Illuminated darkness: Molecular signatures of Congo River dissolved organic matter and its photochemical alteration as revealed by ultrahigh precision mass spectrometry

Abstract

Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole‐water (i.e., unfractionated) DOM was analyzed pre‐ and post‐irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT‐ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo‐resistant, (2) photo‐labile, and (3) photo‐produced. Photo‐resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with > 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT‐ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic‐rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon‐like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo‐labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean.

Chemodiversity of dissolved organic matter in lakes driven by climate and hydrology

“…This finding is further ARTICLE substantiated by an independent photodegradation experiment on DOM in a tropical river system 13 . Direct comparison of compounds in our study with those from this independent photodegradation experiment 13 revealed that a striking 78% of compounds associated with lower levels of precipitation and 55% of compounds associated with longer WRT in our study were identified as photoresistant 13 ( Fig. 4b,d).…”

“…4b,d). In addition, 64% of compounds associated with high precipitation and 77% of compounds associated with shorter WRT in our study were independently identified as photolabile 13 (Fig. 4a,c).…”

“…The burgeoning use of mass spectrometry in DOM studies has tremendously advanced our molecular-level understanding of DOM dynamics [11][12][13][14][15] . Recent developments in data processing now enable us to analyze the detailed molecular composition of DOM as a function of environmental gradients over a large number of samples.…”

“…As the chemical composition of DOM constrains its microbial degradation 17 , chemodiversity may be useful in setting a baseline for the degradation potential of a system. Compounds range from photoreactive molecules like combustion-derived polycyclic aromatics, vascular plant-derived polyphenols, and highly unsaturated and phenolic compounds, to photoresistant aliphatic compounds and peptides 13 . Photoreactive compounds are often thought to be biologically recalcitrant, with aliphatic compounds and peptides being more bioavailable; however, allochthonous carbon has also been shown to be an important substrate for bacterial growth 18 , and photochemical transformations can enhance the bacterial mineralization of previously recalcitrant DOM 19 .…”

Deciphering associations between dissolved organic molecules and bacterial communities in a pelagic marine system

“…In accordance to their findings, we hypothesize that the DOM compounds and the total BC that reach the North Sea undergo mixing and transformation processes from the coast toward the Atlantic on similar timescales of several months and thus retain the intermediate history of the water masses, the 'riverine legacy'. Although taking place on similar timescales, processes influencing DOM and total BC composition are probably different: for example, the BC may respond to changes in temperature or nutrient supply (Fuhrman et al, 2008), whereas the DOM may be more affected by abiotic adsorption/desorption to surfaces (Gogou and Repeta, 2010), flocculation (Mikes et al, 2004) or photochemical processes (Stubbins et al, 2010).…”