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Thermodynamic characterization of proton‐ionizable functional groups on the cell surfaces of ammonia‐oxidizing bacteria and archaea
Author(s) -
GormanLewis D.,
MartensHabbena W.,
Stahl D. A.
Publication year - 2014
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12075
Subject(s) - archaea , chemistry , protonation , cell envelope , inorganic chemistry , ammonia , organic chemistry , biochemistry , ion , escherichia coli , gene
The ammonia‐oxidizing archaeon N itrosopumilus maritimus strain SCM 1 (strain SCM 1), a representative of the T haumarchaeota archaeal phylum, can sustain high specific rates of ammonia oxidation at ammonia concentrations too low to sustain metabolism by ammonia‐oxidizing bacteria ( AOB ). One structural and biochemical difference between N . maritimus and AOB that might be related to the oligotrophic adaptation of strain SCM 1 is the cell surface. A proteinaceous surface layer (S‐layer) comprises the outermost boundary of the strain SCM1 cell envelope, as opposed to the lipopolysaccharide coat of Gram‐negative AOB . In this work, we compared the surface reactivities of two archaea having an S‐layer (strain SCM 1 and S ulfolobus acidocaldarius ) with those of four representative AOB ( N itrosospira briensis , N itrosomonas europaea , N itrosolobus multiformis , and N itrosococcus oceani ) using potentiometric and calorimetric titrations to evaluate differences in proton‐ionizable surface sites. Strain SCM 1 and S . acidocaldarius have a wider range of proton buffering (approximately pH 10–3.5) than the AOB (approximately pH 10–4), under the conditions investigated. Thermodynamic parameters describing proton‐ionizable sites (acidity constants, enthalpies, and entropies of protonation) are consistent with these archaea having proton‐ionizable amino acid side chains containing carboxyl, imidazole, thiol, hydroxyl, and amine functional groups. Phosphorous‐bearing acidic functional groups, which might also be present, could be masked by imidazole and thiol functional groups. Parameters for the AOB are consistent with surface structures containing anionic oxygen ligands (carboxyl‐ and phosphorous‐bearing acidic functional groups), thiols, and amines. In addition, our results showed that strain SCM 1 has more reactive surface sites than the AOB and a high concentration of sites consistent with aspartic and/or glutamic acid. Because these alternative boundary layers mediate interaction with the local external environment, these data provide the basis for further comparisons of the thermodynamic behavior of surface reactivity toward essential nutrients.