bLipids can be anaerobically digested to methane, but methanogens are often considered to be highly sensitive to the long-chain fatty acids (LCFA) deriving from lipids hydrolysis. In this study, the effect of unsaturated (oleate [C 18:1 ]) and saturated (stearate [C 18:0 ] and palmitate [C 16:0 ]) LCFA toward methanogenic archaea was studied in batch enrichments and in pure cultures. Overall, oleate had a more stringent effect on methanogens than saturated LCFA, and the degree of tolerance to LCFA was different among distinct species of methanogens. Methanobacterium formicicum was able to grow in both oleate-and palmitate-degrading enrichments (OM and PM cultures, respectively), whereas Methanospirillum hungatei only survived in a PM culture. The two acetoclastic methanogens tested, Methanosarcina mazei and Methanosaeta concilii, could be detected in both enrichment cultures, with better survival in PM cultures than in OM cultures. Viability tests using live/dead staining further confirmed that exponential growth-phase cultures of M. hungatei are more sensitive to oleate than are M. formicicum cultures; exposure to 0.5 mM oleate damaged 99% ؎ 1% of the cell membranes of M. hungatei and 53% ؎ 10% of the cell membranes of M. formicicum. In terms of methanogenic activity, M. hungatei was inhibited for 50% by 0.3, 0.4, and 1 mM oleate, stearate, and palmitate, respectively. M. formicicum was more resilient, since 1 mM oleate and >4 mM stearate or palmitate was needed to cause 50% inhibition on methanogenic activity.A naerobic degradation of long-chain fatty acids (LCFA) is essential for efficient biogas production from complex lipidcontaining wastewaters (1-3). However, there is still concern regarding the potential toxic effect of LCFA toward methanogenic communities. Toxicity of LCFA is the main reason for insufficient treatment results of LCFA-containing wastewaters (4-8). Studies of methanogenic inhibition in the rumen confirm the toxicity of these compounds. Soliva et al. (9) showed the antimethanogenic potential of saturated fatty acids of medium chain length, specifically myristate (C 14:0 ) and laurate (C 12:0 ). The adverse effect of fatty acids toward methanogens appears to be more pronounced at longer chain lengths and more unsaturated bonds (10, 11). Moreover, methanogens inhibition by fatty acids is a rapid phenomenon with long recuperation time, as shown by Koster (12): a 50% loss of methanogenic activity was observed after exposing methanogenic sludge to 7.5 mM laurate for only 7.5 min.Sensitivity of microorganisms to LCFA seems to be related to their cell wall structure, with Gram-positive bacteria and methanogens being more easily inhibited than Gram-negative bacteria (13). Inhibition of pure cultures of methanogens by fatty acids is reported in literature (11, 14). Nevertheless, methanogens are ubiquitous in anaerobic bioreactors treating LCFA-rich wastewaters, even over prolonged continuous LCFA loading (15,16). Sousa et al. (16) identified three predominant genera of methanogens in LCFA-degradi...

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Background Microbial production of propionate from diluted streams of ethanol (e.g., deriving from syngas fermentation) is a sustainable alternative to the petrochemical production route. Yet, few ethanol-fermenting propionigenic bacteria are known, and understanding of their metabolism is limited. Anaerotignum neopropionicum is a propionate-producing bacterium that uses the acrylate pathway to ferment ethanol and CO2 to propionate and acetate. In this work, we used computational and experimental methods to study the metabolism of A. neopropionicum and, in particular, the pathway for conversion of ethanol into propionate. Results Our work describes iANEO_SB607, the first genome-scale metabolic model (GEM) of A. neopropionicum. The model was built combining the use of automatic tools with an extensive manual curation process, and it was validated with experimental data from this and published studies. The model predicted growth of A. neopropionicum on ethanol, lactate, sugars and amino acids, matching observed phenotypes. In addition, the model was used to implement a dynamic flux balance analysis (dFBA) approach that accurately predicted the fermentation profile of A. neopropionicum during batch growth on ethanol. A systematic analysis of the metabolism of A. neopropionicum combined with model simulations shed light into the mechanism of ethanol fermentation via the acrylate pathway, and revealed the presence of the electron-transferring complexes NADH-dependent reduced ferredoxin:NADP+ oxidoreductase (Nfn) and acryloyl-CoA reductase-EtfAB, identified for the first time in this bacterium. Conclusions The realisation of the GEM iANEO_SB607 is a stepping stone towards the understanding of the metabolism of the propionate-producer A. neopropionicum. With it, we have gained insight into the functioning of the acrylate pathway and energetic aspects of the cell, with focus on the fermentation of ethanol. Overall, this study provides a basis to further exploit the potential of propionigenic bacteria as microbial cell factories.

Acetate is the most important precursor for methane in the degradation of organic matter. Only two genera of methanogenic archaea, Methanosarcina and Methanothrix (former Methanosaeta), are able to grow with acetate as sole energy and carbon source. Phylogenetically, Methanosarcina and Methanothrix both belong to the Methanosarcinales. These two genera show besides morphological differences, interesting differences in physiology. Methanosarcina is a generalist that can grow on a variety of substrates, while Methanothrix