Interplay of iron and sulfur with syntrophic propionate oxidation

In anaerobic environments, different chemical forms of iron and sulfur influence microbial community composition and functions. This study employs mesophilic and thermophilic ammonia-tolerant syntrophic propionate-oxidizing (SPO) cultures to investigate how different iron and sulfur species influence propionate oxidation, as well as downstream syntrophic acetate oxidation and methanogenesis. Elevated concentrations of both Fe³⁺ and Fe²⁺ species strongly inhibited SPO activity and suppressed propionate oxidation by the mesophilic culture. In contrast, FeSO₄ addition to the thermophilic SPO culture markedly enhanced propionate oxidation and methane formation. Notably, neither Na₂SO₄ nor FeCl₂ alone produced comparable stimulation, suggesting that the observed response was driven by a synergistic effect of Fe²⁺ and SO₄ ^2- on the SPO microbial network. Following Fe²⁺ amendment of thermophilic cultures, a bacterium associated with the glycine cleavage pathway became enriched. Subsequently, with the onset of syntrophic propionate and acetate oxidation, the SPO candidate "Candidatus Thermosyntrophopropionicum ammoniitolerans," a syntrophic acetate-oxidizing bacterium affiliated with the family Thermacetogeniaceae, and a hydrogenotrophic methanogen affiliated with the genus Methanothermobacter increased in relative abundance. Overall, the study demonstrates that predicting the outcomes of iron amendments to the anaerobic microbiome demands careful consideration of the prevailing iron and sulfur chemical speciation and their relative molar concentrations, as these factors drive divergent microbial responses under mesophilic and thermophilic conditions. The outcomes support developing targeted strategies to optimize anaerobic digestion and enhance renewable methane yields in high-ammonia biogas systems.