Moss and Peat Leachate Degradability by Heterotrophic Bacteria: The Fate of Organic Carbon and Trace Metals

The respiration of dissolved organic matter (DOM) by aerobic heterotrophic bacterioplankton in boreal surface waters is one of the major factors that regulate CO₂ exchange of lakes and rivers with the atmosphere in arctic and subarctic zones. The DOM that originates from topsoil leaching and vegetation degradation is brought to the lakes by surface flow and is subjected to coagulation and degradation by heterotrophic bacteria, which are well-established processes in the majority of boreal aquatic settings. The behavior of colloids and organic complexes of trace metals during this process is virtually unknown. In this work, we studied the interaction of two model heterotrophic bacteria, soil Pseudomonas aureofaciens and aquatic Pseudomonas reactans, with peat and Sphagnum moss leachates from the permafrost region under controlled laboratory conditions in nutrient-free media. The moss leachate was the better substrate for bacterial survival, with P. reactans exhibiting an order of magnitude higher live cell number compared with P. aureofaciens. In eight-day experiments, we analyzed organic carbon and ∼40 major and trace elements (TEs) during heterotrophic bacteria growth. The total net decrease in the concentration of dissolved organic carbon (DOC) was similar for both bacteria and ranged from 30 mg g_wet⁻¹ to ≤10 mg g_wet⁻¹ during 8 days for the moss and peat leachate, respectively. Despite significant evolutions of pH, DOC, dissolved inorganic carbon (DIC), and cell number, most major (Mg, K, and Ca) and TEs remained nearly constant (within ±30% of the control). Only Fe, Al, P, Zn, Mn, Co, and Ba and to a much lesser extent Cd, Pb, Rare Earth Elements (REEs), U, Ti, and Zr were affected (p < 0.05) by the presence of bacteria relative to the control and exhibited slight to moderate decreases during the experiment. Adsorption onto bacterial surfaces produced fast initial removal of Al, Mn, and Ba and to a lesser degree Cd, Pb, REEs, and U. Intracellular metabolic assimilation mostly affected P, Zn, and Co and progressively decreased their concentrations. Finally, coagulation as individual Fe/Al hydroxides due to DOM removal or pH change could also affect elements that were precipitated with organomineral colloids (Ti and Zr). The degrees of major and TE susceptibility to bacterial activity based on concentration changes during the experiment in both substrates ranged over three orders of magnitude from mg L⁻¹ to µg L⁻¹ and followed the order DOC >> P >> Ba > Zn ≥ Fe ≥ Al > Mn > Cu ≥ Sr > Zr ≥ Ti > Ni ≥ Co > REEs ≥ U > Hf∼Th, which reflected the abundance of the elements in the two substrates. Generally, the soil exopolysaccharide producing bacterium P. aureofaciens in the peat leachate had the greatest impact of the four combinations investigated in this study (two bacteria with two substrates). Under ongoing environmental changes in the boreal zone, the autochthonous processes of bacterioplankton activity are able to decrease the concentrations of a very limited number of TEs, including mainly Fe and several macro- (P) and micro- (Zn, Mn, and Ba) nutrients.