Iron Reduction Controls Carbon Mineralization in Aquaculture Shrimp Pond Sediments in Subtropical Estuaries

Expanding worldwide aquaculture has greatly increased greenhouse gas emissions; however, the underlying microbial mechanisms are poorly understood. In particular, the role of ferric iron [Fe(III)] (hydro)oxides in carbon mineralization in aquaculture pond sediments remains unclear. Here, we studied the rates of microbial Fe(III) reduction, sulfate reduction, methanogenesis, and carbon mineralization in aquaculture shrimp (Litopenaeus vannamei) ponds of various salinities before, during, and after shrimp farming in subtropical estuaries in southeast China. Sediment samples (0-10 cm) were collected to investigate the content of iron species, characteristics of organic matter, and abundance of Geobacter, a proxy of iron reducers. Overall, Fe(III) reduction (46.1% 19.1%) dominated carbon mineralization, followed by sulfate reduction (39.6% 16.8%) and methanogenesis (1.5% 1.1%). Microbial Fe(III) reduction contributed more to carbon mineralization during farming than before and after farming. This enhancement in Fe(III) reduction is attributed to a significant increase in Fe(III) content during farming. Additionally, the contributions of microbial Fe(III) reduction to carbon mineralization were lower in the high-salinity ponds than in the low-salinity ponds due to the suppression of sulfate reduction, abiotic Fe(III) reduction by sulfides, and lower oxidation-reduction potential. Our findings demonstrate that microbial Fe(III) reduction plays a significant role in carbon mineralization in aquaculture pond sediments. Future carbon flux prediction models of aquaculture pond systems should fully integrate microbial Fe(III) reduction. Plain Language Summary The expanding global aquaculture industry has greatly increased the carbon mineralization potential (i.e., production of carbon dioxide and methane). Carbon mineralization is mediated by various types of microbial respiration, including iron reduction, sulfate reduction, and methanogenesis. Aquatic sediments are enriched with iron oxides; however, the contribution of microbial iron reduction to carbon mineralization in the aquaculture sediments is poorly understood. Here, we studied the rates and pathways of carbon mineralization in aquaculture shrimp (Litopenaeus vannamei) ponds of varying salinities before, during, and after shrimp farming in subtropical estuaries in southeast China. Our results show that microbial iron reduction has a larger contribution to carbon mineralization during farming than before and after farming. In addition, the contributions of microbial iron reduction to carbon mineralization were lower in the high-salinity ponds than in the low-salinity ponds. Overall, microbial iron reduction contributed approximately 46% to carbon mineralization, followed by sulfate reduction (approximately 40%) and methanogenesis (approximately 2%). Our findings demonstrate that microbial iron reduction plays a significant role in carbon mineralization in aquaculture pond sediments. Microbial Fe(III) reduction should be involved in future carbon flux prediction models of aquaculture pond systems.