Microbial activity response to hydrogen injection in thermophilic anaerobic digesters revealed by genome-centric metatranscriptomics

Alessandra Fontana,Irini Angelidaki,Adam Kovalovszki,Laura Treu,Stefano Campanaro,Lorenzo Morelli,Fabrizio Cappa,Giorgio Valle,Panagiotis G Kougias

doi:10.1186/s40168-018-0583-4

Alessandra Fontana, Irini Angelidaki + Show 7 more

Open Access

https://doi.org/10.1186/s40168-018-0583-4

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Abstract

BackgroundThe expansion of renewable energy produced by windmills and photovoltaic panels has generated a considerable electricity surplus, which can be utilized in water electrolysis systems for hydrogen production. The resulting hydrogen can then be funneled to anaerobic digesters for biogas upgrading (biomethanation) purposes (power-to-methane) or to produce high value-added compounds such as short-chain fatty acids (power-to-chemicals).Genome-centric metagenomics and metatranscriptomic analyses were performed to better understand the metabolic dynamics associated with H2 injection in two different configurations of anaerobic digesters treating acidic wastes, specifically cheese manufacturing byproducts. These approaches revealed the key-genes involved in methanation and carbon fixation pathways at species level.ResultsThe biogas upgrading process in the single-stage configuration increased the CH4 content by 7%. The dominant methanogenic species responsible for the upregulation of the hydrogenotrophic pathway in this reactor was Methanothermobacter wolfeii UC0008. In the two-stage configuration, H2 injection induced an upregulation of CO2 fixation pathways producing short-chain fatty acids, mainly acetate and butyrate. In this configuration, the abundant species Anaerobaculum hydrogeniformans UC0046 and Defluviitoga tunisiensis UC0050 primarily upregulated genes related to electron transport chains, suggesting putative syntrophisms with hydrogen scavenger microbes. Interestingly, Tepidanaerobacter acetatoxydans UC0018 did not act as an acetate-oxidizer in either reactor configurations, and instead regulated pathways involved in acetate production and uptake. A putative syntrophic association between Coprothermobacter proteolyticus UC0011 and M. wolfeii UC0008 was proposed in the two-stage reactor. In order to support the transcriptomic findings regarding the hydrogen utilization routes, an advanced bioconversion model was adapted for the simulation of the single- and two-stage reactor setups.ConclusionsThis is the first study investigating biogas reactor metatranscriptome dynamics following hydrogen injection for biomethanation and carbon fixation to short-chain fatty acids purposes. The same microbes showed different patterns of metabolic regulation in the two reactor configurations. It was observed an effect of the specialized acidogenic reactor on the overall microbial consortium composition and activity in the two-stage digester. There were also suggested the main species responsible for methanation, short-chain fatty acids production, and electron transport chain mechanisms, in both reactor configurations.

Highlights

The expansion of renewable energy produced by windmills and photovoltaic panels has generated a considerable electricity surplus, which can be utilized in water electrolysis systems for hydrogen production
Metagenomic and metatranscriptomic investigations were performed at two time points; the first point referred to the reactors’ steady-state performance before H2 injection and the second occurred 1 week after H2 injection
To verify the stability of the microbial community during the reactors’ stable operation, an additional set of metagenomic samples was collected from R1 and R3 at multiple time points and was analyzed using 16S rRNA gene amplicon sequencing

Summary

Introduction

The expansion of renewable energy produced by windmills and photovoltaic panels has generated a considerable electricity surplus, which can be utilized in water electrolysis systems for hydrogen production. The installed global wind capacity increased by 10.8% in 2017, with China and the USA as the major producers of electricity from wind [1] This expansion has led to the production of a considerable electricity surplus, which cannot be stored in batteries due to high cost or injected into the national grid, since it could cause electrical instabilities. Hydrogen is highly volatile and difficult to store and transport, and is associated with some environmental risks This H2 could be used for biogas upgrading purposes (power-to-methane) or for the production of high value-added compounds such as fatty acids (mainly short-chain carboxylates) and alcohols (power-to-chemicals), via anaerobic digestion (AD), generating an energy gain in the form of methane along with organic waste valorization [3,4,5,6]

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