The subsurface microbial loop for nutrient recycling in hydrocarbon-contaminated aquifers
Final Report Abstract
The microbial loop is a concept stemming from marine science stating that all dead biomass is recycled by microorganisms which themselves get subject to the food chain feeding higher trophic levels. Hence, organic carbon and nutrients are recycled. In the subsurface, however, there is only a very truncated food chain since higher organisms such as vertebrates are mostly missing and even invertebrates are rare. In this proposal, we hypothesized that microorganisms (bacteria) built an internal microbial loop where bacteria die due to viral lysis or spontaneous cell death and the necromass is directly degraded by again by other bacteria. We first established suitable surrogates of bacterial necromass. We labelled E. coli by growth with 13C-glucose and tested the degradation of pasteurized cells, autoclaved cells, Frenchpressed cells as surrogate for viral lysis, and UV-treated cells as surrogate for spontaneous cell death remaining intact cells. Necromass degradation was measured by 13CO2 evolution with a stable isotope analyzer and the latter two types of necromass turned out suitable and were used for further studies. The bacterial necromass was quickly turned over in natural sediments (hypothesis 1). Half of the CO2 evolution occurred in only three days but it took up to 50 days for complete degradation. The main degrading organisms were indeed bacteria (hypothesis 1 and 3) and not protists which was determined in experiments with added antibiotics against bacteria or protists. Adding necromass to sediments also increased the microbial diversity significantly as determined with 16S rRNA gene amplicon sequencing (hypothesis 3). All experiments evaluating if the recycling of bacterial necromass can stimulate anaerobic hydrocarbon degradation failed and hypothesis 2 could not be tested.
