Extremophiles are microorganisms live and thrive in extreme environments that are harsh and hostile to most forms of life on earth (e.g. low pH, low temperature, high pH and high salinity). They have developed strategies to obtain nutrients and conserve energy to sustain life under these adverse conditions. Such metabolic capabilities are valuable to be exploit for industrial applications such as the remediation of environmental pollutions, which typically bring about extreme physicochemical conditions. The advancing technology bioelectrochemical systems can utilize the microbial metabolism to oxidize a substrate while simultaneously recover electrical energy or produce a useful product in an electrochemical set-up. It enables the remediation of pollutions, and its integration with extremophiles has opened up a wide range of possibilities to tackle various industrial waste streams with extreme conditions in an environmentally friendly manner. Inorganic sulfur compounds such as tetrathionate, thiocyanate and sulfide that originate from mining, metal refinery and petroleum industries are toxic and hazardous to the recipient water body and human health if discharged untreated. The remediation of these three compounds with bioelectrochemical systems that incorporates extremophiles was investigated in three separate studies of this thesis. 16S rRNA gene amplicon sequencing, metagenomics and metatranscriptomics are utilized to profile the microbial communities, and to understand their metabolic potential and states.

Tetrathionate degradation with acidophilic microorganisms in microbial fuel cells at pH 2 was demonstrated in the first study of this thesis. Electricity was produced from the oxidation of tetrathionate, facilitated by the anodic microbiome. 16S rRNA gene amplicon sequencing showed that this community was dominated by members of the genera Thermoplasma, Ferroplasma, Leptospirillum, Sulfobacillus and Acidithiobacillus. Metagenomic analysis reconstructed genomes that were most similar to the genera Ferroplasma, Acidithiobacillus, Sulfobacillus and Cuniculiplasma. Together with metatranscriptomic analysis, it was indicated that this microbial community was metabolizing tetrathionate and other intermediate sulfur compounds via multiple pathways, the electrons released from oxidation were suggested to be transferred to the electrode via soluble electron shuttles. In addition, the Ferroplasma-like population in this study was suggested to be active in metabolising inorganic sulfur compounds and synthesizing soluble electron shuttles. Since characterized Ferroplasma species do not utilize inorganic sulfur compounds, the anodic compartment might have selected a novel Ferroplasma population.

Next, thiocyanate degradation with psychrophilic microorganisms in microbial fuel cells at 8 °C was demonstrated for the first time. Electricity generation alongside with thiocyanate degradation facilitated by the anodic microbiome was observed. 16S rRNA gene amplicon sequencing and metatranscriptomics suggested that Thiobacillus was the predominant and most active population. mRNA analysis revealed that thiocyanate was metabolized primarily via the ‘cyanate’ degradation pathway; the resultant sulfide was oxidized; ammonium was assimilated; carbon dioxide was fixed as carbon source. It was also suggested by mRNA analysis that the consortium used multiple mechanisms to acclimate low temperature such as the synthesis of cold shock proteins, cold inducible proteins and molecular chaperones.

Finally, sulfide removal with haloalkaliphilic microorganisms in microbial electrolysis cells operated at pH 8.8 to 9.5 and with 1.0 M sodium ion was investigated. The anodic microbiome was hypothesized to facilitate current generation by the oxidation of sulfide and of intermediate sulfur compounds to sulfate, which was supported by chemical analysis and microbial profiling. Dominant populations from the anode had 16S rRNA gene sequences that aligned within the genera Thioalkalivibrio, Thioalkalimicrobium, and Desulfurivibrio, which are known for sulfide oxidation. Intriguingly, Desulfurivibrio dominated the electrode-attached community, possibly enriched by the electrode as a selecting pressure. This finding suggested a novel role of this organism to carry out sulfide oxidation coupled to electron transfer to the electrode.

These three studies demonstrated the possibilities of utilizing extremophilic bioelectrochemical systems to remediate various inorganic sulfur pollution streams. The advancing molecular microbiological tools facilitated the investigation towards the composition and metabolic state of the microbial community. Challenges remain in a more thorough understanding regarding the metabolism of extremophiles (e.g. sulfur metabolism and extracellular electron transfer) and better energy recovery in bioelectrochemical systems.