OD600 reached 10.03 at 30 h. At 42 h, 18.3 g/L ABE was
produced and 49.5 g/L glucose was consumed (Fig. 3a,
b). The results proved that C. acetobutylicum TSH1 was
obligate anaerobic bacteria. It was unable to grow and
produce butanol under microaerobic conditions. In contrast,
18.1 g/L ABE was produced and 50.4 g/L glucose
was consumed by the symbiotic system under static culture
without anaerobic pretreatment. Furthermore, cells
grew well and produced 17.8 g/L ABE in symbiotic system
with air sparging. Though some papers have reported
that aerating a small amounts of air into the fermentation
broth didn’t affect the cell growth of Clostridium, these
researches either used periodic aeration to control ORP
(oxidoreduction potential) level or aerated with 5 %
O2/95 % N2 mixed gas [28, 29]. In this study, even when
air was continuously aerated into bioreactor, the symbiotic
system still produced ABE solvents (Fig. 3b). To our
knowledge, this is the first report about butanol production
by a symbiotic system with continuous air sparging.
The results also confirmed that oxygen tolerance of
symbiotic system was much higher than that of the single
culture. Compared with the condition of anaerobic pretreatment,
fermentation rate of symbiotic system were
slower. The fermentation of C. acetobutylicum TSH1 with
nitrogen pretreatment finished at 42 h. And the fermentation
of with symbiotic system air sparging and without
nitrogen pretreatment finished at 48 h.
Product titers, yields, and productivities were shown
in Table 4. Butanol titers obtained in these cultures were
similar, but yields and productivities were slightly different.
For C. acetobutylicum TSH1, about 11.1 g/L butanol
was obtained, and for the symbiotic system, 11.0 g/L
and 11.2 g/L butanol was obtained, respectively. Butanol
yield of C. acetobutylicum TSH1 was 0.22 g/g, which was
similar with artificial symbiotic system without anaerobic
pretreatment. But yield of symbiotic system with air
flushing was 0.18 g/g. Compared with the nitrogen pretreatment
and static culture, continuous air flushing
could take away a little solvent from the broth, so the
solvent loss rate was higher. And another possible reason
is excessive oxygen is disadvantage to the butanol
production under microaerobic condition. Butanol productivity
of C. acetobutylicum TSH1 with anaerobic pretreatment
was 0.26 g/L h. In contrast, productivities of
symbiotic system without anaerobic pretreatment and
with air flushing were 0.21 and 0.23 g/L h, respectively.
This phenomenon was corresponding to the slower cell
growth and glucose consumption in symbiotic system
cultures (Fig. 3). The slower fermentation rate of symbiotic
system cultures could be attributed to inhibition of
oxygen, as both of the cultures were carried out without
anaerobic pretreatment. These results were agreed with
the observations of Dwidar M et al. [30] and indicated
that co-culture of C. acetobutylicum and Bacillus produced
butyric acid after a longer lag time compared with
nitrogen purging pretreatment.