Selenium (Se) nanoparticles are often synthesized by anaerobes. However, anaerobic bacteria cannot be directly applied for bioremediation of contaminated top soil which is generally aerobic. In this study, a selenite-reducing bacterium, Citrobacter freundii Y9, demonstrated high selenite reducing power and produced elemental nano-selenium nanoparticles (nano-Se-0) under both aerobic and anaerobic conditions. The biogenic nano-Se-0 converted 45.8-57.1% and 39.1-48.6% of elemental mercury (Hg-0) in the contaminated soil to insoluble mercuric selenide (HgSe) under anaerobic and aerobic conditions, respectively. Addition of sodium dodecyl sulfonate enhanced Hg-0 remediation, probably owing to the release of intracellular nano-Se-0 from the bacterial cells for Hg fixation. The reaction product after remediation was identified as non-reactive HgSe that was formed by amalgamation of nano-Se-0 and Hg-0. Biosynthesis of nano-Se-0 both aerobically and anaerobically therefore provides a versatile and cost-effective remediation approach for Hg-0-contaminated surface and subsurface soils, where the redox potential often changes dramatically. (C) 2016 Published by Elsevier Ltd.