From the peat bog to the estuarine mixing zone: Common features and variances in riverine dissolved organic matter determined by non-targeted analysis

Abstract

Abstract Land-derived dissolved organic matter (DOM) is able to transport iron and other trace metals from natural organic matter (NOM)-rich areas like peat bogs via rivers to the open ocean and therefore plays a pivotal role in the iron supply of coastal marine organisms. Iron is distributed over all DOM size-fractions and predominantly found in large clusters in river waters, but a shift towards lower molecular weight is observed on the way to the mixing zone. As the involved ligands are still largely unknown, a workflow was established targeting low molecular weight DOM constituents between 100 and 700 Da to provide an additional tool for DOM characterization employing the high performance of time-of-flight mass spectrometry (TOFMS) in the low mass range. An estuarine mixing zone sample from the Halladale River with high iron content, a natural organic matter-(NOM) rich creek water sample from Craggie Burn, which is a tributary of the Halladale River (Scotland, UK), and a NOM sample from the Suwannee River (Georgia, USA) were investigated by reversed phase and hydrophilic interaction liquid chromatography combined with electrospray-ionization time-of-flight mass spectrometry. Non-targeted evaluation of the accurate mass data employing state-of-the-art feature detection and alignment tools was performed. Several compounds could be identified utilizing authentic standards clearly demonstrating that the employed workflow is able to provide valid information about low molecular weight constituents of DOM. It was shown that the three investigated samples share a significant proportion of the annotated molecular formulas though they vary in seawater content and origin, especially in the protein type region of the van Krevelen plot. Those compounds seem to be resistant to salting out or further degradation. Moreover, the lignin and tannin type region is largely overlapping in the Craggie Burn and the Suwannee River samples, but almost completely absent in the Halladale River sample. Therefore, our findings support the view that land-derived lignin and tannin structures can transport iron to the estuary, where they are degraded and ligand exchange takes place.