Lipid turnover through lipophagy in the newly identified extremophilic green microalga Chlamydomonas urium

Abstract

Autophagy is a central degradative pathway highly conserved among eukaryotes, includingmicroalgae, which remains unexplored in extremophilic organisms. In this study, we describedand characterized autophagy in the newly identified extremophilic green microalga Chlamy-domonas urium, which was isolated from an acidic environment. The nuclear genome of C. urium was sequenced, assembled and annotated in order toidentify autophagy-related genes. Transmission electron microscopy, immunoblotting, meta-bolomic and photosynthetic analyses were performed to investigate autophagy in this extre-mophilic microalga. The analysis of the C. urium genome revealed the conservation of core autophagy-relatedgenes. We investigated the role of autophagy in C. urium by blocking autophagic flux withthe vacuolar ATPase inhibitor concanamycin A. Our results indicated that inhibition of autop-hagic flux in this microalga resulted in a pronounced accumulation of triacylglycerols and lipiddroplets (LDs). Metabolomic and photosynthetic analyses indicated that C. urium cells withimpaired vacuolar function maintained an active metabolism. Such effects were not observedin the neutrophilic microalga Chlamydomonas reinhardtii. Inhibition of autophagic flux in C. urium uncovered an active recycling of LDs through lipo-phagy, a selective autophagy pathway for lipid turnover. This study provided the metabolicbasis by which extremophilic algae are able to catabolize lipids in the vacuole