Methylotrophic methanogenesis induced by ammonia nitrogen in an anaerobic digestion system

Objectives

This lab-scale study aimed to investigate the effect of total ammonia nitrogen (TAN) stress on the methanogenic activity and the taxonomic and functional profiles of the microbial community of anaerobic sludge (AS) from a full-scale bioreactor.

Methods

The AS was subjected to a stepwise increase in TAN every 14 days at concentrations of 1, 2, 2.5, 3, 3.5, and 4 g TAN/L (Acclimated-AS or AAS). This acclimation stage was followed by an ammonia stress stage (4 g/L). A blank-AS (BAS) was maintained without TAN during the acclimation stage. In the second stress stage (ST), the BAS was divided into two new treatments: a control (BAS') and one that received a shock load of TAN of 4 g/L (SBAS'). Methane production was measured, and a metagenomic analysis was conducted to describe the microbial community.

Results

A decrease in the relative abundance of Methanothrix soehngenii of 16 % was related to a decrease of 23 % in the methanogenic capacity of AAS when comparing with the final stage of BAS. However, recovery was observed at 3.5 g TAN/L, and a shift to methylotrophic metabolism occurred, indicated by a 4-fold increase in abundance of Methanosarcina mazei. The functional analysis of sludge metagenomes indicated that no statistical differences (p > 0.05, RM ANOVA) were found in the relative abundance of methanogenic genes that initiate acetoclastic and hydrogenotrophic pathways (acetyl-CoA synthetase, ACSS; acetate kinase, ackA; phosphate acetyltransferase, pta; and formylmethanofuran dehydrogenase subunit A, fwdA) into the BAS and AAS during the acclimation phase. The same was observed between groups of genes associated with methanogenesis from methylated compounds. In contrast, statistical differences (p < 0.05, one-way ANOVA) in the relative abundance of these genes were recorded during ST. The functional profiles of the genes involved in acetoclastic, hydrogenotrophic, and methylotrophic methanogenic pathways were brought to light for acclimatation and stress experimental stages.

Conclusions

TAN inhibited methanogenic activity and acetoclastic metabolism. The gradual acclimatization to TAN leads to metabolic and taxonomic changes that allow for the subsequent recovery of methanogenic functionality. The study highlights the importance of adequate management of anaerobic bioprocesses with high nitrogen loads to maintain the methanogenic functionality of the microbial community.