Culture Independent Geochemical Tools for Adressing Microbial Activity

Abstract:

Decades of ocean drilling have demonstrated wide-spread microbial life in deep sub-seafloor sediment, and surprisingly high numbers of microbial cells and endospores. Despite the ubiquity of life in the deep biosphere, the large community sizes are not yet understood given the extremely low energy fluxes. We have developed and applied new approaches to the deep sub-seafloor to quantify distributions and turnover times of living microbial biomass, endospores and microbial necromass. The approach combines sensitive analyses of unique bacterial marker molecules (muramic acid and d-amino acids) and the bacterial endospore marker (dipicolinic acid) with a series of models that link microscopic (e.g., racemization dynamics of stereo-isomeric amino acids) and macroscopic (e.g., porewater geochemistry) properties. Model output includes production rates and turnover times of microbial biomass and necromass, concentration profiles of reactive organic carbon, and rates of organic carbon decomposition. In combination, these results allow us to assess the role of microbial activity in the sub-seafloor carbon budget. One key result is that the turnover time of biomass is far longer than turnover times found in cultures and active surface sediments.