Nutrient regulation of biological nitrogen fixation across the tropical western North Pacific

Abstract

Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N 2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N 2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N 2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N 2 fixation across the tropical ocean.

Phytoplankton photophysiology across tropical eddies: Deconvolving nutrient, light, and community signals

“…For example, our correction using blank fluorescence from deionized water instead of filtered seawater might have underestimated F v /F m m e a s u r e m e n t s ( C u l l e n a n d D a v i s , 2 0 0 3 ) . I f t h e underestimation of F v /F m in this study was true, the categorization of the SCS should possibly be the regime "ironsufficient with low macronutrients," consistent with the reported Fe and nitrate data in the SCS waters in literatures (Wu et al, 2003;Wen et al, 2022). Such speculation should be examined in future nutrient addition experiments.…”

“…Antenna proteins similar with IsiA are also found in lowlight ecotypes of Prochorococcus (Bibby et al, 2003). However, this hypothesis of Fe deficiency in surface waters of the SCS basin appears unlikely, as the measured nitrate concentrations were 0.002-0.2 μM and previously reported Fe concentrations for this region were 0.2-0.3 nM (Wu et al, 2003;Wen et al, 2022), by assuming a threshold of 10:1 N:Fe ratio (μM:nm) for Fe limitation (Browning et al, 2017). Perhaps more likely is that absolute values of F v /F m are influenced by the active fluorescence measurement protocol and the instrument used and as such should be evaluated with caution (as discussed in Section 4.2).…”

Aerosol Nutrients and Their Biological Influence on the Northwest Pacific Ocean (NWPO) and Its Marginal Seas

“…For example, addition of Saharan mineral dust with a lower N:P ratio into the Eastern Mediterranean stimulated N 2 fixation rates more prominently compared to anthropogenic European aerosols with a higher N:P ratio [ 94 ]. A recent study reported supply ratio of Fe:N from subsurface layers is the most important factor in regulating diazotroph abundances and N 2 fixation rates across the tropical NWPO, while phosphate availability sets an upper limit of total amount of fixed N [ 95 ]. As the atmospheric P inputs were strongly depleted relative to N and Fe in the context of the stoichiometry of phytoplankton Fe, N, P requirements, especially in anthropogenic aerosols [ 96 ], deposition of anthropogenic East Asian aerosol may fuel diazotrophs with more stoichiometrically available Fe than P in the NWPO region and its marginal seas by providing anthropogenic East Asian aerosols, causing the switch of N 2 fixation from Fe to P limitation.…”

“…As the atmospheric P inputs were strongly depleted relative to N and Fe in the context of the stoichiometry of phytoplankton Fe, N, P requirements, especially in anthropogenic aerosols [ 96 ], deposition of anthropogenic East Asian aerosol may fuel diazotrophs with more stoichiometrically available Fe than P in the NWPO region and its marginal seas by providing anthropogenic East Asian aerosols, causing the switch of N 2 fixation from Fe to P limitation. In the northern SCS receiving East Asian aerosols, co-limitation of N 2 fixation by both Fe and P have been demonstrated by nutrient addition assays [ 95 ]. However, the effect of atmospheric input on N 2 fixation in NWPO is still unknown.…”

A novel porous graphitic carbon nitride (g-C7N3) substrate: prediction of metal-based π–d conjugated nanosheets toward the highly active and selective electrocatalytic nitrogen reduction reaction

“…To improve the NRR performance of SACs, finding suitable substrates to support single transition metal (TM) atoms and further form SACs featuring TM–N coordinate sites that mimic biological nitrogen fixation is crucial. 27–30 With the superiority of maximizing single-atom active centers, two-dimensional (2D) materials, such as graphene, 31 graphitic carbon nitrides (g-C x N y , such as g-CN, 1 g-C 2 N 33 and g-C 3 N 4 35 ), and graphdiyne 36,37 have been explored as supporting materials of SACs.…”