Abstract
BackgroundLignin intermediates resulting from lignocellulose degradation have been suspected to hinder anaerobic mineralisation of organic materials to biogas. Phenyl acids like phenylacetate (PAA) are early detectable intermediates during anaerobic digestion (AD) of aromatic compounds. Studying the phenyl acid formation dynamics and concomitant microbial community shifts can help to understand the microbial interdependencies during AD of aromatic compounds and may be beneficial to counteract disturbances.ResultsThe length of the aliphatic side chain and chemical structure of the benzene side group(s) had an influence on the methanogenic system. PAA, phenylpropionate (PPA), and phenylbutyrate (PBA) accumulations showed that the respective lignin intermediate was degraded but that there were metabolic restrictions as the phenyl acids were not effectively processed. Metagenomic analyses confirmed that mesophilic genera like Fastidiosipila or Syntrophomonas and thermophilic genera like Lactobacillus, Bacillus, Geobacillus, and Tissierella are associated with phenyl acid formation. Acetoclastic methanogenesis was prevalent in mesophilic samples at low and medium overload conditions, whereas Methanoculleus spp. dominated at high overload conditions when methane production was restricted. In medium carbon load reactors under thermophilic conditions, syntrophic acetate oxidation (SAO)-induced hydrogenotrophic methanogenesis was the most important process despite the fact that acetoclastic methanogenesis would thermodynamically be more favourable. As acetoclastic methanogens were restricted at medium and high overload conditions, syntrophic acetate oxidising bacteria and their hydrogenotrophic partners could step in for acetate consumption.ConclusionsPAA, PPA, and PBA were early indicators for upcoming process failures. Acetoclastic methanogens were one of the first microorganisms to be impaired by aromatic compounds, and shifts to syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis occurred in thermophilic reactors. Previously assumed associations of specific meso- and thermophilic genera with anaerobic phenyl acid formation could be confirmed.
Highlights
Lignin intermediates resulting from lignocellulose degradation have been suspected to hinder anaerobic mineralisation of organic materials to biogas
With regard to the mesophilic controls, which produced up to 85 NmL methane within 28 days, no significant differences in biogas production could be observed when the lignin intermediates were added under low carbon load (LCL) conditions
The highest cumulative methane production after 28 days could be observed in syringic acid samples under medium carbon load (MCL) conditions and in gallic acid MCL samples (130 ± 7.08 and 111 ± 3.40 NmL CH4 cum, respectively), followed by syringic acid LCL and gallic acid LCL samples (98.1 ± 4.38 and 95.6 ± 6.41 NmL C H4 cum, respectively)
Summary
Lignin intermediates resulting from lignocellulose degradation have been suspected to hinder anaerobic mineralisation of organic materials to biogas. Phenyl acids like phenylacetate (PAA) are early detectable intermediates during anaerobic digestion (AD) of aromatic compounds. The fact that methane as end product of anaerobic digestion (AD) can still be used for energy exploitation shows that degradation processes under anoxic conditions [2, 3] yield far less energy for the microorganisms. A variety of anaerobic microorganisms depend on each other to overcome thermodynamic restrictions [4]. These microbial interdependencies, especially the obligatory mutualistic (syntrophic) co-operations, are still not sufficiently understood despite their significance for maintaining anaerobic systems [5]. A low methanogenic activity can restrict upstream degradation steps as metabolic intermediates accumulate and reactions become endergonic [5]
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