Appraisal of strategies to improve thermophilic hydrogen production exploiting Caldicellulosiruptor species

Forskningsoutput: AvhandlingDoktorsavhandling (sammanläggning)

Abstract

The transition from a fossil to a bio-based economy is of vital importance to stem the effects of ongoing climate change.
This bio-based economy will necessitate the production of both biofuels and chemical compounds from biological
sources. Hydrogen is a promising candidate as a renewable energy carrier due to its high energy density, carbon
neutrality when combusted and its potential use as a reducing agent to produce biochemicals. Caldicellulosiruptor is a
genus of extreme thermophilic bacteria capable of producing hydrogen close to the theoretical maximum of 4 mol
H2/mol hexose from an array of different mono-, oligo- and polymeric sugars permitting the utilisation of a diverse range
of lignocellulose substrates. Although promising, the advancement of Caldicellulosiruptor towards becoming a model
organism for hydrogen production relies on overcoming the intrinsic hurdles that exist in this genus, such as
osmosensitivity, low QH2 and the high financial cost of nutrient addition.
Throughout this work, techniques for improving the Caldicellulosiruptor process were studied in detail. Adaptive
laboratory evolution was employed as a technique to increase osmotolerance by incremental adaption to elevated
glucose concentrations, thereby facilitating development of osmotolerant strains belonging to five species of
Caldicellulosiruptor. Notably, the degree of adaption to higher osmolarity varies depending on the species. The
osmotolerant strain C. owensensis CO80 was demonstrated to grow at glucose concentrations up to 80 g/l;
nevertheless, the highest QH2 values for this strain were observed with lower osmolarity media. C. owensensis CO80
was further implemented with the osmotolerant strain, C. saccharolyticus G5, as a co-culture to produce hydrogen from
concentrated hydrolysates, in which the wild-type strains are unable to be cultivated.
Several strategies to optimise of nutrient addition to the fermentation process were studied, including designed coculture
with Coprothermobacter proteolyticus, vitamin removal and the design of a new trace element solution. The
implementation of a novel trace element solution (EB-1) permitted a phosphorus reduction of 90% compared to
previous cultivations. However, further optimisation of the trace elements and vitamin addition are required to increase
QH2.
Process integration was undertaken by further fermentation of the effluent of Caldicellulosiruptor cultivated on
lignocellulose hydrolysates to yield methane and polyhydroxybutyrate by a methanogenic consortium and Ralstonia
eutropha, respectively. These studies illustrate that acetate produced by Caldicellulosiruptor can be further fermented
into industrially relevant compounds.
In addition, several key physiological attributes of C. saccharolyticus fermentation were also studied. Although, C.
saccharolyticus co-ferments sugars, a diauxic-like production of hydrogen occurs when C. saccharolyticus is batch
cultivated with a mixture of glucose, xylose and arabinose. This effect is amplified when wheat straw hydrolysate is
used as a substrate. It was also observed that xylose is fully consumed after 25 hours while full glucose consumption
requires over 80 hours. In addition, continuous cultivation of osmotolerant strains on the studied hydrolysates displayed
full xylose consumption, while a significant portion of glucose remained in the effluent. Co-culturing of C.
saccharolyticus with the proteolytic hydrogen-producing Coprothermobacter proteolyticus marginally increased QH2
compared to the mono-culture. However, a significant reduction in QH2 was observed when C. saccharolyticus was
cultivated at 60°C, due to the lower growth temperature causing a significant metabolic shift from acetate to lactate.
The compiled findings of this thesis opens up new avenues of research and will act as a stepping stone for further
strain development, media optimisation and process integration exploiting the Caldicellulosiruptor genus.

Detaljer

Författare
Enheter & grupper
Forskningsområden

Ämnesklassifikation (UKÄ) – OBLIGATORISK

  • Naturvetenskap

Nyckelord

Originalspråkengelska
KvalifikationDoktor
Handledare/Biträdande handledare
  • Ed van Niel, handledare
  • Karin Willquist, Biträdande handledare
Tilldelningsdatum2019 apr 26
UtgivningsortLund
Förlag
  • Department of Applied Microbiology, Lund University
Tryckta ISBN978-91-7422-642-3
Elektroniska ISBN978-91-7422-650-8
StatusPublished - 2019 apr 26
PublikationskategoriForskning

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