Genomics and transcriptomics of the hydrogen producing extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus

As fossil fuels are depleting, there is a clear need for alternative sustainable fuel sources. One of the interesting alternatives is hydrogen, which can be produced from biomass by bacteria and archaea. To make the application feasible, organisms are needed which have high hydrogen productivities as well as the capacity to handle a wide variety of substrates. Caldicellulosiruptor saccharolyticus has these abilities and this was the reason to select this organism to study hydrogen production at elevated temperatures. The aim of this study was to clarify the pathways involved in hydrogen production as well as studying the regulation of the hydrogen production on different levels. Especially the regulation of the disposal of reducing equivalents (NADH, reduced ferredoxin, NADPH), was an important aspect. The genome sequence was annotated and compared with transcriptome data. This revealed the presence of the Emden-Meyerhof pathway, and the non-oxidative part of the pentose phosphate pathway for the breakdown of glucose and xylose. The genomic data explained the high hydrolyzing capacity of this organism by the presence of a large number of carbohydrate active enzymes as well as a large variety of ABC transporters. With transcriptomics the substrate specificity of several of these transporters was predicted. The absence from the genome of some components involved in classical carbon catabolite repression explained the capacity of C. saccharolyticus to grow simultaneously on C5 and C6 sugars. When C. saccharolyticus was cultured on rhamnose, 1.2 propanediol production was observed. This observation could be explained by the presence of a rhamnose pathway, which could be unraveled using the genome sequence as well as the transcriptome data. As hydrogen pressure seems to be an important factor for hydrogen production by C. saccharolyticus, the effect of a high P(H2) was studied in a continuous culture. This showed a clear shift to lactate and ethanol production at higher P(H2). By comparing the transcriptome for the high and low P(H2) situation, this shift could be explained by the upregulation of the genes encoding an alcohol and a lactate dehydrogenase. This suggested that the electrons of NADH are most likely disposed of via lactate production, while the electrons of reduced ferredoxin might be shuttled via an oxidoreductase to produce ethanol.Overall this study revealed several new insights in the regulation of hydrogen production by thermophilic anaerobic organisms.

Bibliographic Details

Main Author:	Verhaart, M.R.A.
Other Authors:	Stams, Fons
Format:	Doctoral thesis biblioteca
Language:	English
Subjects:	bioenergy, genomics, hydrogen, thermophilic bacteria, transcriptomics, bio-energie, genexpressieanalyse, thermofiele bacteriën, waterstof,
Online Access:	https://research.wur.nl/en/publications/genomics-and-transcriptomics-of-the-hydrogen-producing-extremely-
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