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BACKGROUND Propionate accumulation may lead to digester failure. This study aimed to investigate the effect of ammonia, a metabolic product of protein, on propionate degradation. The shift of microbial community was also investigated. RESULTS Propionate accumulated over the experimental period in the reactor with a total ammonia nitrogen (TAN) concentration of 2.5 g N L−1, as a result the digester failed after 4 hydraulic retention times (HRT) at an organic load rate (OLR) of 0.8 g propionic acid (HPr) L−1 d−1. The average HPr degradation rate was below 54% during the fourth HRT, while >97% of the degraded HPr was converted to methane. The reactor without ammonia stress did not experience HPr accumulation and OLR was increased stepwise to 1.2 g L−1 d−1 at the 8th HRT. The average HPr degradation rate and methane recovery rate of this reactor in the last HRT was 99% and 74%, respectively. According to the shifts of microbial community, acetoclastic methanogen was more vulnerable to ammonia than hydrogenotrophic methanogen CONCLUSION TAN concentration of 2.5 g N L−1 inhibited propionate degradation more severely than methanogenesis. The loss of abundance of Clostridiaceae and Syntrophobacter might be the main reason for the poor performance under ammonia stress. © 2017 Society of Chemical Industry

BACKGROUND The main obstacle associated with methane production in anaerobic digestion processes from marine macroalgae is inhibited by the sea salts. This study aims to investigate the anaerobic digestion performance of marine macroalgae by acclimating inoculum to varying salinity, and to analyze the influence of salinity on microbial community by high-throughput gene sequencing. RESULTS The relationship between specific methane yield and salinity followed the third-order polynomial equation: y = 271.533 + 6.449X − 0.261X2 + 0.002X3. The acclimating inoculum produced methane efficiently at 35 g L−1 with a specific methane yield of 220.88 mL CH4 g−1 VS. Methanogenesis was considerably impeded at salinity greater than 55 g L−1. The bacterial phyla Bacteroidetes, Firmicutes, and Proteobacteria and the Methanobacterium, Methanosaeta, and Methanosarcina genera in archaea were predominant at different salinities. Hydrogenotrophic methanogens such as Methanobacterium can tolerate salinity up to 85 g L−1, whereas acetoclastic methanogens, Methanosaeta and Methanosarcina were severely inhibited at salinity greater than 65 g L−1. CONCLUSION The acclimating inoculum can be used as anaerobic microbial resources for efficient methane production from marine macroalgae under seawater conditions of 35 g L−1. The hydrolytic, acidogenic, acetogenic bacteria and hydrogenotrophic methanogens normally metabolized at different salinities, whereas acetoclastic methanogens were severely inhibited at high saline conditions. © 2017 Society of Chemical Industry