Ammonia oxidation to nitrite and its subsequent oxidation to nitrate provides energy to the two populations of nitrifying chemoautotrophs in the energy-starved dark ocean, driving a coupling between reduced inorganic nitrogen (N) pools and production of new organic carbon (C) in the dark ocean. However, the relationship between the flux of new C production and the fluxes of N of the two steps of oxidation remains unclear. Here, we show that, despite orders-of-magnitude difference in cell abundances between ammonia oxidizers and nitrite oxidizers, the two populations sustain similar bulk N-oxidation rates throughout the deep waters with similarly high affinities for ammonia and nitrite under increasing substrate limitation, thus maintaining overall homeostasis in the oceanic nitrification pathway. Our observations confirm the theoretical predictions of a redox-informed ecosystem model. Using balances from this model, we suggest that consistently low ammonia and nitrite concentrations are maintained when the two populations have similarly high substrate affinities and their loss rates are proportional to their maximum growth rates. The stoichiometric relations between the fluxes of C and N indicate a threefold to fourfold higher C-fixation efficiency per mole of N oxidized by ammonia oxidizers compared to nitrite oxidizers due to nearly identical apparent energetic requirements for C fixation of the two populations. We estimate that the rate of chemoautotrophic C fixation amounts to ∼1 × 1013to ∼2 × 1013mol of C per year globally through the flux of ∼1 × 1014to ∼2 × 1014mol of N per year of the two steps of oxidation throughout the dark ocean.
Water desalination
performance of capacitive deionization (CDI)
largely depends on electrode materials properties. Rational design
and regulation of the structure and composition of electrode materials
to acquire high CDI performance is of great significance. Herein,
nitrogen-doped hollow mesoporous carbon spheres (N-HMCSs) were investigated
as electrode material for CDI application. To understand the effect
of structure and composition on CDI performance, another two CDI electrode
materials, i.e., hollow mesoporous carbon spheres (HMCSs) and solid
mesoporous carbon spheres (SMCSs) were prepared for comparison. The
obtained N-HMCSs possessed unique hollow cavity and excellent nitrogen
doping property, resulting in fast ion diffusion, good charge transfers
ability and fine wettability. Compared with HMCSs and SMCSs electrodes,
N-HMCSs electrode exhibited an improved electrosorption capacity and
rate, demonstrating the dependence of CDI performance on the synergistic
effect of hollow structure and nitrogen doping property. N-HMCSs electrode
also present excellent cycle stability over 20 adsorption–desorption
cycles. These results indicate the promising prospect of N-HMCSs for
CDI application.
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