Molecular profile of labile and recalcitrant dissolved organic matter in coastal vegetated communities

Vegetated coastal communities are main sources of the marine dissolved organic matter (DOM), which may enter into the food chain (i.e. labile DOM) or remain stored in the ocean for longer periods (i.e. recalcitrant DOM), contributing to the blue carbon pool [1]. In particular, microbial utilization and processing of labile molecules of DOM is a key process that modifies the chemical composition and reactivity of DOM, ultimately resulting in the accumulation of resistant molecules [2]. In recent years, a growing number of studies have shown that the chemical characterization of DOM at molecular level using ultra-high resolution mass spectrometry (UHRMS) can provide key information on the sources, transformations, and fate of marine DOM [3]. This study was aimed to characterize at molecular level the labile, bacterial metabolism-derived, and recalcitrant fractions of DOM associated to three blue-carbon communities: the seagrasses Cymodocea nodosa and Zostera noltei, and the macroalga Caulerpa prolifera. For this purpose, a bioavailability experiment was conducted using seawater (free of microorganisms) from each community and a coastal bacterial inoculum. The viability of the cultures was confirmed by the decrease of dissolved organic carbon concentration and the increase of bacterial abundance observed in all communities at the end of the experiment. The solid-phase extraction of DOM followed by UHRMS analyses allowed the assignment of molecular formulas to compounds present in DOM at the beginning and at the end of experiment. The results showed that the percentage of molecular formulae that disappeared during bacterial cultivation (i.e., labile compounds) varied among communities, with the following trend: C. prolifera (55%) > C. nodosa (50%) > Z. noltei (38%). Representation of these molecular formulae in a van Krevelen diagram showed that a significant number of them were in the regions of compounds considered to be easily bioavailable, such as lipid-, peptide-, amino sugar- and carbohydrate-like compounds. On the other hand, the molecular formulae that were detected at the beginning and at the end of the culture (9-12%) were assigned to compounds resistant to degradation, and most of them fell in the diagram within the chemical classes expected for recalcitrant molecules (lignin- and tannin-like regions). These results provide insights into the molecular composition of DOM in blue carbon ecosystems, showing that the lability/recalcitrance of DOM, and hence the potential contribution to the blue carbon pool, seems to depend on the dominant species. [1] Carlson, C. A. and Hansell, D. A. 2015. “DOM sources, sinks, reactivity, and budgets” In Biogeochemistry of marine dissolved organic matter (second edition), edited by D. A. Hansell and C. A. Carlson. Academic Press, Boston, MA, 65-126 pp.[2] Li, H., Zhang Z., Xiong, T., Tang, K., He, C., Shi, Q., Jiao, N., Zhang, Y. 2022. Carbon sequestration on the form of recalcitrant dissolved organic carbon in a seaweed (kelp) farming environment. Environ. Sci. Technol. 56: 9112-9122.[3] Qi, Y., Q. Xie, J. J. Wang, et al. 2022. “Deciphering dissolved organic matter by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS): from bulk to fractions and individuals.” Carbon Res. 1: 3.