PhD defence by Alexandre Nguyen-tiêt

On Friday, 24 October 2025 at 11:00, Alexandre Nguyen-tiêt from DTU Aqua will defend his PhD thesis “Microbiology of hydrogen sulfide production in recirculating aquaculture systems.”

You can follow the defence online in Zoom via this link or in person in Atriumsalen at DTU Hirtshals Campus, Willemoesvej 2, 9850 Hirtshals. There will be a lunch break from 12:00-13:00.

Alexandre Nguyen-tiêt’s PhD thesis explores the microbiology behind hydrogen sulfide (H₂S) production in recirculating aquaculture systems, a method for farming fish and aquatic organisms in land-based tanks. While recirculating aquaculture systems offer environmental benefits and efficient resource use, they are vulnerable to sudden outbreaks of H₂S—a toxic gas that can kill fish within hours and pose serious health risks to workers.

Traditionally, H₂S formation in recirculating aquaculture systems has been linked to sulfate-reducing bacteria. However, Alexandre Nguyen-tiêt’s research reveals an alternative and previously overlooked pathway: the microbial degradation of the sulfur-containing amino acid cysteine. Using advanced molecular biology tools and high-resolution microprofiling, he identified the bacterial groups, genes, and enzymes involved in this process.

Overall, the thesis highlights the importance of understanding both sulfate- and cysteine-driven H₂S production. This knowledge can help the aquaculture industry prevent toxic gas buildup.

Examiners

• Senior Researcher Sebastian Politis, DTU Aqua (chair)
• Professor Jeppe Lund Nielsen, Center for Microbial Communities, Section of Biotechnology, Department of Chemistry and Bioscience, Aalborg University
• Assistant Professor Fotini Kokou, Aquaculture & Fisheries Group, Department of Animal Sciences, Wageningen University, The Netherlands

Chair at defence

• Senior Researcher Manuel Gesto, DTU Aqua

Supervisors

• Principal Supervisor: Senior Researcher Sanni-Leea Hellevi Aalto, DTU Aqua
• Co-supervisor: Professor Stefan Bertilsson, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), Sweden

Learn more

Read the popular science summary of Alexandre Nguyen-tiêt PhD thesis

A copy of the thesis is available for reading at DTU Aqua. Please contact Susan Zumbach Johannesen, szjo@aqua.dtu.dk