PNNL

Abstract

Propionate accumulation is a common indicator of process imbalances in anaerobic bioreactor systems. The accumulation of propionate can occur due to low retention rates, hydrogen accumulation, or mechanical changes affecting the proximity between propionate oxidizers and partner species, thereby preventing necessary electron transfer. Few studies, however, have observed the changes in microbial community structure during propionate accumulation. We used 454 pyrosequencing of 16S rDNA to evaluate the community membership during propionate accumulations in replicate bioreactors with rumen based cultures. Half of the culture volume from a parent reactor was transferred to a sterile “daughter” reactor, and both systems were run identically. Both reactors experienced a propionate accumulation after roughly 10 days, with the propionate accumulation being less pronounced in the parent reactor as compared to the daughter reactor. Non-metric multidimensional scaling (NMDS) was used to determine clustering patterns of the samples, and correlative methods were used to determine which OTUs were significantly associated with the movements of samples along the NMDS axes. The presence of Saccharofermentans characterized the position of early samples, whereas the presence of Ruminococcus and Succiniclasticum were more indicative of the positions of later samples. Hydrogen accumulation and low sequence counts indicated low methanogen activity. Although both reactor systems were closed to microbial inputs due to the sterilization of influent media, we recorded significant increases in reactor diversity over time. This suggests that changes in the abundances of dominant community members may affect the sequencing of rare taxa within samples.