Response of Fe(III)-reducing kinetics, microbial community structure and Fe(III)-related functional genes to Fe(III)-organic matter complexes and ferrihydrite in lake sediment

Microbial Fe(III) reduction significantly influences the fate of various elements and contaminants. Previous research has employed different Fe(III)-OM complexes and ferrihydrite to study Fe(III)-reduction-related biogeochemistry processes. However, the effects of adding specific Fe(III)-OM complexes and ferrihydrite on the Fe(III)-reducing bacterial community, Fe(III)-reducing kinetics, and Fe(III)-related functional genes remain largely unexplored. This study applied microcosm experiments and metagenomic analysis of lake sediments with and without amendments of ferrihydrite, Fe(III)-citrate, or Fe(III)-EDTA. Results showed that sediments amended with Fe(III)-citrate and Fe(III)-EDTA exhibited faster Fe(III) reduction rates and more significant changes in bacterial community structures compared to those amended with ferrihydrite. Geobacter and Clostridium were enriched in the sediments amended with Fe(III)-EDTA and Fe(III)-citrate, respectively. Despite a slower reduction rate and lack of enrichment of specific Fe(III)-reducing bacteria, ferrihydrite still led to an increase in the copy numbers of genes related to Fe(III) reduction and iron assimilation in the metagenomes, suggesting an increase in these capacities. These results suggest that introducing various Fe(III)-OM complexes and ferrihydrite into the environment would result in differences in not only Fe(III) reduction rates and Fe(III)-reducing bacterial communities but also in iron-related functional genes. Meanwhile, variations in Fe(III) reduction rates and Fe(III)-reducing bacterial communities do not necessarily correlate with changes in the abundances of functional genes relevant to Fe(III) reduction and iron assimilation in the metagenomes. These results provide a better understanding of the adaptive mechanisms of Fe(III)-reducing bacteria in different environmental systems.