Probing rapid carbon fixation in fast-growing seaweed Ulva meridionalis using stable isotope ¹³C-labelling

The high growth rate of Ulva seaweeds makes it a potential algal biomass resource. In particular, Ulva meridionalis grows up to fourfold a day. Here, we demonstrated strong carbon fixation by U. meridionalis using ¹³C stable isotope labelling and traced the ¹³C flux through sugar metabolites with isotope-ratio mass spectrometry (IR-MS), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), ¹³C-nuclear magnetic resonance spectrometry (¹³C-NMR), and gas chromatography-mass spectrometry (GC-MS). U. meridionalis was first cultured in ¹³C-labelled enriched artificial seawater for 0-12 h, and the algae were collected every 4 h. U. meridionalis grew 1.8-fold (dry weight), and the ¹³C ratio reached 40% in 12 h, whereas ¹³C incorporation hardly occurred under darkness. At the beginning of the light period, ¹³C was incorporated into nucleic diphosphate (NDP) sugars in 4 h, and ¹³C labelled peaks were identified using FT-ICR-MS spectra. Using semiquantitative ¹³C-NMR measurements and GC-MS, ¹³C was detected in starch and matrix polysaccharides after the formation of NDP sugars. Moreover, the 14:10 light:dark regime resulted into 85% of ¹³C labelling was achieved after 72 h of cultivation. The rapid ¹³C uptake by U. meridionalis shows its strong carbon fixation capacity as a promising seaweed biomass feedstock.