Influence of pH on Ammonia Accumulation and Toxicity in Halophilic, Methylotrophic Methanogens

Abstract

We studied the effects of pH and ammonia concentration on the growth of three methanogens. These three halophilic, methylotrophic methanogens, Methanolobus bombayensis, Methanolobus taylorii, and Methanohalophilus zhilinaeae, grew at environmental pH ranges that overlapped with each other and spanned the pH range from 7.0 to 9.5. During growth they had reversed membrane pH gradients (⌬pH) at all pH values tested. The ⌬pH was in the range of ؊0.4 to ؊0.9 pH units, with the cytosol being more acidic than the environmental pH. Methanohalophilus zhilinaeae had the most negative ⌬pH (؊0.9 pH units). These negative pH gradients resulted in the accumulation of ammonium (NH 4 ؉), and when grown at the highest external ammonia concentrations that allowed good growth, cells had cytosolic NH 4 ؉ concentrations as high as 180 mM. The high concentrations of cytosolic NH 4 ؉ were accompanied by greater ⌬pH and lower concentrations of the major cytosolic cation K ؉ (compared with cells grown in medium with only 5 mM ammonia). Methanolobus bombayensis and Methanolobus taylorii were more sensitive to total external ammonia at higher external pH values, but the inhibitory concentration of un-ionized ammonia that resulted in a 50% reduction of the growth rate was about 2 to 5 mM, regardless of the pH. This is consistent with growth inhibition by ammonia in other bacteria. However, Methanohalophilus zhilinaeae was more resistant to un-ionized ammonia than any other known organism. It had a 50% inhibitory concentration for un-ionized ammonia of 13 mM at pH 8.5 and 45 mM at pH 9.5. We examined the effects of pH on three ammonia-assimilating activities (glutamine synthetase, glutamate dehydrogenase, and alanine dehydrogenase) in cell lysates and found that the pH ranges were consistent with the observed ranges of intracellular pH.

Reduction of accumulated volatile fatty acids by an acetate-degrading enrichment culture

“…They determined a 50% reduction in the growth rate of acetoclastic methanogens at levels of about 300 mg L À1 free NH 3 . As pH and NH 4 1 /NH 3 levels measured within the present investigation correspond with the data reported in the literature for undisturbed digesters (Kadam & Boone, 1996), these parameters can be excluded from being a likely reason for the accumulation of the VFAs, as shown in Figs 1 and 2.…”

Prevention of microbial hazard on fresh‐cut lettuce through adoption of food safety and hygienic practices by lettuce farmers

“…Organic inputs alone may not meet the nutritional needs of crops because they contain a comparatively less quantity of nutrients compared to inorganic fertilizers, therefore, there is no need to integrate the two forms in order to achieve better crop yields (Frankenberger and Abdelmagid 1985). As a small, uncharged molecule, ammonia can cross the bacterial membrane and cause damage to the cell either by causing rapid alkalization of the cytoplasm or through a decrease in intracellular K + concentration (Kadam and Boone 1996;Park and Diez-Gonzalez 2003). Furthermore, the insignificant correlation can be associated with other sources of contamination as highlighted by Quadrous Rodrigues et al (2014) that seedlings also may be contaminated, especially when they are not treated with chemicals or have not undergone heat treatments before use, as in the case of some investigated organic farms that was studied.…”

Comparative transcriptome analysis of responses of Methanothermobacter thermautotrophicus to different environmental stimuli

“…We used hydrogen peroxide for exposing cells to an oxidative stress. Ammonia sometimes accumulates in anaerobic digesters (Nishio and Nakashimada, 2007) and is known to be toxic to methanogens (Jarrell et al, 1987;Kadam and Boone, 1996). A heat shock gene (dnaK) of M. mazei has been reported to be upregulated in the presence of ammonia (Lange et al, 1997a), although mechanisms of ammonia toxicity have not been well understood yet.…”