Magnetite nanoparticles modulate microbial nitrate reduction pathways: Implications for nitrogen cycle in ferruginous oceans

Abstract Nitrogen (N) is an essential element for living organisms that limits the productivity of biospheres through time. It has been suggested that ferruginous anoxic waters could enhance the fraction of dissimilatory nitrate reduction to ammonium (DNRA) and conserve the biologically available N in the ecosystems. However, the role of mixed-valent iron [Fe(II)-Fe(III)] minerals (e.g., magnetite) in the microbially-mediated N cycle has been largely overlooked. Here we examine the microbial nitrate reduction in the presence/absence of magnetite nanoparticles (NPs) through a series of anaerobic bench experiments with a common denitrifying bacterium- Pseudogulbenkiania sp. strain 2002. Our results show that NH 4 + yield is remarkably enhanced by the presence of magnetite NPs; otherwise the reduction products are mainly N 2 O/N 2. Upon microbial nitrate reduction, magnetite NPs are partially oxidized to form a maghemite-magnetite solid solution. These findings demonstrate that magnetite NPs can partially modulate microbial nitrate reduction from denitrification to DNRA. This metabolic shift has significant implications for N retention in ferruginous water bodies that predominated the Precambrian oceans.