Effect of Sub-Stoichiometric Fe(III) Amounts on LCFA Degradation by Methanogenic Communities.

Ana J Cavaleiro,Sérgio A Silva,João C Sequeira,Alfons J M Stams,M Madalena Alves,Ana P Guedes,Diana Z Sousa,Ana L Arantes,Andreia F Salvador

doi:10.3390/microorganisms8091375

Ana J Cavaleiro, Sérgio A Silva + Show 7 more

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https://doi.org/10.3390/microorganisms8091375

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Abstract

Long-chain fatty acids (LCFA) are common contaminants in municipal and industrial wastewater that can be converted anaerobically to methane. A low hydrogen partial pressure is required for LCFA degradation by anaerobic bacteria, requiring the establishment of syntrophic relationships with hydrogenotrophic methanogens. However, high LCFA loads can inhibit methanogens, hindering biodegradation. Because it has been suggested that anaerobic degradation of these compounds may be enhanced by the presence of alternative electron acceptors, such as iron, we investigated the effect of sub-stoichiometric amounts of Fe(III) on oleate (C18:1 LCFA) degradation by suspended and granular methanogenic sludge. Fe(III) accelerated oleate biodegradation and hydrogenotrophic methanogenesis in the assays with suspended sludge, with H2-consuming methanogens coexisting with iron-reducing bacteria. On the other hand, acetoclastic methanogenesis was delayed by Fe(III). These effects were less evident with granular sludge, possibly due to its higher initial methanogenic activity relative to suspended sludge. Enrichments with close-to-stoichiometric amounts of Fe(III) resulted in a microbial community mainly composed of Geobacter, Syntrophomonas, and Methanobacterium genera, with relative abundances of 83–89%, 3–6%, and 0.2–10%, respectively. In these enrichments, oleate was biodegraded to acetate and coupled to iron-reduction and methane production, revealing novel microbial interactions between syntrophic LCFA-degrading bacteria, iron-reducing bacteria, and methanogens.

Highlights

Lipids and long-chain fatty acids (LCFA) are frequently present as contaminants in municipal and industrial wastewater [1]
At the beginning of the incubations, Fe(II) concentrations around 4 mmol L−1 were measured in all the bottles, and total iron concentrations of 105 ± 5 and 115 ± 14 mmol L−1 were quantified in the assays with suspended and granular sludge, respectively
Oleate biodegradation was verified in all the conditions studied, i.e., initial oleate concentrations decreased to undetectable levels in the assays inoculated with suspended and granular sludge, both in the presence and absence of Fe(III) (Figure 2)

Summary

Introduction

Lipids and long-chain fatty acids (LCFA) are frequently present as contaminants in municipal and industrial wastewater [1]. Due to their hydrophobic character, these compounds are generally associated with sludge flotation and foaming in conventional aerobic wastewater treatment plants [2]. In AD, lipids are hydrolyzed to glycerol and LCFA, which are further converted to acetate and hydrogen/formate by syntrophic bacteria. This reaction is thermodynamically feasible only when hydrogen/formate concentrations are kept low [3,4]. The activity of methanogenic archaea, which act as hydrogen/formate scavengers, is crucial for maintaining low concentrations of these intermediate compounds

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