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

Shuntaro Tsubaki,Tomoya Imai,Ayumu Onda,Masanori Hiraoka,Hiroshi Nishimura

doi:10.1038/s41598-020-77237-1

Shuntaro Tsubaki, Tomoya Imai + Show 3 more

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https://doi.org/10.1038/s41598-020-77237-1

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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.

Highlights

The high growth rate of Ulva seaweeds makes it a potential algal biomass resource
Ulva seaweeds are characterised by matrix polysaccharides that are heteropolysaccharides consisting of rhamnose (Rha), glucuronic acid (GlcA), iduronic acid (IdoA), and s ulfates[22,23,24]
Biosynthesis of ulvan occurs from nucleic diphosphate (NDP) sugar, similar to pectin biosynthesis in higher plants23. d-fructose-6-P supplied from photosynthesis and glucogenesis is converted to uridine diphosphate glucose (UDP-Glc) through several steps

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Introduction

The high growth rate of Ulva seaweeds makes it a potential algal biomass resource. In particular, Ulva meridionalis grows up to fourfold a day. The “germling cluster method” allows mass production of Ulva by clustering the algae bodies and cultivating them in high density floating Ulva clusters in a land tank This method provides efficient exposure for Ulva growth by three-dimensionally stirring the clusters. Ulva are collected by a coarse filtration, which is much easier than harvesting microalgae biomass that requires flocculation and centrifugation This efficient cultivation system enhances Ulva growth and captures more CO2. Efficient photosynthesis can be maintained under strong light by protecting photosystems I and II from excess electrons ( e−) by trapping them in x anthophyll[20,21] Various polysaccharides such as storage, cell wall, and matrix polysaccharides are formed through biosynthetic pathways after carbon fixation in the Calvin cycle.

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