPS), which is involved in in the precipitation of both hydroxyapatite and calcite, whilst the cells seem more directly involved in the former, likely due to local pH increases. Furthermore, we find that the hydroxyapatite precipitates seem to form under energy-stressed conditions and may therefore be a consequence of a stress response mechanism. The influence of methanogenesis on phosphate solubility raises interesting questions with regards to their role in phosphorus cycling and bioavailability, in particular during the early stages of life.