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Highlights • Molecular composition of Posidonia oceanica and P. australis seagrasses compared. • Lignin of P. oceanica has an extraordinary abundance of p -hydroxybenzoic acid. • p -hydroxybenzoates attach to the γ-OH of the lignin side-chains. • Lignin composition may explain higher carbon storage capacity of P. oceanica. Abstract There is considerable variability in the ability of seagrass ecosystems to sequester organic carbon (C org) 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 C org 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 (p BA) 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 p BA, a component that was completely absent in P. australis. 2D-NMR and DFRC data further revealed that p BA 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 C org storage capacity of P. oceanica ecosystems might be related to the higher abundance of p BA-rich lignin and its recalcitrant nature. [ABSTRACT FROM AUTHOR]

Posidonia oceanica forms extensive peat-like deposits (mats) in Mediterranean coastal waters, which have a potential as carbon sinks and archives of environmental change. Nonetheless, the organic chemistry of both P. oceanica plant materials, as well as the environmental and diagenetic effects on the composition of its detritus, is poorly understood. We analyzed plant organs of P. oceanica and the coarse organic matter from a mat core spanning 750 yrs using pyrolysis techniques (PY-GC–MS and THM-GC–MS) to improve our understanding of their molecular properties and their preservation upon mat development. It appeared that leaf sheaths, roots and the outer parts of rhizomes were composed predominantly of phenolic constituents based on p -hydroxybenzoic acid ( p -HBA), which is atypical for vascular plants, in addition to carbohydrates and lignin. The inner rhizome and leaf blades had a different composition, with predominance of carbohydrates. The seagrass detritus in the mat was composed mainly of p -HBA phenolic material and carbohydrates, confirming earlier studies showing that the coarse detritus in the mat originates largely from Posidonia sheath, root and rhizome debris. The intermolecular arrangement of the p -HBA remains unclear, as they seem to correspond to ester-bound phenols yet their persistence in the mat attributes them a refractory nature. Variations in molecular composition within the mat are probably associated with diagenetic alteration of P. oceanica detritus, such as the decomposition of minor amounts of fatty acids, chlorophyll and syringyl lignin, and selective preservation of p -HBA relative to carbohydrates. This work lays the foundations for a molecular understanding of carbon storage within the mats and the environmental changes recorded therein. [ABSTRACT FROM AUTHOR]

• Molecular analysis of 3200 yr Posidonia australis seagrass soil record (Australia). • Tracing diagenetic effects on sheaths and root/rhizome detritus. • Ageing-induced selective degradation of polysaccharides and enrichment of lignin. • Minor constituents cutin, suberin and tannin also preferentially depleted. • Overall C storage capacity associated with lignocellulose chemical recalcitrance. Seagrass ecosystems are recognised for their role in climate change mitigation, due to their capacity to form organic-rich sediments. The chemical recalcitrance of seagrass organs is one characteristic driving carbon storage, but the molecular background of this feature is poorly understood. We assessed molecular composition changes of Posidonia australis sheaths (SH) and roots plus rhizomes (RR) along a sediment core, encompassing 3200 cal. yr BP, by means of nuclear magnetic resonance spectroscopy (13C NMR), conventional analytical pyrolysis (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). Significant trends with depth (age) in the composition of both SH and RR remains of P. australis were observed from all methods. In general terms, polysaccharides become depleted (degraded) and lignin enriched (selectively preserved) as age increases, and the minor constituents cutin, suberin and condensed tannin are also preferentially depleted during ageing in both fractions. Molecular changes with ageing were smaller in SH, especially regarding polysaccharides, indicative of a superior stability compared to RR. The molecular changes observed are most pronounced within the first 75 cm of the record, which reflects the recalcitrance of P. australis detritus once it is buried below that depth (corresponding to approximately 700 cal. yr BP). The capacity of P. australis to act as a long-term carbon sink seems to be mainly related to the resistance of buried lignocellulose materials to decomposition. The results on diagenetic effects on the molecular fingerprint of seagrass detritus contribute to our understanding of carbon sequestration in Blue Carbon ecosystems. Furthermore, data comparison of the methods applied using principal component analysis (PCA) allowed us to identify consistencies, discrepancies and complementarities. [ABSTRACT FROM AUTHOR]

Blue carbon ecosystems (BCE) play an essential role in the global carbon cycle by removing atmospheric carbon dioxide and storing it as organic carbon (OC) in biomass and sediments. However, organic matter (OM) deposition and degradation/preservation processes are poorly understood, especially on the long-term and at molecular scales. We analysed sediment samples from six cores collected in tidal marshes, mangroves and seagrasses (up to 150 cm long cores spanning up to 10,000 yrs of OC accumulation) from Spencer Gulf (South Australia), by pyrolysis (Py-GC–MS and THM-GC–MS), and we compared the results with elemental and stable isotope data, to decipher OM provenance and to assess degradation/preservation dynamics. The results showed that: (1) the major biopolymers preserved were polysaccharides, polyphenolic moieties (lignin and tannin) and polymethylenic moieties (e.g. cutin, suberin, chlorophyll) with smaller apportions of proteins and resins; (2) the OM originates predominantly from vascular plant materials (in particular lignocellulose) that have been well-preserved, even in some of the oldest sediments; (3) mangroves were found to be the most efficient OC sinks, partially explained by syringyl lignin preservation; (4) seagrasses were shown to store polysaccharide-enriched OM; (5) large proportions of polycyclic aromatic hydrocarbons (PAHs) in surficial tidal marsh and mangrove sediments probably reflect pyrogenic OM from industrial combustion, and; (6) "ecosystem shifts", i.e. mangrove encroachment in tidal marsh and transition from seagrass to mangrove, were detected. Deposition environment and source vegetation control OC sequestration and there is no specific recalcitrant form of OM that is selectively preserved. For the first time, we demonstrate how analytical pyrolysis in combination with stable isotope analysis can be used to reconstruct (palaeo-)ecological shifts between different BCE. This study improves our knowledge on OC accumulation dynamics and the response of BCE to environmental change, which can inform the implementation of strategies for climate change mitigation. Unlabelled Image • Organic matter (OM) sources in blue carbon ecosystems studied by molecular analysis • Preservation of OM allows deciphering ecosystem shifts on a millennial timescale. • Polysaccharides, lignin and suberin moieties are major OM constituents. • Mangrove encroachment in marsh and seagrass-mangrove transition enhanced C storage. • Industrial activities and possibly palaeofires were revealed by molecular analysis. [ABSTRACT FROM AUTHOR]