Role of Siderophores in Dissimilatory Iron Reduction in Arctic Soils : Effect of Direct Amendment of Siderophores to Arctic Soil

Abstract:

Dissimilatory iron reduction (DIR), where ferric iron (Fe³⁺) is reduced to ferrous iron (Fe²⁺) anaerobically, is an important respiratory pathway used by soil bacteria. DIR contributes to carbon dioxide (CO₂) efflux from the wet sedge tundra biome in the Arctic Coastal Plain (ACP) in Alaska, and could competitively inhibit the production of methane, a stronger greenhouse gas than CO₂, from arctic soils. The occurrence of DIR as a dominant anaerobic process depends on the availability of substantial levels of Fe³⁺ in soils. Siderophores are metabolites made by microbes to dissolve Fe³⁺ from soil minerals in iron deficient systems, making Fe³⁺ soluble for micronutrient uptake. However, as the ACP is not iron deficient, siderophores in arctic soils may play a vital role in anaerobic respiration by dissolving Fe³⁺ for DIR. We studied the effects of direct siderophore addition to arctic soils through a field study conducted in Barrow, Alaska, and a laboratory incubation study conducted at San Diego State University. In the field experiment, 50μM deferroxamine mesylate (a siderophore), 50μM trisodium nitrilotriacetate (an organic chelator) or an equal volume of water was added to isolated experimental plots, replicated in clusters across the landscape. Fe²⁺ concentrations were measured in soil pore water samples collected periodically to measure DIR over time in each. In the laboratory experiment, frozen soil samples obtained from drained thaw lake basins in the ACP, were cut into cores and treated with the above-mentioned compounds to the same final concentrations. Along with measuring Fe²⁺ concentrations, CO₂ output was also measured to monitor DIR over time in each core. Experimental addition of siderophores to soils in both the field and laboratory resulted in increased concentrations of soluble Fe³⁺ and a sustained increase in Fe²⁺concentrations over time, along with increased respiration rates in siderophore-amended cores. These results show increased DIR in siderophore treated cores compared to the other treatments. From the results of these experiments, we conclude that arctic soil microbes can use siderophores to maintain a pool of dissolved Fe³⁺ for DIR. This study provides insight into the mechanisms of DIR in this ecosystem, and has relevance for understanding anaerobic soil respiration in the Arctic.