Potassium channel KCN11 is required for maintaining cellular osmolarity during nitrogen starvation to control proper cell physiology and TAG accumulation in Chlamydomonas reinhardtii

Abstract

Background: Nitrogen (N) starvation in algae induces a variety of structural and metabolic changes including accumulation of triacylglycerol (TAG). Given the promising prospect of using algae as feedstock for biofuel production, accumulation of TAG upon N starvation becomes an ideal system to study TAG biosynthesis. Under nitrogen-depleted conditions, algae also accumulate compatible solutes such as sugar and certain amino acids, which is expected to elevate osmolarity in the cytoplasm. However, how osmoregulation is maintained and how it impacts on carbon metabolism, especially TAG accumulation under N starvation, are not well understood. Results: We show here that potassium channel KCN11 localized in the contractile vacuole (CV) mediates osmoregulation during N starvation and loss of KCN11 profoundly affects cell physiology and TAG biosynthesis. KCN11 level is increased and the CV pulsation is accelerated. Loss of KCN11 induces aberrant CV cycle, inhibition of cell growth, increase of cell size, inhibition of chlorophyll loss and TAG accumulation. These effects are rescued by addition of sucrose to raise osmolarity in the culture medium, indicating that osmoregulation is required for cell adaptation to N starvation. Metabolomic analysis shows reduction of acetyl-CoA and accumulation of glyceraldehyde-3-phosphate in kcn11 mutant relative to the control under N starvation, indicating that defects in acetyl-CoA biosynthesis and some metabolic steps from glyceraldehyde-3-phosphate to TAG contribute to the decreased TAG accumulation due to loss of osmoregulation. Conclusions: This work provides novel insight of osmoregulation during N starvation in the control of cell physiology and metabolism especially TAG accumulation. According to these findings, we propose that osmolarity should be carefully monitored during the industrial production of biodiesel.