INTERNATIONAL JOURNAL OF AGRICULTURE & BIOLOGY ISSN Print: 1560–8530; ISSN Online: 1814–9596 12–315/MFA/2012/14–5–843–850 http://www.fspublishers.org Review Article Syntrophic Propionate Degradation in Anaerobic Digestion: A Review JIANZHENG LI1, QIAOYING BAN, LIGUO ZHANG AND AJAY KUMAR JHA State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China 1Corresponding author’s e-mail: [email protected] ABSTRACT Degradation of propionate is a central issue for improving performance of an anaerobic digestion system, because propionate is an intermediate product, which is generally accumulated in anaerobic digesters. Many factors containing pH, temperature, volatile fatty acids, hydrogen partial pressure, and toxins would inhibit the biodegradation of propionate under anaerobic conditions. Propionate can be oxidized only if a syntrophic association is carried out by propionate-oxidizing bacteria and hydrogen-consumming bacteria. As propionate degraders, syntrophic propionate-oxidizing bacteria (SPOB) play an important role in anaerobic food chain and anaerobic global carbon cycles. However, these microorganisms are often difficult to be isolated and cultured as result of extremely fastidious metabolism type. To date, only ten species were identified as SPOB, which belonged to class Deltaproteobacteria or Firmicutes. Most identified SPOB degrade propionate through methylmalonyl coenzyme A pathway (MMC) pathway and genomic information indicated the catabolic pathways are controlled by ecological factors and/or global cellular conditions. Further investigation is needed on the response mechanism of SPOB to the environmental factors. To prevent anaerobic digestor from propionate accumulation, it is important to detect SPOB in time and forecast the accumulation of propionate with mathematical models. © 2012 Friends Science Publishers Key Words: Anaerobic digestion; Propionate degradation; Syntrophic propionate-oxidizing bacteria; Ecophysiology; Phylogeny INTRODUCTION (Kaspar & Wuhrmann, 1978; Van Lier et al., 1996; Shah et al., 2009; Ghasimi et al., 2009; Liu et al., 2010). Anaerobic Anaerobic digestion technology was paid attention by oxidation of propionate is affected by many factors, more researchers, because of the removal of organic including pH, temperature, hydrogen partial pressure, pollutants in solid waste and wastewater and simultaneously volatile fatty acids (VFAs), reactor configuration and produces methane as an energy resource (Wang et al., 2004; organic compounds (Kim et al., 2002; Dhaked et al., 2003; Mahmood et al., 2011). It is complex with a number of Siegert & Banks, 2005; Liu et al., 2006). sequential and parallel steps that are carried out by mainly The biodegradation of propionate depends on four groups of microorganisms including primary syntrophy between propionate-oxidizing bacteria and fermenting bacteria, syntrophic acetogens, homoacetogens hydrogenotrophic methanogenic archaea in methanogenic and methanogenic archaea (Kosaka et al., 2006; Jha et al., environments (Worm et al., 2011). The syntrophic 2011). Each group of microbes have specific metabolic metabolism with other bacteria result in the pure cultivation functions (Fig. 1). Propionate is an important intermediate of syntrophic propionate-oxidizing bacteria (SPOB) is very during anaerobic digestion of organic polymers. Its difficult. To date, only ten SPOB species have been isolated degradation into acetate and H2/CO2 (and then to CH4) and identified (Table II). The analysis of propionate- accounts for 6% ~ 35% in the total methanogenesis oxidizing pathways provides information about these (Glissmann & Conrad, 2000). However, the oxidation of microorganisms in depth, including intermediates, enzymes propionate is energetically unfavorable with a standard and energy conservation. Furthermore, the genomes of change in Gibbs free energy (ΔG°′) of +76 kJ per mol several representative bacteria present the prospects to reaction as illustrated in Table I (Müller et al., 2010). assess their unexplored functions (McInerney et al., 2007; Thermodynamically, propionate is more difficult to be Kosaka et al., 2008). anaerobically oxidized than butyrate, lactate and ethanol. So, Propionate is always produced in anaerobic digestion it is usually accumulated in an anaerobic digester, even and oxidized only if a syntrophic association is carried out resulting in failure of the anaerobic digestion process by propionate-oxidizing bacteria and hydrogen- To cite this paper: Li, J., Q. Ban, L. Zhang and Jha, A.K. 2012. Syntrophic propionate degradation in anaerobic digestion: a review. Int. J. Agric. Biol., 14: 843–850 LI et al. / Int. J. Agric. Biol., Vol. 14, No. 5, 2012 consumming bacteria (Chen et al., 