Abstract
BackgroundProteinaceous wastes exhibit high theoretical methane yields and their residues are considered valuable fertilisers. The routine anaerobic degradation of proteins often raises problems like high aromatic compound concentrations caused by the entry of aromatic amino acids into the system. A profound investigation of the consequences of aromatic compound exposure on various microorganisms, which cascade-like and interdependently degrade complex molecules to biogas, is still pending.ResultsIn mesophilic samples, methane was predominantly produced via acetoclastic methanogenesis. The highest positive correlation was observed between phenylacetate (PAA) and Psychrobacter spp. and between phenylpropionate (PPA) and Haloimpatiens spp. Moreover, Syntrophus spp. negatively correlated with PAA (Spearman’s rank correlations coefficient (rs) = − 0.46, p < 0.05) and PPA concentrations (rs = − 0.44, p < 0.05) and was also associated with anaerobic benzene ring cleavage. In thermophilic samples, acetate was predominantly oxidised by Tepidanaerobacter spp. or Syntrophaceticus spp. in syntrophic association with a hydrogenotrophic methanogen. The genera Sedimentibacter and Syntrophaceticus correlated positively with both PAA and PPA concentrations. Moreover, Sedimentibacter spp., Tepidanaerobacter spp., Acetomicrobium spp., and Sporanaerobacter spp. were significant LEfSe (linear discriminant analysis effect size) biomarkers for high meso- as well as thermophilic phenyl acid concentrations. Direct negative effects of phenyl acids on methanogenic properties could not be proven.ConclusionsAnaerobic phenyl acid formation is not restricted to specific microbial taxa, but rather done by various meso- and thermophilic bacteria. The cleavage of the highly inert benzene ring is possible in methanogenic batch reactors—at least in mesophilic fermentation processes. The results indicated that phenyl acids rather affect microorganisms engaged in preceding degradation steps than the ones involved in methanogenesis.
Highlights
Proteinaceous wastes exhibit high theoretical methane yields and their residues are considered valuable fertilisers
Thereafter, 659 Operational taxonomic unit (OTU) and 282 OTUs remained for further analyses in mesophilic and thermophilic samples, respectively
The relative abundance of Candidatus Caldatribacterium and the PAA concentration [15] were highest in medium load Phe samples, which indicates that this microorganism was directly or at least indirectly involved in the conversion of phenylalanine to phenylacetate
Summary
Proteinaceous wastes exhibit high theoretical methane yields and their residues are considered valuable fertilisers. The routine anaerobic degradation of proteins often raises problems like high aromatic compound concentrations caused by the entry of aromatic amino acids into the system. A profound investigation of the consequences of aromatic compound exposure on various microorganisms, which cascade-like and interdependently degrade complex molecules to biogas, is still pending. An increased use of waste products can be challenging due to undesirable compounds entering biogas plants [4,5,6,7]. Protein-rich waste products like slaughterhouse waste, thin stillage, or pig manure have a high theoretical methane yield [8,9,10] and the resulting residues are considered desirable fertilisers [11]. Free ammonia is toxic to acetoclastic methanogens; syntrophic acetate oxidation (SAO) combined with hydrogenotrophic methanogenesis is a common pathway in ammonia-rich anaerobic reactors [8,9,10]
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