Methanogenesis and the K+ transport system are activated by divalent cations in ammonia-treated cells of Methanospirillum hungatei

Thermodynamic restrictions determine ammonia tolerance of methanogenic pathways in Methanosarcina barkeri

Ammonia is a ubiquitous potential inhibitor of anaerobic digestion processes, mainly exhibiting inhibition towards methanogenic activity. However, knowledge as to how ammonia affects the methanogens is still limited. In this study, we cultured a multitrophic methanogen, Methanosarcina barkeri DSM 800, with acetate, H₂/CO₂, and methanol to evaluate the influence of ammonia on different methanogenic pathways. Aceticlastic methanogenesis was more sensitive to increased ammonia concentrations than hydrogenotrophic and methylotrophic methanogenesis. Theoretical maximum NH₃ tolerances of M. barkeri fed with acetate, H₂/CO₂, and methanol were calculated to be 39.1 ± 9.0, 104.3 ± 7.4, and 85.7 ± 1.0 mg/L, respectively. The order of the ΔG range of M. barkeri under three methanogenic pathways reflected the order of ammonia tolerance of M. barkeri. Our results provide insights into the role of the thermodynamic potential of methanogenesis on the tolerance of ammonia stress; and shed light on the mechanism of ammonia inhibition on anaerobic digestion.

Effect of water regime on the dynamics of free ammonia during high solid anaerobic digestion of pig manure

Free ammonia (FAN) inhibition is commonly encountered during high solid anaerobic digestion (HSAD) of pig manure. The performance of HSAD is highly related to its operational water regime; however, little information is available regarding the dynamics of free ammonia with varied water regimes. In this work, four treatments were set with equal amount of water supply but varied addition frequencies, i.e. add once but at different times in treatments T1 and T2, add twice in T3 while it was three times in treatment T4. Results showed that the whole methanogenic process ran smoothly with the average methane gas production rate maintaining at 191.1 LCH₄/kgVS_added. Although a higher methane gas production rate of 217.4 LCH₄/kgVS_added was achieved in T1, one time water addition triggered a higher ammonia inhibition potential. Cumulative FAN release was 6.03 mgFAN/kgVS_added in T1 while the balance between FAN and ammonia tended to the fraction of FAN. In T4, cumulative FAN of 5.07 mgFAN/kgVS_added was evolved, which was lower than that in T1 but similar to the situation in T2. The lowest FAN was observed in T3, indicating that a moderate frequency of dilution might be conducive to alleviate free ammonia inhibition.

Low-affinity potassium uptake system in the archaeon Methanobacterium thermoautotrophicum: overproduction of a 31-kilodalton membrane protein during growth on low-potassium medium

During growth on low-K+ medium (1 mM K+), Methanobacterium thermoautotrophicum accumulated K+ up to concentration gradients ([K+]intracellular/[K+]extracellular) of 25,000- to 50,000-fold. At these gradients ([K+]extracellular of < 20 microM), growth ceased but could be reinitiated by the addition of K+ or Rb+. During K+ starvation, the levels of a protein with an apparent molecular weight of 31,000 increased about sixfold. The protein was associated with the membrane and could be extracted by detergents. Cell suspensions of M. thermoautotrophicum obtained after K+-limited growth catalyzed the transport of both K+ and Rb+ with apparent Km and Vmax values of 0.13 mM and 140 nmol/min/mg, respectively, for K+ and 3.4 mM and 140 nmol/min/mg, respectively, for Rb+. Rb+ competitively inhibited K+ uptake with an inhibitor constant of about 10 mM. Membranes of K+-starved cells did not exhibit K+-stimulated ATPase activity. Immunoblotting with antisera against Escherichia coli Kdp-ATPase did not reveal any specific cross-reactivity against membrane proteins of K+-starved cells. Cells of M. thermoautotrophicum grown at a high potassium concentration (50 mM) catalyzed K+ and Rb+ transport at similar apparent Km values (0.13 mM for K+ and 3.3 mM for Rb+) but at significantly lower apparent Vmax values (about 60 nmol/min/mg for both K+ and Rb+) compared with K+-starved cells. From these data, it is concluded that the archaeon M. thermoautotrophicum contains a low-affinity K+ uptake system which is overproduced during growth on low-K+ medium.

Alanine as an end product during fermentation of monosaccharides by Clostridium strain P2

The thermophilic Clostridium P2 was isolated from a semi-continuously fed reactor with high ammonium concentration. This bacterium formed substantial amounts of L-alanine as a major fermentation product from glucose, fructose and mannose. Low amounts of acetate, butyrate, carbon dioxide and hydrogen were also formed. A high partial pressure of hydrogen inhibited the degradation of the monosaccharides, whereas hydrogen removal, in the form of methanogenesis was found to be stimulatory. However, the amount of alanine produced per mole of hexose degraded did not change. Hexose degradation and alanine production were favoured by high ammonium concentrations. Nuclear magnetic resonance spectroscopy studies provided strong evidence that an active Embden-Meyerhof-Parnas pathway existed and that alanine was produced via an amination of pyruvate.

Methanogenesis and ATP synthesis in methanogenic bacteria at low electrochemical proton potentials. An explanation for the apparent uncoupler insensitivity of ATP synthesis

The rate of methane formation from H2 and CO2, the intracellular ATP content and the electrochemical proton potential (delta mu H+) were determined in cell suspensions of Methanobacterium thermoautotrophicum, which were permeabilized for K+ with valinomycin (1.2 mumol/mg protein). In the absence of extracellular K+ the cells formed methane at a rate of 4 mumol min-1 (mg protein)-1, the intracellular ATP content was 20 nmol/mg protein and the delta mu H+ was 200 mV (inside negative). When K+ was added to the suspensions the measured delta mu H+ decreased to the value calculated from the [K+]in/[K+]out ratio. Using this method of delta mu H+ adjustment, it was found that lowering delta mu H+ from 200 mV ([K+]in/[K+]out = 1000) to 100 mV ([K+]in/[K+]out = 40) had no effect on the rate of methane formation and on the intracellular ATP content. At delta mu H+ values below 100 mV ([K+]in/[K+]out less than 40) both the rate of methanogenesis and the ATP content decreased. Methanogenesis completely ceased and the ATP content was 2 nmol/mg when delta mu H+ was adjusted to values lower 50 mV ([K+]in/[K+]out less than 7). The data show that methanogenesis from H2 and CO2 and ATP synthesis in M. thermoautotrophicum are possible at relatively low electrochemical proton potentials. Similar results were obtained with Methanosarcina barkeri. Protonophoric uncouplers like 3,5,3',4'-tetrachlorosalicylanilide (TCS) or 3,5-di-tert-butyl-4-hydroxy-benzylidenemalononitrile (SF 6847) were found not to dissipate delta mu H+ below 100 mV in M. thermoautotrophicum even when used at high concentrations (400 nmol/mg protein). This finding explains the observed uncoupler insensitivity of methanogenesis and ATP synthesis in this organism.