2005; de Bok et al., 2005; Table I: Reactions involved in syntrophic propionate Ghasimi et al., 2009). It is accumulated in anaerobic metabolism digestion process whenever, there is a change in organic loading rate, influent pH and temperature, etc., because the Reaction ΔG°′ syntrophic propionate-oxidizing is more difficult than other (kJ mol-1) Proton-reducing bacteria intermediate products such as butyrate and ethanol (Müller – – Propionate + 2 H2O → acetate + CO2 + 3 H2 +76.0 et al., 2010). Thus, degradation of propionate is very - + Propionate- + 2 H2O + 2 CO2 →Acetate- + 3 HCOO + 3 H +65.3 important for improving performance of anaerobic digestion Methanogens process. However, only a few species have been obtained by 4 H2 + CO2 → CH4 + 2 H2O –131.7 - + 4 HCOO + 4 H → CH4 + 3 CO2 + 2 H2O –144.5 now, leading to a lack in deep understanding of syntrophic – Acetate + H+ → CO2 + CH4 –36.0 propionate degradation. Based on the analysis of effect of ΔG°′ (Standard Gibbs free energy change): concentration of 1 M, pH 7.0 propionate accumulation on the performance of anaerobic and T = 25°C (Müller et al., 2010) digestion, main factors affecting its biodegradation were discussed in this work. The currently isolated SPOB and Fig. 1: Anaerobic food chain of the conversion of their ecophysiological, phylogenetic and genomics were organic matter to methane. The major microbial focused on as well. This summary should help to isolate groups catalyzing the reactions are in oval. SPOB: more SPOB and reveal the biochemical mechanism in Syntrophic propionate-oxidizing bacteria, SBOB: propionate oxidation process. syntrophic butyrate-oxidizing bacteria (Modified from Effect of propionate accunulation on anaerobic digestion: Liu and Whitman, 2008) Under high operational performance, the conversion rate is proportional to the generation rate of the intermediate Organic polymers products, so these compounds are hardly accumulated. However, overloading, toxicity and process parameters fluctuation disturb the process and consequently cause Heterotrophic anaerobes process instable. The process imbalance generally results in accumulation of VFAs including propionate (Pullammanppallil et al., 2001). The degradation of Monomers or oligomers propionate into acetate is considered as one rate limiting step in anaerobic digestion system (Amani et al., 2011a). Furthermore, its high concentration (> 3000 mg L-1) may Heterotrophic anaerobes cease the fermentation process (Boone & Xun, 1987). An increase of propionate has been observed before the failure Acetogens of anaerobic digesters in treating swine, municipal sludge, Short fatty acids, Alcohols and food processing water (Kaspar & Wuhrmann, 1978). Several researchers presented that propionate Syntrophs (SPOB, SBOB) accumulation affect growth, diversity and activity of the methanogens. One previous research suggested that a lot of methanogens was influenced when the propionate -1 concentration is as low as 1500-2220 mg L and 2 orders of H +CO magnitude of methanogens were decreased at the 2 2 Homoacetogenic Formate Acetate concentration of propionate above 5920 mg L-1 (Barredo & bacteria Evison, 1991). Another research also indicated that methane Aceticlastic generation was decreased by 62~78% compared with the Hydrogentrophic -1 methanogens control at neutral pH when propionate was 5000 mg L methanogens (Hajarnis & Ranade, 1994). The inhibition extent CH +CO dramatically increased when the pH was decreased, 4 2 indicating that undissociated propionate is the most toxic. In addition, Dhaked et al. (2003) found a two-log reduction in microbial growth and metabolism. Dhaked et al. (2003) methanogenic counts in the slurry fermentation at pH 6.0 or reported that the propionate anaerobic conversion is much 7.0 after propionate of 15000 mg L-1 was added into slurry. faster at neutral or weak alkaline pH (7 to 8) than at weak They also found that methane content of biogas decreased acid (pH 6). Pure culture of SPOB or co-culture of SPOB with the increase in propionate concentration (Dhaked et al., and methanogens grow in a neutral environment with the 2003). pH ranged from 6 to 8.8 (Table II). We found that Factors Affecting Syntrophic Propionate Degradation propionate degradation was much faster at pH 7.0 to 8.5 pH: The pH is the most important operational parameter for than that of pH 6.5 or below; the propionate anaerobic anaerobic digestion processes. It has direct influence on oxidation hardly occurred at pH 5.5 (unpublished data). 844 SYNTROPHIC DEGRADATION OF PROPIONATE / Int. J. Agric. Biol., Vol. 14, No. 5, 2012 Table II: Characteristics of syntrophic propionate-oxidizing bacteria Organism Cell morphology pH Tempe G+C Substrates Reference