Identification of ecosystem-specific markers in dissolved organic matter by ultrahigh resolution tandem mass spectrometry

Ultrahigh resolution mass spectrometry (FTMS) has become an indispensable tool for the study of molecular detail in dissolved organic matter (DOM) but many challenges remain in order to identify overarching ecosystem imprints and unknown marker compounds. Tandem mass spectrometry (FTMSⁿ) has emerged as a suitable tool to achieve these aims. We studied the composition of solid-phase extracted DOM from groundwater transects in two contrasting headwater catchments within the Rio Negro basin (wet riparian valley/ dry upland slope), and compared them to known basin-specific DOM signatures. We hypothesized that the unique climatic and geological setting of the basin would allow for the detection of ecosystem imprints within the aquatic continuum. Isotope information (¹⁸O, ²H, ¹³C, ¹⁴C) was used to assess water and DOC sources. DOM showed ¹⁴C ages < 11 years but indicated molecular differences between sites. N-containing formulae were characteristic of the wet riparian system, probably due to poor drainage, and overprinted the aliphatic DOM signal of adjacent plateau groundwater. In contrast, indicative markers of the dry Campina forests did show strong overlap with known Rio Negro markers. These markers were predicted to be highly oxidized, relatively large (> 300 Da) and aromatic, indicating also similarity with reported markers from temperate podzolic sites. We argue that high DOC levels and low pH (~ 4) likely explain the molecular similarity between temperate conifer forests, tropical broadleaf Campina forests and blackwater streams. Molecular differences between clayey plateau soils and sandy podzols moreover suggested the existence of two major pools of DOM signals: A surface ecosystem signal which is dominated by phenolic compounds and rapidly lost in fine-textured soil, and a microbial signal dominated by higher mass aliphatic and N-containing compounds. Surprisingly, these trends paralleled observations from temperate soil profiles and large-scale hydrological gradients. However, FTMS cannot discern structural isomers, and thus we tested a novel approach to deconvolute FTMS² spectra of mixtures of unknown ecosystem markers through comparison with delta masses of known plant polyphenols. Rare delta masses emerge as a novel tool to trace and identify ecosystem imprints of unknown markers in DOM.