Cultivation ofMethylosinus trichosporiumOB3b: III. Production of particulate methane monooxygenase in continuous culture

Methane Monooxygenase Gene Transcripts as Quantitative Biomarkers of Methanotrophic Activity in Methylosinus trichosporium OB3b

Methanotrophic microorganisms are characterized by their ability to oxidize methane. Globally they have a significant impact on methane emissions by attenuating net methane fluxes to the atmosphere in natural and engineered systems, though the populations are dynamic in their activity level in soils and waters. Methanotrophs oxidize methane using methane monooxygenase (MMO) enzymes, and selected subunit genes of the most common MMOs, specifically pmoA and mmoX, are used as biomarkers for the presence and abundance of populations of bacterial methanotrophs. The relative expression of these biomarker genes is dependent on copper-to-biomass ratios. Empirically derived quantitative relationships between methane oxidation biomarker transcript amounts and methanotrophic activity could facilitate determination of methane oxidation rates. In this study, pure cultures of a model type II methanotroph, Methylosinus trichosporium OB3b, were grown in hollow-fiber membrane bioreactors (HFMBR) under different steady-state methane oxidation conditions. Methanotroph biomass (DNA based) and methane oxidation biomarker mRNA transcript amounts were determined using quantitative PCR (qPCR) and reverse transcription-PCR (RT-qPCR), respectively. Under both copper-present and copper-limited conditions, per-cell pmoA mRNA transcript levels positively correlated with measured per-cell methane oxidation rates across 3 orders of magnitude. These correlations, if maintained across different methanotrophs, could prove valuable for inferring in situ oxidation rates of methanotrophs and understanding the dynamics of their impact on net methane emissions.IMPORTANCE Methanotrophs are naturally occurring microorganisms capable of oxidizing methane and have an impact on global net methane emissions. The genes pmoA and mmoX are used as biomarkers for bacterial methanotrophs. Quantitative relationships between transcript amounts of these genes and methane oxidation rates could facilitate estimation of methanotrophic activity. In this study, a strong correlation was observed between per-cell pmoA transcript levels and per-cell methane oxidation rates for pure cultures of the aerobic methanotroph M. trichosporium OB3b grown in bioreactors. If similar relationships exist across different methanotrophs, they could prove valuable for inferring in situ oxidation rates of methanotrophs and better understanding their impact on net methane emissions.

Methanotrophic production of polyhydroxybutyrate-co-hydroxyvalerate with high hydroxyvalerate content

Type II methanotrophic bacteria are a promising production platform for PHA biopolymers. These bacteria are known to produce pure poly-3-hydroxybutyrate homopolymer (PHB). We isolated a strain, Methylocystis sp. WRRC1, that was capable of producing a wide range of polyhydroxybutyrate-co-hydroxyvalerate copolymers (PHB-co-HV) when co-fed methane and valerate or n-pentanol. The ratio of HB to HV monomer was directly related to the concentration of valeric acid in the PHA accumulation media. We observed increased incorporation of HV and total polymer under copper-free growth conditions. The PHB-co-HV copolymers produced had decreased melting temperatures and crystallinity compared with methanotroph-produced PHB.

Particulate methane monooxygenase from Methylosinus trichosporium OB3b

Particulate methane monooxygenase (pMMO) catalyzes methane hydroxylation to methanol at ambient temperature and pressure. pMMO from Methylosinus trichosporium OB3b is one of the two pMMOs for which the protein structure was determined by X-ray crystallography. Because purified pMMO is inherently instable in vitro, it is difficult to use for time-consuming analysis. Therefore, investigations using crude enzyme preparations of pMMO are useful in some cases. In this chapter, methods for preparing pMMO from M. trichosporium OB3b to varying degrees of purity, including bacterial cells expressing pMMO, membrane fractions containing pMMO, and highly purified pMMO, are described.

Development and mathematical modeling of a two-stage reactor system for trichloroethylene degradation using Methylosinus trichosporium OB3b

A two-stage reactor system was developed for the continuous degradation of gas-phase trichloroethylene (TCE). Methylosinus trichosporium OB3b was immobilized on activated carbon in a TCE degradation reactor, trickling biofilter (TBF). The TBF was coupled with a continuous stirred tank reactor (CSTR) to allow recirculation of microbial cells from/to the TBF for the reactivation of inactivated cells during TCE degradation. The mass transfer aspect of the TBF was analyzed, and mass transfer coefficient of 3.9 h(-1) was estimated. The loss of soluble methane monooxygenase (sMMO) activity was modeled based on a material balance on the CSTR and TBF, and transformation capacity (T (c)) was determined to be 20.2 micromol mg(-1). Maximum TCE degradation rate of 525 mg 1(-1) d(-1) was obtained and reactor has been stably operated for more than 270 days.

Methanotrophs, Methylosinus trichosporium OB3b, sMMO, and their application to bioremediation

One of the most problematic groups of the USEPA and EU priority pollutants are the halogenated organic compounds. These substances have a wide range of industrial applications, such as solvents and cleaners. Inadequate disposal techniques and accidental spillages have led to their detection in soil, groundwater, and river sediments. Persistence of these compounds in the environment has resulted from low levels of biodegradation due to chemical structural features that preclude or retard biological attack. Research has indicated the idea that treatment systems based on methanotrophic co-metabolic transformation may be a cost-effective and efficient alternative to physical methods because of the potential for high transformation rates, the possibility of complete compound degradation without the formation of toxic metabolites, applicability to a broad spectrum of compounds, and the use of a widely available and inexpensive growth substrate. A substantial amount of work concerning methanotrophic cometabolic transformations has been carried out using the soluble form of methane monooxygenase (sMMO) from the obligate methanotroph Methylosinus trichosporium OB3b. This NADH-dependent monooxygenase is derepressed when cells are grown under copper stress. sMMO has a wider specificity than the particulate form. sMMO has been shown to degrade trichloroethylene (TCE) at a rate of at least one order of magnitude faster than obtained with other mixed and pure cultures, suggesting it has a wider application to bioremediation. Furthermore, sMMO catalyzes an unusually wide range of oxidation reactions, including the hydroxylation of alkanes, epoxidation of alkenes, ethers, halogenated methanes, cyclic and aromatic compounds including compounds, that are resistant to degradation in the environment. However, the practical application of methantrophs and Methylosinus trichosporium OB3b to the treatment of chlorinated organics has met with mixed success. Although oxidation rates are rapid, compound oxidation with M. trichosporium OB3b is difficult. This fastidious organism grows relatively slowly, which limits the speed with which sMMO expressing biomass can be generated. Furthermore, product toxicity toward the cell, affecting the stability of the enzyme when transforming certain compounds has been observed, for example, by the products of 1,2,3 trichlorobenzene hydroxylation (2,3,4- and 3,4,5-trichlorophenol) and of TCE degradation (chloral hydrate). Because of this toxicity and the inability of sMMO to further oxidize its own hydroxylation products, the ability of methane monoxygenase to carry out the monooxygenation of a wide variety of substituted aromatics and polyaromatics cannot be fully exploited in M. trichosporium OB3b. Many of these problems could be overcome by the use of either a mixed downstream heterotrophic population of organisms that could accommodate the products of hydroxylation or to express sMMO in an organism that could metabolize the products of hydroxylation. The latter of these two approaches would have several advantages. The main benefit would be the removal of the need for methane, which is required to induce sMMO in M. trichosporium OB3b, and supply carbon and energy to the cells that continuously oxidise the target compound, but also acts as a competitive inhibitor of sMMO. Instead, the recombinant could utilize the products of sMMO-mediated hydroxylation as a carbon source.