(Santamauro et al. 2014). However, at lower
temperatures, octanoic and decanoic acid are
also more toxic (Viegas and Sa ´-Correia 1995;
Viegas 1997). Furthermore, low temperatures
might not be feasible in large-scale production,
in which high gravity can lead to elevated temperatures, and cooling of big fermentation
plants is cost-intensive (Gibson et al. 2007).
Therefore, it might be more desirable to generate thermotolerant S. cerevisiae strains by
introducing traits of thermotolerant yeasts,
such as Kluyveromyces marxianus (Cernak
et al. 2018), or even thermotolerant oleaginous
yeasts, like R. toruloides (Wu et al. 2018).
Another important cultivation parameter for
FA production is the pH and media buffering.
Short- and medium-chain FAs are more toxic
to S. cerevisiae at lower pH values, causing
decreased viability and yields (Viegas et al.
1989). Therefore, it was shown that the addition
of potassium phosphate buffer to complex
medium elevated short- and medium-chain
FA production substantially (Gajewski et al.
2017).
A promising approach to avoid the reuptake of the products by the cells and simultaneously decrease the effects of product toxicity
is an in situ extraction. The addition of dodecane to the culture of a long-chain fatty alcoholproducing S. cerevisiae was successfully applied
for the extraction of these compounds (Runguphan and Keasling 2014; d’Espaux et al. 2017).
Upon addition of dodecane to a culture broth of
a 1-octanol producing S. cerevisiae strain, however, production was reduced—likely due to
the extraction of the precursor octanoic acid
by dodecane (Henritzi et al. 2018). Therefore,
when considering in situ extraction, an agent
needs to be found, which is highly specific for
each product, and does not remove important
pathway intermediates from the culture.
Current efforts for yeast FA production
were carried out in minimal (Zhou et al.
2016b; Yu et al. 2018) as well as complex
medium (Leber et al. 2015; Gajewski et al.
2017). To be truly more sustainable than
petroleum-based production, and to prevent a
competition with food supply, starting materials, such as lignocellulosic biomass from agricultural waste, need to be utilized. To extract
fermentable sugars from such biomass, it first
must undergo pretreatment, in the process of
which fermentation inhibitors arise. Therefore,
the ideal FA producing yeast needs to be resistant to these inhibitors, as well as to be able to
utilize glucose and five-carbon sugars, such as
xylose, as carbon sources (Peralta-Yahya et al.
2012). D’Espaux et al. (2017) presented the
first—and, so far, only—report about FA/alcohol production with S. cerevisiae solely from
lignocellulosic feedstock. They fed the nonfood crops sorghum and switchgrass, which
were pretreated with ionic liquids, to an engineered S. cerevisiae strain and obtained up to
0.7 g L
À1 fatty alcohols (d’Espaux et al. 2017).
By combining the different engineering and
cultivation strategies mentioned above, a further increase in FA titers produced from lignocellulosic biomass can be foreseen.
VII. Conclusions
Much progress has been made in recent years in
the development of strategies for microbial
production of FA and their derivatives from
renewable feedstocks. The available literature
shows that extensive interventions into the central carbon metabolism, relying on expression
of heterologous pathways and manipulation of
the activity of endogenous enzymes, are necessary to enable a high yield of oleochemicals in
different chassis organisms. The highest yields
of microbially produced oleochemicals
reported to date were achieved in S. cerevisiae,
owing to the great body of knowledge regarding
the physiology of this yeast (0.1 g free FA/g
glucose, i.e., approximately 30% of the theoretical yield). Moreover, FA production in S. cerevisiae has become more chain length-specific,
and many FAs and a great variety of derivatives
have already been produced successfully in labscale fermentations. The development of biosensors and high-throughput screening methods has become increasingly important and will
considerably accelerate the development for
well-performing strains. Once such challenges
360
L. Baumann et al.
temperatures, octanoic and decanoic acid are
also more toxic (Viegas and Sa ´-Correia 1995;
Viegas 1997). Furthermore, low temperatures
might not be feasible in large-scale production,
in which high gravity can lead to elevated temperatures, and cooling of big fermentation
plants is cost-intensive (Gibson et al. 2007).
Therefore, it might be more desirable to generate thermotolerant S. cerevisiae strains by
introducing traits of thermotolerant yeasts,
such as Kluyveromyces marxianus (Cernak
et al. 2018), or even thermotolerant oleaginous
yeasts, like R. toruloides (Wu et al. 2018).
Another important cultivation parameter for
FA production is the pH and media buffering.
Short- and medium-chain FAs are more toxic
to S. cerevisiae at lower pH values, causing
decreased viability and yields (Viegas et al.
1989). Therefore, it was shown that the addition
of potassium phosphate buffer to complex
medium elevated short- and medium-chain
FA production substantially (Gajewski et al.
2017).
A promising approach to avoid the reuptake of the products by the cells and simultaneously decrease the effects of product toxicity
is an in situ extraction. The addition of dodecane to the culture of a long-chain fatty alcoholproducing S. cerevisiae was successfully applied
for the extraction of these compounds (Runguphan and Keasling 2014; d’Espaux et al. 2017).
Upon addition of dodecane to a culture broth of
a 1-octanol producing S. cerevisiae strain, however, production was reduced—likely due to
the extraction of the precursor octanoic acid
by dodecane (Henritzi et al. 2018). Therefore,
when considering in situ extraction, an agent
needs to be found, which is highly specific for
each product, and does not remove important
pathway intermediates from the culture.
Current efforts for yeast FA production
were carried out in minimal (Zhou et al.
2016b; Yu et al. 2018) as well as complex
medium (Leber et al. 2015; Gajewski et al.
2017). To be truly more sustainable than
petroleum-based production, and to prevent a
competition with food supply, starting materials, such as lignocellulosic biomass from agricultural waste, need to be utilized. To extract
fermentable sugars from such biomass, it first
must undergo pretreatment, in the process of
which fermentation inhibitors arise. Therefore,
the ideal FA producing yeast needs to be resistant to these inhibitors, as well as to be able to
utilize glucose and five-carbon sugars, such as
xylose, as carbon sources (Peralta-Yahya et al.
2012). D’Espaux et al. (2017) presented the
first—and, so far, only—report about FA/alcohol production with S. cerevisiae solely from
lignocellulosic feedstock. They fed the nonfood crops sorghum and switchgrass, which
were pretreated with ionic liquids, to an engineered S. cerevisiae strain and obtained up to
0.7 g L
À1 fatty alcohols (d’Espaux et al. 2017).
By combining the different engineering and
cultivation strategies mentioned above, a further increase in FA titers produced from lignocellulosic biomass can be foreseen.
VII. Conclusions
Much progress has been made in recent years in
the development of strategies for microbial
production of FA and their derivatives from
renewable feedstocks. The available literature
shows that extensive interventions into the central carbon metabolism, relying on expression
of heterologous pathways and manipulation of
the activity of endogenous enzymes, are necessary to enable a high yield of oleochemicals in
different chassis organisms. The highest yields
of microbially produced oleochemicals
reported to date were achieved in S. cerevisiae,
owing to the great body of knowledge regarding
the physiology of this yeast (0.1 g free FA/g
glucose, i.e., approximately 30% of the theoretical yield). Moreover, FA production in S. cerevisiae has become more chain length-specific,
and many FAs and a great variety of derivatives
have already been produced successfully in labscale fermentations. The development of biosensors and high-throughput screening methods has become increasingly important and will
considerably accelerate the development for
well-performing strains. Once such challenges
360
L. Baumann et al.