Doctoral Defense in bioengineering - Yixi Su

Doctoral candidate:
Yixi Su

Title of thesis:
Decoding the model diatom Phaeodactylum tricornutum for developing photosynthetic cell factories

Opponents:
Dr. Maria Barbosa, Professor at Wageningen University, Dr Tryggvi Stefansson Chief Operating Officer at Algalíf Iceland 

Advisors:
Dr. Weiqi Fu, Adjunct Professor at the University of Iceland and Professor at Zhejiang University, Dr. Sigurður Brynjólfsson, Professor of Mechanical Engineering, biomechanics and bioengineering at the University of Iceland

Also in the doctoral committee:
Dr. Óttar Rolfsson,Professor of Biochemistry and Systems Biology at the University of Iceland, dr. Ólafur Eysteinn Sigurjónsson Professor and Dean of the School of Technology at Reykjavík University

Chair of Ceremony:
Dr. Rúnar Unnþórsson Head of faculty of Industrial Engineering, Mechanical Engineering and Computer Science at the University of Iceland

Abstract:
Climate change, caused by greenhouse gas emissions, has jeopardized sustainability. The urgent demand for carbon neutrality is driving the development of microalgae-based carbon capture technology. There is a desire to utilize untapped carbon sources from industrial flue gas to produce value-added microalgal biomass. However, direct use of flue gas poses challenges to microalgae cultivation such as inhibitory impacts of acidic pH on algal growth. Adaptive laboratory evolution was conducted on the model diatom Phaeodactylum tricornutum under constant acidic conditions, resulting in adapted strains with improved growth. Meanwhile, stress responses and adaptive mechanisms were investigated through genomics and transcriptomics, reveling important pathways that could confer higher acid resistance on the adapted strains. Several ion/electron carrier genes, which are likely to influence intracellular pH homeostasis, were selected for further reverse genetics studies. Overexpression modifications of the ferredoxin, cation/proton antiporter, and bicarbonate transporter enabled positive growths of corresponding transgenic strains under acidic stress that completely inhibited the wild-type. These modifications alleviated oxidative stress and partially rescued photosynthetic function in the transgenics. While the PtFDX transgenic exhibited transcriptome profiles similar to the WT, overexpressing the proton and bicarbonate pumps caused elevated expression of various transmembrane transporters, which may be responsible for the acidic tolerant trait. Furthermore, the photosynthetic carbon fixation efficiency is the main obstacle to be addressed for developing photosynthetic cell factories. We first decode the circadian regulation mechanism underlying P. tricornutum under conditions without environment cues (i.e., constant light and temperature conditions). These findings pave the way for improving photosynthetic efficiency in microalgae.

About the candidate:
Yixi Su was born in China 1989. He completed his BSc degree in Microbiology from the University of Liverpool in July 2012. Then, he completed am MSc degree in Biological and Chemical Engineering at the University of Sheffield and a MSc degree in Biotechnology at the Technical University of Denmark (DTU). Since 2017 he has been doing doctoral studies in Bioengineering at the Faculty of Industrial Engineering, Mechanical Engineering and Computer Science at the University of Iceland.