None of the other carbon substrates tested supported growth. Nitrate
ammonia, urea, lysine, peptone and yeast extract were
utilized as sole nitrogen sources. No growth was observed
in nitrogen-free medium under a reduced oxygen level and
the acetylene reduction test was negative, although the nifH
gene was amplified by PCR. A positive control assay under
the same atmospheric conditions, where acetylene reduction
was verified, was carried out using Methylococcus
capsulatus strain ‘Bath’. NaCl concentrations above 0.5%
(w/v) were inhibitory. Growth occurred at 20–37 uC but
not at 18 uC or at 39 uC. Optimal growth occurred at 30–
35 uC with a specific growth rate of 0.14 h21 (equivalent to
a doubling time of 5 h). The pH range for growth was
pH 5–8 with optimum growth at pH 6.5–6.8.