There is considerable variability in the ability of seagrass ecosystems to sequester organic carbon (Corg) in their sediments, which act as natural carbon sinks contributing to climate change mitigation. In this work, we studied the chemistry of two Posidonia seagrass species aiming to elucidate whether differences in chemical composition might explain differences in their Corg sequestration capacity. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and Thermally assisted Hydrolysis and Methylation (THM) GC-MS data showed a remarkable difference in phenolic compound patterns between P. oceanica and P. australis bulk plants and individual organs (leaves, sheaths, roots and rhizomes). The lignin of P. australis generates a series of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) products that are typical of herbaceous plants, whereas P. oceanica is particularly rich in p-hydroxybenzoic acid (pBA) derivatives. The structural characteristics of the lignins were further investigated by two-dimensional Nuclear Magnetic Resonance (2D-NMR) spectroscopy and Derivatization Followed by Reductive Cleavage (DFRC), focusing on sheath tissues. The analyses confirmed important differences in the lignin content (19.8% in P. australis and 29.5% in P. oceanica) and composition between the two species; intriguingly, the cell-walls of P. oceanica sheaths were highly enriched in pBA, a component that was completely absent in P. australis. 2D-NMR and DFRC data further revealed that pBA was esterified to the lignin, acylating the γ-OH of the lignin side-chain. Interestingly, P. oceanica lignin presented an extremely high degree of p-hydroxybenzoylation in both guaiacyl (73%) and syringyl (61%) lignin units; the highest p-hydroxybenzoylation degree reported in plant lignins to date. It is tempting to conclude that the higher Corg storage capacity of P. oceanica ecosystems might be related to the higher abundance of pBA-rich lignin and its recalcitrant nature.

This study has been funded by the Spanish projects CTQ2014-60764-JIN and AGL2017-83036-R (co-financed by FEDER funds). OS was supported by an ARC DECRA DE170101524.

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