Chemical characterization of dissolved organic matter in three blue carbon communities

Abstract

Vegetated coastal (i.e., blue carbon) communities actively contribute to marine dissolved organic matter (DOM), which may enter the food chain (labile DOM) or remain for longer periods, for example stored in the deep ocean, and contribute to the blue carbon pool (recalcitrant DOM). In recent years, the chemical characterization of DOM at molecular level has proven to provide key information about the sources, transformations, and fate of marine DOM. This study aimed to investigate the molecular composition of DOM in three coastal communities, the seagrasses Cymodocea nodosa and Zostera noltei , and the macroalga Caulerpa prolifera , using nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS). 1H NMR spectra were closely similar for these communities and revealed aromatic features consistent with terrestrial DOM inputs. HRMS analysis of DOM assigned a total of 2492 molecular formulae. All the communities showed similar chemical profiles with most formulae attributable to highly unsaturated (58.8-62.3%) and aromatic (16.9-24.7%) compounds. In a bioavailability assay, C. prolifera community had the highest percentage of labile compounds (55%), followed by C. nodosa (50%) and Z. noltei (38%). According to van Krevelen diagrams, a significant fraction of labile compounds in all communities belonged to lipid-, peptide-, amino sugar-, and carbohydrate-like classes. These results provide novel insights into the molecular composition of DOM in blue carbon ecosystems and are consistent with inputs from seagrasses, terrestrial sources, and plants from adjacent salt marshes. The percentage of labile compounds in DOM seems dependent on the dominant species, and therefore, their potential contribution to the blue carbon pool.