Mesophilic Mineral-Weathering Bacteria Inhabit the Critical-Zone of a Perennially Cold Basaltic Environment

The weathering of silicate in the world's critical-zone (rock-soil interface) is a natural mechanism providing a feedback on atmospheric CO₂ concentrations through the carbonate-silicate cycle. We examined culturable bacterial communities from a critical-zone in western Iceland to determine the optimum growth temperature and their ability to solubilize phosphate-containing minerals, which are abundant within the critical-zone area examined here. The majority of isolated bacteria were able to solubilize mineral-state phosphate. Almost all bacterial isolates were mesophilic (growth optima of 20–45°C), despite critical-zone temperatures that were continuously below 15°C, although all isolates could grow at temperatures associated with the critical-zone (−2.8–13.1°C). Only three isolates were shown to have thermal optima for growth that were within temperatures experienced at the critical-zone. These findings show that the bacteria that inhabit the western Icelandic critical-zone have temperature growth optima suboptimally adapted to their environment, implying that other adaptations may be more important for their long-term persistance in this environment. Moreover, our study showed that the cold basaltic critical-zone is a region of active phosphate mineral-weathering.