Resilience toward environmental stress is important for the function of both natural and engineered microbial ecosystems. Engineered microbial ecosystems are usually designed to maintain processes given a prescribed set of environmental conditions. Operation outside of these conditions may cause stress and failure of the desired system processes. Resources (nutrients) may become a source of stress in microbial bioreactors that have become adapted to particular substrates. Methanogenic bioreactors make excellent models for experimentally studying the resilience of microbial community structure and function because the major steps in the food web are readily measured. Anaerobic food webs consist of three broad trophic levels: hydrolysis/ primary fermentation, secondary fermentation (including syntrophic interactions), and methanogenesis. To better understand how stressors impact anaerobic ecosystem resilience, we have subjected methanogenic bioreactors to pulse disturbances with a unique substrate that induces stress when applied at high levels. We evaluated whether prior exposure to non-stressful pulses of the new substrate improved resilience toward a large, stressful pulse. The experiment used five replicate 10 liter methanogenic bioreactors that were established with inoculum from a pilot-scale reactor. The basal feedstock was poultry litter which has a low C/N ratio, high ammonia, and high lignocellulose. Reactors had a 20-day hydraulic retention time (HRT). Crude glycerol which has an extremely high C/N ratio was applied as pulse disturbances. Anaerobic food web linkages were evaluated with standard HACH tests and Gas Chromatography for fatty acids and gases. Microbial community structure dynamics were evaluated with Illumina sequencing of 16S rRNA gene diversity.
Results/Conclusions
The effects of small pulses of crude glycerol could be followed through the food web as rapid increases in total volatile acids (VA), particularly acetate and propionate, decreases in methane, and increases in biogas. Functional resilience occurred quickly as the systems returned to steady state conditions. Beta diversity analyses (UniFrac and Bray Curtis) showed modest changes following the small pulses. A final large stressful pulse was applied which more than doubled VAs. The reactor with the greatest community divergence after the small pulses showed the greatest functional resilience toward the stressful pulse: VA levels recovered most quickly. We found that prior exposure to non-stressful pulses of a new substrate was associated with a change in community structure and an increase in functional resilience toward later stressful-levels of the same disturbance.