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A system capable of handling a large volumetric gas fraction while providing a high gas to liquid mass transfer is a necessity if the metanotrophic bacterium Methylococcus capsulatus is to be used in single cell protein (SCP) production. In this study, mixing time and mass transfer coefficients were determined in a 0.15 m 3 forced flow U-loop fermenter of a novel construction. The effect on the impeller drawn power when a gas was introduced into the system was also studied. Mixing time decreased and mass transfer increased with increasing volumetric liquid flow rate and specific power input. This happened also for a large volume fraction of the gas, which was shown to have only minor effect on the power drawn from the pump impeller. Very large mass transfer coefficients, considerably higher than those obtainable in an STR and previous tubular loop reactors, could be achieved in the U-loop fermenter equipped with static mixers at modest volumetric liquid and gas flow rates. Biotechnol. Bioeng. 2017;114: 344-354. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

Structural and functional characteristics of the regular glycoprotein layers in prokaryotes are analyzed with a special emphasis on aerobic methanotrophic bacteria. S-layers are present at the surfaces of Methylococcus, Methylothermus, and Methylomicrobium cells. Different Methylomicrobium species either synthesize S-layers with planar (p2, p4) symmetry or form cup-shaped or conicalstructures with hexagonal (p6) symmetry. A unique, copper-binding polypeptide 'CorA'/MopE (27/45 kDa), which is coexpressed with the diheme periplasmic cytochrome c peroxidase 'CorB'/Mca (80 kDa) was found in Methylomicrobium album BG8, Methylomicrobium alcaliphilum 20Z, and Methylococcus capsulatus Bath. This tandem of the surface proteins is functionally analogous to a new siderophore, methanobactin. Importantly, no 'CorA'/MopE homologue was found in methanotrophs not forming S-layers. The role of surface proteins in copper metabolism and initial methane oxidation is discussed.

Methane-oxidizing bacteria are ubiquitous in the environment and are globally important in oxidizing the potent greenhouse gas methane. It is also well recognized that they have wide potential for bioremediation of organic and chlorinated organic pollutants, thanks to the wide substrate ranges of the methane monooxygenase enzymes that they produce. Here we have demonstrated that the well characterized model methanotroph Methylococcus capsulatus (Bath) is able to bioremediate chromium(VI) pollution over a wide range of concentrations (1.4-1000 mg L(-1) of Cr(6+)), thus extending the bioremediation potential of this major group of microorganisms to include an important heavy-metal pollutant. The chromium(VI) reduction reaction was dependent on the availability of reducing equivalents from the growth substrate methane and was partially inhibited by the metabolic poison sodium azide. X-ray spectroscopy showed that the cell-associated chromium was predominantly in the +3 oxidation state and associated with cell- or medium-derived moieties that were most likely phosphate groups. The genome sequence of Mc. capsulatus (Bath) suggests at least five candidate genes for the chromium(VI) reductase activity in this organism.

Proteins can coordinate metal ions with endogenous nitrogen and oxygen ligands through backbone amino and carbonyl groups, but the amino acid side chains coordinating metals do not include tryptophan. Here we show for the first time the involvement of the tryptophan metabolite kynurenine in a protein metal-binding site. The crystal structure to 1.35 angstroms of MopE* from the methane-oxidizing Methylococcus capsulatus (Bath) provided detailed information about its structure and mononuclear copper-binding site. MopE* contains a novel protein fold of which only one-third of the structure displays similarities to other known folds. The geometry around the copper ion is distorted tetrahedral with one oxygen ligand from a water molecule, two histidine imidazoles (His-132 and His-203), and at the fourth distorted tetrahedral position, the N1 atom of the kynurenine, an oxidation product of Trp-130. Trp-130 was not oxidized to kynurenine in MopE* heterologously expressed in Escherichia coli, nor did this protein bind copper. Our findings indicate that the modification of tryptophan to kynurenine and its involvement in copper binding is an innate property of M. capsulatus MopE*.

Bacteria that produce exopolysaccharides (EPS) and use methane as the only source of carbon were selected by studying a collection of methanotroph strains: Methylococcus capsulatus E 494, 874, and 3009; M. thermophilus 111p, 112p, and 119p; Methylobacter ucrainicus 159 and 161; M. luteus 57v and 12b; Methylobacter sp. 100; Methylomonas rubra 15 sh and SK-32; Methylosinus trichosporium OV3b, OV5b and 4e; M. sporium 5, 12, A20d, and 90v; and Methylocystis parvus OVVP. Mesophilic methanotroph strains with the ribulose monophosphate way of C1-compound assimilation synthesized EPS more actively than bacteria operating the serine cycle. The dynamics of EPS synthesis by methanotrophs during chemostat cultivation was studied.