Transformation of dissolved organic matter and associated metals in boreal mire and river waters: Effects of biota and sunlight

Despite the undeniable importance of bio- and photo-degradation of dissolved organic matter (DOM) in the carbon (C) cycle of natural waters, the combined effects of these processes on C and trace metal concentrations remain largely unexplored. In particular, a central question remains on the partitioning of major and trace metals and DOM in dissolved, colloidal, and particular fractions in the presence of sunlight and microorganism. In this study, we selected two contrasting yet representative water samples from a European boreal region: an acidic mire (pH = 4.3, DOC = 130 mg L-1) and a neutral boreal river (pH = 7.6, DOC = 39 mg L-1). By analyzing major and trace metal solutes in particulate (0.22-100 µm), colloidal (1 kDa-0.22 µm), and low molecular weight (LMW<1kDa) fractions, we tracked changes in DOM quality and concentrations of major DOC (dissolved organic carbon) pools and trace metals during a 10-day exposure to sunlight and incubation with a native bacterial consortium (with and without sunlight). The susceptibility of organic and inorganic colloids to photo- and biodegradation was influenced by the pH of the aqueous solutions, with significant differences in the behavior of DOC, Fe, and trace metals between mire and river water. However, in both cases, photodegradation was the dominant process over biodegradation, with the most substantial solute concentration changes observed during combined sunlight and bacterial treatment Collectively, the experiments demonstrate the dominant effect of photolysis on the partitioning of DOM, Fe, and trace metals among particulate, colloidal, and low molecular weight (LMW) 'truly' dissolved fractions. Significant changes, ranging from 1.5 to 3 times in solute concentrations, can occur over relatively short exposure periods (< 1 week) and vary substantially between small water bodies with contrasting acidity and DOM concentrations. These findings highlight the need for further research into photolysis and biodegradation processes, with a focus on seasonal variations, residence times, and biotic activity.