Environmental Science and Engineering Seminar

Microorganisms control the bulk of chemical cycling on our planet, but measurements of microbial growth in natural environments are rare. Microbial growth rate is a simple yet profound parameter: it is not only a mediator of geochemical transformation but is also an indicator of fitness and an environment's relative habitability. The fundamental question guiding the research presented here is – how do we quantitatively measure microbial growth in natural systems and what does this rate tell us about the feedback between organism and environment? In this talk, I focus on two research areas 1) microbial growth in alpine soils and permafrost 2) carbon utilization in subsurface hyperalkaline groundwaters.

The biomass of microorganisms in soil rivals that of above ground plants and animals, yet fundamental questions remain regarding the activity of soil microorganisms. We apply lipid stable isotope probing (lipid-SIP), demonstrating the dominance of slow-growing taxa in the soil microbiome, in contrast to prior studies that were limited to measuring primarily fast-growing taxa. These results challenge the pervasive assumption that soils with more microbial biomass are more productive than those with less. We extend our lipid-SIP methodology to Alaskan subsurface permafrost samples from the Last Glacial Maximum to test factors governing microbial mobilization of recalcitrant organic C and the rates of microbial resuscitation after thaw.

The second topic of this discussion is the carbon preferences of microorganisms hosted in serpentinizing groundwater. Serpentinites, hydrated ultramafic rocks that produce [hyper]alkaline, reducing, H2-rich groundwaters, are environments of newfound industrial interest due to their reserves of geological hydrogen and their ability to sequester CO2 (via direct-air-capture coupled to subsurface injection). However, it is assumed that microbial life functions at slow rates in these extreme environments, where groundwater pH can exceed 12. Here, we demonstrate that cell-specific anabolic rates vary by five orders of magnitude, with corresponding energetic flux encompassing most of the range of life on Earth. Moreover, despite [hyper]alkaline conditions, we detect robust activity of methanogenic archaea that are poised to use hydrogen to convert dissolved inorganic carbon to methane. This study indicates that regions of the Earth's subsurface undergoing water/rock reactions may be far more habitable than previously postulated. Such vigorous microbial activity in serpentinites has profound implications for industrial perturbation of the Earth's subsurface, where recovery and burial of H2 and CO2 respectively may be disrupted by microbial activity.