(2020) The Thermoacidophilic Methanotroph Methylacidiphilum Fumariolicum SolV Oxidizes Subatmospheric H2 with a High-Affinity [NiFe] Hydrogenase
Schmitz R, Pol A, Mohammadi S, Hogendoorn C, van Gelder T, Jetten M, Daumann L & Op den Camp H
08f: Room 3, Tuesday 23rd June 05:33 - 05:36
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Thank you for the fascinating insight into experimental constraints on H2-oxidizing . Given that these are tolerant of high T, and give their NiFe hydrogenase – can you comment on what understanding there is of the evolutionary history of this organism? Has it been identified in systems deeper than soils? Thank you – question posed by Barbara Sherwood Lollar (firstname.lastname@example.org)
Dear Barbara, thanks for your question. The methanotrophs that we study (of the acidophilic genus Methylacidiphilum) grow optimally at 55 degrees Celsius and a pH of around 3. All strains/clones are found in environments that are hot (40 to 60 degrees Celsius) and acidic (pH 1 to 5). Usually, they are present in volcanic ecosystems, living in features such as fumaroles and mud pots. In these systems, methane and hydrogen gas are emitted, which are used as energy source. I have never read about these bacteria present in deep soil systems, but I can make a prediction: if the temperatures are sufficiently high (40 to 60 degrees Celsius), the pH is sufficiently low (pH 1 to 5), and sufficient energy and carbon sources are present (either CH4, or H2 and CO2), they could also be present in other systems than geothermal/volcanic ecosystems. In addition, they are aerobic microorganisms, so they need oxygen, and they require lanthanides, because one of the essential enzymes (the methanol dehydrogenase) needs lanthanides as a metal inside the enzyme's cofactor. One of the few examples of these bacteria outside the geothermal habitats, is a corroded, acidic, methane-rich sewage pipe on Hawaii (10.1111/jam.12491). Therefore, I believe in time we will find these bacteria in more habitats than just geothermal systems.
It's interesting that the membrane-associated enzyme possesses a higher affinity for H2 than the isolated enzyme. Do you have any hypotheses on why this would be?
Dear Elizabeth, good question! This has probably to do with oxygen poisoning and/or the connection with the membrane itself. The isolated enzyme has an affinity for H2 of about 300 nM. The membrane fraction containing this enzyme shows an affinity of 140 nM, which is significantly lower. What we noticed is that the isolated enzyme is quite vulnerable. If in an assay H2 has become depleted and O2 is present in small concentrations, oxygen seems to damage the isolated enzyme. This effect of oxygen does not take place when the enzyme is in association with the membrane. Therefore, the membrane could render a protective environment for this enzyme. The enzyme is located in the cytoplasm without a membrane anchor, but the hydrogen oxidizing activity is primarily found in the membrane. It is conceivable that another protein (a redox partner), couples the hydrogenase to the membrane, which is needed for energy conservation. Once this connection is broken, the enzyme works suboptimally, which is reflected by a lower affinity.
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