ferment 5-carbon sugars such as xylose and
arabinose, which are abundant in hemicellulose. Acquisition of the ability to ferment pentose sugars by yeasts is a major priority for
bioethanol production from second generation
energy crops (Yang et al. 2007). It has long been
known that anaerobic fungi deconstruct hemicellulose and ferment both pentose and hexose
sugars (Theodorou et al. 1996b). In an attempt
to overcome the inability of yeast to ferment
pentose sugars, the xylose isomerase gene from
Piromyces sp. E2 was successfully expressed in
Saccharomyces cerevisiae (Yang et al. 2007).
However, ethanol production was still hampered due to the clear preferences of the
isomerization equilibrium towards xylose formation. Kuyper et al. (2005) overcame this limitation by supplementing genes for all of the
enzymes involved in the conversion of xylulose
to glycolytic intermediates. In order to minimize xylitol production, the endogenous gene
encoding aldose reductase was deleted. Under
anaerobic conditions, the engineered yeast
strain entirely consumed mixtures of glucose
and xylose sequentially, with glucose as the
preferred substrate.
C. Biogas Production with Anaerobic Fungi
Biogas and digestate production by anaerobic
digestion (AD) of plant biomass is a commercially viable strategy to generate sustainable
renewable energy and fertilizer/soil improver
(Strzalka et al. 2017), but common strategies
omit anaerobic fungi. During AD a mixed
microbial consortium, comprising of bacteria
(such as hydrolytic and syntrophic acetate oxidizing Clostridia) and methanogenic archaea
converts substrates such as lignocellulosic residues, animal manure/slurry, and food processing waste to biogas (Schnu ¨rer 2018). Biogas,
the main energy product from AD, is a mixture
of methane (50–75%) and carbon dioxide (25–
50%) with traces of other gases including
hydrogen, hydrogen sulfide, and ammonia.
Biogas can be upgraded to bio-methane, for
injection to the gas network for electricity production and use in domestic, commercial, and
industrial heating or use as a fuel for road
transport (Schnu ¨rer 2016).
To achieve an adequate amount and quality
of biogas, and more importantly a high methane yield, the process conditions in biogas reactors should try to resemble those found in the
natural model for biogas plants, the rumen. In
the rumen the conditions are constant temperatures (such as 39
C), low redox potential
(–0.4 V), nearly neutral pH values, an anoxic
environment and constant influx of fresh or
undigested feed (Weimer et al. 2009). All these
parameters are very similar to those that occur
in mesophilic anaerobic digesters (Weimer
et al. 2009). However, it should be noted that
longer hydraulic retention times are necessary
in biogas reactors to achieve sufficient digestion of feedstocks. Compared to the rumen
anaerobic fungi are usually missing in the biogas producing microbial consortium. While
anaerobic fungi do not produce bio-methane
as a fermentation end product, they do produce
biochemical substrates which can be used by
methanogenic archaea (Cheng et al. 2009; see
below).
Therefore, bioaugmentation of biogas reactors with anaerobic fungi or microbial consortia derived from rumen fluid (Nagler et al. 2019)
could help to enhance and accelerate the conversion of lignocellulose to biogas. To date,
most bioaugmentation research has involved
the addition of anaerobic fungi to laboratoryscale AD bioreactors. These bioaugmentation
trials are summarized in Table 17.3.
While the results in Table 17.2 appear
encouraging, many studies are inconclusive
due to the inclusion of spent culture medium
alongside anaerobic fungal cultures in augmented reactors. This confounded bias was
avoided in recent studies employing the semicontinuous fermentation of maize silage and
anaerobic sludge treated with a 1:1 mixture of
Anaeromyces KF8 and Piromyces KF9 (Procha ´zka et al. 2012) and the two-stage batch
fermentation of hay with two individual Neocallimastix frontalis isolates (Dollhofer et al.
2018). In both experiments, a control reaction
with the respective heat-inactivated culture was
performed. In these augmented trials, increased
biogas production (Procha ´zka et al. 2012) or
accelerated biomass degradation and higher
initial biogas yields (Dollhofer et al. 2018)
were achieved.
