5:15 PM - 6:30 PM
[AOS12-P01] Nitrogen cycling in the primary nitrite maximum of the Indian Ocean
Keywords:Trace metal, Nitrogen cycle, Indian Ocean, Nitrite, Iron
The mechanisms that maintains a peak in nitrite concentrations at the bottom of the euphotic zone (primary nitrite maximum, PNM) remains unclear, despite data supporting either microbial nitrification or phytoplankton release of nitrite. In this study, based on field observations and on board incubation experiments conducted at three stations in the Bay of Bengal (BB), Equatorial Region (ER), and Subtropical South Indian Ocean (SI) along 88 degree E in the Indian Ocean, we tested the hypothesis that iron limitation stimulates the release of nitrite from phytoplankton cells into ambient water near the subsurface chlorophyll maximum (SCM) by inhibiting the nitrite metabolic pathway.
In BB, the PNM layer was shallower than 1% light depth and appeared near the bottom of the subsurface chlorophyll maximum. In contrast, in ER and SI, the PNM layer was deeper than 1% light depth and was formed between the subsurface chlorophyll maximum and the upper part of the nitracline. Ammonia oxidation rates measured under dark conditions increased with depth within the euphotic layer in BB. However, in ER and SI, it was not detected within the euphotic layer and increased from layers deeper than 1% light depth. Among the ammonia-oxidizing archaea, Shallow Marine Clade (SMC) appeared in all subsurface layers and showed a tendency to increase in abundance with depth, while Deep Marine Clade (DMC) was detected only in layers deeper than around 1% light depth. The SMC accounted for more than 95% of the ammonia-oxidizing archaeal community detected near the PNM layer, suggesting a relationship with nitrite accumulation. The effects of Fe and light on the plankton community near the PNM suggested that a lack of light or iron in BB, a lack of light in ER, and a lack of both Fe and light in SI may be responsible for the release of nitrite. The changes in nitrite concentration during the incubation experiments indicated that the regulation of ammonia oxidation by light and nitrite oxidation by iron in ER. Based on these results, it is clear that the effects of light intensity and Fe availability on microbial nitrification and phytoplankton nitrogen metabolism near the PNM are different in the northern and southern Indian Ocean.
In BB, the PNM layer was shallower than 1% light depth and appeared near the bottom of the subsurface chlorophyll maximum. In contrast, in ER and SI, the PNM layer was deeper than 1% light depth and was formed between the subsurface chlorophyll maximum and the upper part of the nitracline. Ammonia oxidation rates measured under dark conditions increased with depth within the euphotic layer in BB. However, in ER and SI, it was not detected within the euphotic layer and increased from layers deeper than 1% light depth. Among the ammonia-oxidizing archaea, Shallow Marine Clade (SMC) appeared in all subsurface layers and showed a tendency to increase in abundance with depth, while Deep Marine Clade (DMC) was detected only in layers deeper than around 1% light depth. The SMC accounted for more than 95% of the ammonia-oxidizing archaeal community detected near the PNM layer, suggesting a relationship with nitrite accumulation. The effects of Fe and light on the plankton community near the PNM suggested that a lack of light or iron in BB, a lack of light in ER, and a lack of both Fe and light in SI may be responsible for the release of nitrite. The changes in nitrite concentration during the incubation experiments indicated that the regulation of ammonia oxidation by light and nitrite oxidation by iron in ER. Based on these results, it is clear that the effects of light intensity and Fe availability on microbial nitrification and phytoplankton nitrogen metabolism near the PNM are different in the northern and southern Indian Ocean.