Iron controls the assembly processes of heterotrophic denitrifying microbial communities

Knowledge of the behavior of microbial communities in ecosystems is limited, yet microbes control partly all element cycles and, in turn, processes influencing pollution and climate change. In particular, heterotrophic denitrification is major process controlling the transformation of nitrogen oxides pollutants, e.g. nitrates (NO4-) and nitrous oxide (N2O), a greenhouse gas. Iron acts as either nutrient or stress in denitrifying microbial communities, yet underlying mechanisms are unclear. We hypothesized that iron plays a vital role in the assembly process of heterotrophic denitrifying microbial communities. To test this hypothesis we cultivated 17 microcosms under carbon- and iron-rich conditions. Geochemical analysis, 16S rRNA gene high-throughput sequencing, and multivariate statistical analysis were implemented to unravel the driving forces that shape the assembly process of microbial communities. Results reveal that iron was the most dominant factor influencing the diversity and composition of heterotrophic denitrifying communities, versus pH, C/N, temperature, nitrate reduction, and iron oxidation rates. Higher Fe concentration induced more clustered microbial interactions. Heterotrophic microbial community assembly is shaped by 90% stochastic processes, whereas deterministic processes rose with Fe(II) concentration. Overall our findings demonstrate that iron controls the balance between the deterministic and stochastic processes of heterotrophic denitrifying microbial communities. These mechanisms may have implications for bioremediation strategies.