Effects of iron and light on microbial nitrogen cycles in the primary nitrite maxima of the eastern Indian Ocean

Abstract

To elucidate the effect of light and iron on the dynamics of nitrogen at the primary nitrite maximum (PNM), observed at the bottom of the euphotic zone of the eastern Indian Ocean, we conducted high-resolution sampling and incubation experiments at three stations. The stations were located at the Bay of Bengal, equatorial region, and South Indian subtropical gyre. The PNM was observed below the subsurface chlorophyll maximum and around the nitracline at all three stations; its magnitude was largest (>1.5 μM) in the Bay of Bengal. The results of the iron enrichment experiments under two different light regimes demonstrated that the growth of phytoplankton communities at the PNM was limited by light availability at all three stations. In the subtropical gyre, co-imitation by iron and light was observed, whereas iron was the secondary limiting factor at the other two stations, suggesting widespread iron limitation on phytoplankton growth in the subsurface waters of the open ocean. However, the changes in the concentrations of ammonium, nitrite, and nitrate suggest that the effect of light and iron deficiency on the release of nitrite by phytoplankton is relatively minor. Instead, active ammonium oxidization by shallow-clade ammonium-oxidizing archaea was observed around the PNM, suggesting that nitrite supply via the tight coupling of ammonium regeneration sustained by high surface productivity and ammonium oxidization maintains the distinct PNM in the subsurface water of the eastern Indian Ocean.