Parallel processing of substrate correlates with greater functional stability in methanogenic bioreactor communities perturbed by glucose

Abstract

Parallel processing is more stable than serial processing in many areas that employ interconnected activities. This hypothesis was tested for microbial community function using two quadruplicate sets of methanogenic communities, each set having substantially different populations. The two communities were maintained at a mean cell residence time of 16 days and a mean glucose loading rate of 0.34 g/liter-day in variable-volume reactors. To test stability to perturbation, they were subjected to an instantaneous glucose pulse that resulted in a 6.8-g/liter reactor concentration. The pattern of accumulated products in response to the perturbation was analyzed for various measures of functional stability, including resistance, resilience, and reactivity for each product. A new stability parameter, "moment of amplification envelope," was used to compare the soluble compound stability. These parameters indicated that the communities with predominantly parallel substrate processing were functionally more stable in response to the perturbation than the communities with predominantly serial substrate processing. The data also indicated that there was good replication of function under perturbed conditions; the degrees of replication were 0.79 and 0.83 for the two test communities.

FIG. 1

Anaerobic food chain as a network of substrate flow through a methanogenic community (a) and ecological parameters of functional stability (b).

FIG. 2

Disappearance of glucose (a) and accumulation of major soluble products (b through e) in response to the perturbation. The data for HS reactors are on the left, and the data for LS reactors are on the right. Symbols: ●, reactor 5; ■, reactor 6; ⧫, reactor 7; ▴, reactor 8; ○, reactor 1; □, reactor 2; ◊, reactor 3; ▵, reactor 4.

FIG. 3

Utilization of glucose and accumulation of the four major soluble intermediate products in the HS set (a) and the LS set (b). The lines indicate averages for three reactors. Insets show magnified data for the first 24 h.