3.5 Succinic acid production from corn cob using co-cultivation
system
Corn
cob was considered to be rich in hemicellulose (Ma et al., 2016), hence,
direct conversion of corn cob through CBP was investigated for succinic
acid production. In this study, corn cob containing approximately 33%
of hemicellulose was directly used as substrate for succinic acid
fermentation with only mechanic crush. As shown in Fig. 6A, the highest
succinic acid concentration of 12.51 g/L was obtained from 80 g/L of
corn cob after 8 days, representing the yield of 0.16 g/g. Before the
dosage of strain 130Z, xylose continued to be accumulated and finally
reached up to 8.68 g/L. In addition, xylose concentration was maintained
at 7-8 g/L during co-cultivation process, while xylanase and
β-xylosidase activities were maintained at 0.37-0.42 and 5.98-7.21 U/mL,
respectively (Fig 6B).
Results showed that strain M5 and 130Z could be good partners to achieve
succinic acid directly from hemicellulosic materials through
CBP. The stable co-existence of
these two strains was mainly based on
competitive cheater and cooperator
interactions (Shahab et al., 2018). When cooperative individuals benefit
more from cooperative behavior than on-cooperators (cheaters), they can
generally maintain cooperative phenotypes, referred to as assortment
(Hol et al., 2013). In this study, strain M5 continuously secreted
xylanase and β-xylosidase, which was a cooperative feature because
xylose obtained by enzymatic hydrolysis was available as public goods
for both strains. Strain 130Z may act as a cheater, because strain 130Z
only utilized xylose for succinic acid production without contributing
energy to the secretion of enzymes. However, strain M5 also benefited
from the presence of strain 130Z, mainly reflected by the continuous
consumption of xylose by strain 130Z, which could relieve product
feedback inhibition to xylanase and β-xylosidase caused by high
concentration of xylose (Hol et al., 2013).
3.6 Comparison of succinic production
fromlignocellulosic
biomass with other studies
So far, researchers have done series of work to realize the conversion
of lignocellulosic biomass to succinic acid (Table 3). However, majority
work involved complex pretreatment processes, such as biological,
chemical, physical or physicochemical methods etc. For example, corn
fiber could be pretreated with aqueous ammonia (SAA), dilute sulfuric
acid and lignocellulosic enzymes
into fermentable monomeric sugars for succinic acid production (Yoo,
Nghiem, & Kim, 2016). 26.3 g/L of succinic acid with yield of 0.53 g/g
could be produced from corn fiber
hydrolysate by Escherichia coli AFP184. In addition, 30.00 g/L
succinic acid with yield of 0.69 g/g was also obtained from dilute-acid
pretreated corn stover hydrolysate by using Basfia
succiniciproducens (Salvachua et al., 2016). Similarly, a maximum
succinic acid titer and yield of 39.60 g/L and 0.78 g/g were achieved
from deacetylated and dilute-acid pretreated corn stover hydrolysate byA. succinogenes (Bradfield et al., 2015). Recently, the
enzymatic extracellular mixtures of two new isolates were shown to
hydrolyze both cellulose and xylan into monosaccharides. The produced
monosaccharides were converted into succinic acid ranging from 15 to 18
g/L by B. succiniciproducens BPP7, with an average yield 0.75 g/g
(Pennacchio et al., 2018). All above examples were accompanied by
complex and expensive pretreatment steps. Particularly, efforts have
also been made to use genetic engineering to transform single bacterium
with capabilities of lignocellulose degradation and succinic acid
production. For example, an E. coli was engineered to secrete
hemicellulases for succinate acid production from beechwood xylan
through CBP (Zheng et al., 2012). 14.44 g/L of succinic acid with yield
of 0.37 g/g was finally produced from 1% xylose and 3% xylan. This
represents the first example CBP strain for succinic acid production
from xylan without using externally supplied hydrolases. In this study,
the designed synthetic microbial co-cultivation system can achieve
higher succinic acid production without the need for complex genetic
engineering and pretreatment process. The application of
co-cultivation-based CBP may pave the way for direct lignocellulosic
biomass conversion into biochemicals and bioenergy.
- CONCLUSIONS
In this study, a synthetic microbial co-cultivation system was designed
to convert lignocellulosic biomass to succinic acid through CBP.
Response surface methodology (RSM) was used to improve succinic acid
titer. After process optimization, T. thermosaccharolyticum M5
and A. succinogenes 130Z produced up to 32.50 g/L of succinic
acid with yield of 0.41 g/g from 80 g/L xylan w. In addition, 12.51 g/L
succinic acid was also obtained from 80 g/L of corn cob, representing
the first successful example for succinic acid production from
lignocellulose by using microbial consortium. Future work was still
needed to improve the hydrolytic rate to match the reducing sugar
releasing and take up rates.