Publicação
Microbial interactions in the transformation of long-chain fatty acids in anaerobic reactors
| Resumo: | Conversion of long-chain fatty acids (LCFA) to methane in continuous bioreactors is not fully understood, in particular for unsaturated LCFA. For example, palmitate (C16:0, saturated LCFA) often accumulates during oleate (C18:1, unsaturated LCFA) biodegradation in anaerobic bioreactors, and the reason why this happens and which microorganisms catalyze this reaction remained unknown. Facultative anaerobic bacteria are frequently found in reactors operated at high loads of LCFA, but their function is also unclear. In this thesis, oleate degradation in continuous bioreactors was studied to get further insights on the microbial interactions occurring in the transformation of LCFA. For that propose we compared bioreactors treating oleate-based wastewater (organic loading rates of 1 and 3 kg COD m-3 d-1), operated under different redox conditions (strictly anaerobic-AnR, -350 mV; microaerophilic-MaR, - 250 mV) (Chapter 3). For the higher load tested, palmitate accumulated 7 times more in the MaR, where facultative anaerobes were more abundant, and only the biomass from this reactor could recover the methanogenic activity, after a transient inhibition. In other experiment, two bioreactors were operated separately and the abundance of facultative anaerobic bacteria, namely Pseudomonas spp. (from which two strains were isolated), was strongly correlated (p<0.05) with palmitate-to-total LCFA percentage in the biofilm formed in a continuous plug flow reactor fed with very high loads of oleate (Chapter 4). The Pseudomonas isolates were further tested in batch and continuous reactors together with a syntrophic LCFA-degrading co-culture of Syntrophomonas zehnderi and Methanobacterium formicicum (Chapter 5). The results obtained show that facultative bacteria play an important role in anaerobic reactors treating oleate-based wastewaters, protecting the strict anaerobes from oxygen toxicity and decreasing the oleate toxicity towards the methanogens helping to improve oleate degradation. An additional study was performed to get insight into the production of biosurfactants and biopolymers by the Pseudomonas isolates and the anaerobic granular sludge during oleate degradation in continuous bioreactors (Chapter 6). The results indicate that possibly glycolipids and hydroxyalkanoates may have been produced, however it was not possible to identify these compounds. In conclusion, this thesis showed that the oxidation-reduction potential (ORP) and therefore the presence of facultative bacteria, regulate oleate to palmitate conversion, decreasing the LCFA toxicity toward methanogens, ultimately accelerating the methane production from lipid-rich wastewaters. |
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| Autores principais: | Duarte, Maria Salomé Lira |
| Assunto: | Engenharia e Tecnologia::Biotecnologia Ambiental |
| Ano: | 2018 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Conversion of long-chain fatty acids (LCFA) to methane in continuous bioreactors is not fully understood, in particular for unsaturated LCFA. For example, palmitate (C16:0, saturated LCFA) often accumulates during oleate (C18:1, unsaturated LCFA) biodegradation in anaerobic bioreactors, and the reason why this happens and which microorganisms catalyze this reaction remained unknown. Facultative anaerobic bacteria are frequently found in reactors operated at high loads of LCFA, but their function is also unclear. In this thesis, oleate degradation in continuous bioreactors was studied to get further insights on the microbial interactions occurring in the transformation of LCFA. For that propose we compared bioreactors treating oleate-based wastewater (organic loading rates of 1 and 3 kg COD m-3 d-1), operated under different redox conditions (strictly anaerobic-AnR, -350 mV; microaerophilic-MaR, - 250 mV) (Chapter 3). For the higher load tested, palmitate accumulated 7 times more in the MaR, where facultative anaerobes were more abundant, and only the biomass from this reactor could recover the methanogenic activity, after a transient inhibition. In other experiment, two bioreactors were operated separately and the abundance of facultative anaerobic bacteria, namely Pseudomonas spp. (from which two strains were isolated), was strongly correlated (p<0.05) with palmitate-to-total LCFA percentage in the biofilm formed in a continuous plug flow reactor fed with very high loads of oleate (Chapter 4). The Pseudomonas isolates were further tested in batch and continuous reactors together with a syntrophic LCFA-degrading co-culture of Syntrophomonas zehnderi and Methanobacterium formicicum (Chapter 5). The results obtained show that facultative bacteria play an important role in anaerobic reactors treating oleate-based wastewaters, protecting the strict anaerobes from oxygen toxicity and decreasing the oleate toxicity towards the methanogens helping to improve oleate degradation. An additional study was performed to get insight into the production of biosurfactants and biopolymers by the Pseudomonas isolates and the anaerobic granular sludge during oleate degradation in continuous bioreactors (Chapter 6). The results indicate that possibly glycolipids and hydroxyalkanoates may have been produced, however it was not possible to identify these compounds. In conclusion, this thesis showed that the oxidation-reduction potential (ORP) and therefore the presence of facultative bacteria, regulate oleate to palmitate conversion, decreasing the LCFA toxicity toward methanogens, ultimately accelerating the methane production from lipid-rich wastewaters. |
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