Page 238 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 10. Co-culturing methanogens and methanotrophs in an MBR abcd
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Figure 5. a–d) Fluorescence in situ hybridization micrographs of the reactor biomass after 1 (a), 22 (b), 49 (c), and 63 days (d) of inoculation with M. barkeri. e–h) Fluorescence in situ hybridization micrographs of the reactor biomass after 3 (e), 16 (f), 30 (g), and 65 days (h) of the second inoculation with M. barkeri. Aerobic methanotrophic bacteria are stained in green (EUB mix), and M. barkeri are stained in magenta (overlay of ARCH-0890 and MSMX-0860 in blue and red, respectively). Scale bars represent 10 μm.
Lower methane/ammonium ratio induced nitrite toxicity event
After reducing the methane influx to 0.62 ml min-1 on t = 205 days, nitrite increased from background levels of 15.6 ± 10.8 (SD) μM to 78.6 ± 4.8 (SD), probably due to the non-specific co-metabolism of ammonia by methane monooxygenase (MMO) of the aerobic methanotrophs under methane-limiting conditions. The nitrite accumulation resulted in a decrease of M. barkeri cells as could be observed by FISH microscopy (data not shown). To minimize the risk of ammonium co-metabolism, ammonium concentration in the inflow medium was lowered from 3.7 to 1 mM. Growth experiments with axenic cultures of M. barkeri indicated that growth was not limited by ammonium concentrations down to 1 mM but was reduced when concentrations fell below 0.5 mM. Although the ammonium source is shared during co- cultivation, reactor liquid ammonium concentrations never dropped below 0.5 mM (data not shown). After system stabilization, a second inoculation with M. barkeri was performed (t = 252 days) to confirm the successful co-cultivation and to show repeated formation of interactions between methanogens and aerobic methanotrophs (Fig. 5e-h).
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