The role of microbial Fe reduction in regulating CO2 and CH4 production in an Arctic ecosystem

Background/Question/Methods

Previous research showed that anaerobic respiration using iron (Fe) oxides as terminal electron acceptor contributed substantially to ecosystem respiration (ER) in a drained thaw lake basin (DTLB) on the Arctic coastal plain, but it was not known how widespread this pathway is across the region. In addition, the high availability of Fe(III) in this ecosystem could potentially represses CH4 production by providing a more thermodynamically favorable metabolic pathway. As DTLB age, the surface organic layer thickens, progressively burying the Fe-rich mineral layers. We therefore hypothesized that Fe(III) availability and Fe reduction would decline with basin age, while CH4 production would increase. We studied four DTLB across an age gradient, comparing seasonal changes in the oxidation state of dissolved and extractable Fe pools and the estimated contribution of Fe reduction to ER. We also measured dissolved CO2 and CH4 in soil pore water from these and other replicate DTLB varying in age and related these data to organic layer thickness and other soil properties.

Results/Conclusions

The organic layer thickness did not strictly increase with age for these four sites, though soil Fe levels decreased with increasing organic layer thickness. However, there were high levels of Fe minerals in organic layers, especially in the ancient basin where cryoturbation may have transported Fe upward through the profile. Net reduction of Fe oxides occurred in the latter half of the summer, and contributed an estimated 43-45% to ecosystem respiration in the sites with the thickest organic layers and 61-63% in the sites with the thinnest organic layers. All sites had high concentrations of soluble Fe(II) and Fe(III), explained by the presence of siderophores, and this pool became progressively more reduced during the first half of the summer. Dissolved CH4 was positively correlated and CO2:CH4 ratio was negatively correlated with organic layer thickness, which in turn predicts the content of Fe oxides in the active layer. CO2:CH4 ratios were generally quite high (averages for individual basins ranged from ~300-1000), indicating the competitive effect of alternative electron acceptors. We conclude that Fe(III) reduction contributes broadly to ER in the Arctic coastal plain, and that variation in Fe(III) plays a role in controlling CH4 production as soils develop in these ecosystems.