Molecular characterization of dissolved organic matter in a North Brazilian mangrove porewater and mangrove-fringed estuaries by ultrahigh resolution Fourier Transform-Ion Cyclotron Resonance mass spectrometry and excitation/emission spectroscopy

Abstract

Electrospray ionization (ESI) combined with ultrahigh resolution Fourier Transform-Ion Cyclotron Resonance mass spectrometry (FT-ICR MS) at 9.4 Tesla has been shown to be an ideal tool for the molecular characterization of dissolved natural organic matter (DOM). Here, we apply this technique for tracing DOM as it moves from mangrove porewaters into a Brazilian estuary and assess the changes that occur in the initial stages of outwelling. Comparisons of molecular features of DOM from the porewater and two sites within the adjacent estuary revealed that mass spacing patterns were far more pronounced in the mangrove porewater DOM, and the average molecular weights of estuarine DOM were lower. Calculation of double bond equivalents and Kendrick mass analysis normalized for both methylene (-CH 2−) and oxygen (O) substitution indicated that the lower molecular weights observed in the estuarine DOM were due primarily to loss of high molecular weight, highly unsaturated and/or aromatic components observed in the porewater DOM. Photodegradation is suggested as one mechanism that removes these high molecular weight components once mangrove-derived DOM reaches the estuary. The mass analyses were supported by UV–Vis and 3-D synchronous scanning fluorescence spectroscopy measurements to aid in the characterization of DOM in this unique ecosystem.