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Abstract

Methane, a non-expensive natural substrate, is used by Methylocystis
spp. as a sole source of carbon and energy. Here, we assessed whether
Methylocystis sp. strain SC2 is able to also utilize hydrogen as an
energy source. The addition of 2% H-2 to the culture headspace had the
most significant positive effect on the growth yield under CH4 (6%) and
O-2 (3%) limited conditions. The SC2 biomass yield doubled from 6.41
(+/- 0.52) to 13.82 (+/- 0.69) mg cell dry weight per mmol CH4, while
CH4 consumption was significantly reduced. Regardless of H-2 addition,
CH4 utilization was increasingly redirected from respiration to
fermentation-based pathways with decreasing O-2/CH4 mixing ratios.
Theoretical thermodynamic calculations confirmed that hydrogen
utilization under oxygen-limited conditions doubles the maximum biomass
yield compared to fully aerobic conditions without H-2 addition.
Hydrogen utilization was linked to significant changes in the SC2
proteome. In addition to hydrogenase accessory proteins, the production
of Group 1d and Group 2b hydrogenases was significantly increased in
both short- and long-term incubations. Both long-term incubation with
H-2 (37 d) and treatments with chemical inhibitors revealed that SC2
growth under hydrogen-utilizing conditions does not require the activity
of complex I. Apparently, strain SC2 has the metabolic capacity to
channel hydrogen-derived electrons into the quinone pool, which provides
a link between hydrogen oxidation and energy production. In summary, H-2
may be a promising alternative energy source in biotechnologically
oriented methanotroph projects that aim to maximize biomass yield from
CH4, such as the production of high-quality feed protein.