17 The Biotechnological Potential of Anaerobic Gut Fungi
425
arabinose, which are abundant in hemicellulose. Acquisition of the ability to ferment pentose sugars by yeasts is a major priority for
bioethanol production from second generation
energy crops (Yang et al. 2007). It has long been
known that anaerobic fungi deconstruct hemicellulose and ferment both pentose and hexose
sugars (Theodorou et al. 1996b). In an attempt
to overcome the inability of yeast to ferment
pentose sugars, the xylose isomerase gene from
Piromyces sp. E2 was successfully expressed in
Saccharomyces cerevisiae (Yang et al. 2007).
However, ethanol production was still hampered due to the clear preferences of the
isomerization equilibrium towards xylose formation. Kuyper et al. (2005) overcame this limitation by supplementing genes for all of the
enzymes involved in the conversion of xylulose
to glycolytic intermediates. In order to minimize xylitol production, the endogenous gene
encoding aldose reductase was deleted. Under
anaerobic conditions, the engineered yeast
strain entirely consumed mixtures of glucose
and xylose sequentially, with glucose as the
preferred substrate.
C. Biogas Production with Anaerobic Fungi
Biogas and digestate production by anaerobic
digestion (AD) of plant biomass is a commercially viable strategy to generate sustainable
renewable energy and fertilizer/soil improver
(Strzalka et al. 2017), but common strategies
omit anaerobic fungi. During AD a mixed
microbial consortium, comprising of bacteria
(such as hydrolytic and syntrophic acetate oxidizing Clostridia) and methanogenic archaea
converts substrates such as lignocellulosic residues, animal manure/slurry, and food processing waste to biogas (Schnu ¨rer 2018). Biogas,
the main energy product from AD, is a mixture
of methane (50–75%) and carbon dioxide (25–
50%) with traces of other gases including
hydrogen, hydrogen sulfide, and ammonia.
Biogas can be upgraded to bio-methane, for
injection to the gas network for electricity production and use in domestic, commercial, and
industrial heating or use as a fuel for road
transport (Schnu ¨rer 2016).
To achieve an adequate amount and quality
of biogas, and more importantly a high methane yield, the process conditions in biogas reactors should try to resemble those found in the
natural model for biogas plants, the rumen. In
the rumen the conditions are constant temperatures (such as 39
C), low redox potential
(–0.4 V), nearly neutral pH values, an anoxic
environment and constant influx of fresh or
undigested feed (Weimer et al. 2009). All these
parameters are very similar to those that occur
in mesophilic anaerobic digesters (Weimer
et al. 2009). However, it should be noted that
longer hydraulic retention times are necessary
in biogas reactors to achieve sufficient digestion of feedstocks. Compared to the rumen
anaerobic fungi are usually missing in the biogas producing microbial consortium. While
anaerobic fungi do not produce bio-methane
as a fermentation end product, they do produce
biochemical substrates which can be used by
methanogenic archaea (Cheng et al. 2009; see
below).
Therefore, bioaugmentation of biogas reactors with anaerobic fungi or microbial consortia derived from rumen fluid (Nagler et al. 2019)
could help to enhance and accelerate the conversion of lignocellulose to biogas. To date,
most bioaugmentation research has involved
the addition of anaerobic fungi to laboratoryscale AD bioreactors. These bioaugmentation
trials are summarized in Table 17.3.
While the results in Table 17.2 appear
encouraging, many studies are inconclusive
due to the inclusion of spent culture medium
alongside anaerobic fungal cultures in augmented reactors. This confounded bias was
avoided in recent studies employing the semicontinuous fermentation of maize silage and
anaerobic sludge treated with a 1:1 mixture of
Anaeromyces KF8 and Piromyces KF9 (Procha ´zka et al. 2012) and the two-stage batch
fermentation of hay with two individual Neocallimastix frontalis isolates (Dollhofer et al.
2018). In both experiments, a control reaction
with the respective heat-inactivated culture was
performed. In these augmented trials, increased
biogas production (Procha ´zka et al. 2012) or
accelerated biomass degradation and higher
initial biogas yields (Dollhofer et al. 2018)
were achieved.
17 The Biotechnological Potential of Anaerobic Gut Fungi
425