Acidification of seawater caused by anthropogenic carbon dioxide (CO) is anticipated to influence the growth of dinitrogen (N)-fixing phytoplankton, which contribute a large fraction of primary production in the tropical and subtropical ocean. We found that growth and N-fixation of the ubiquitous cyanobacterium decreased under acidified conditions, notwithstanding a beneficial effect of high CO Acidification resulted in low cytosolic pH and reduced N-fixation rates despite elevated nitrogenase concentrations. Low cytosolic pH required increased proton pumping across the thylakoid membrane and elevated adenosine triphosphate production. These requirements were not satisfied under field or experimental iron-limiting conditions, which greatly amplified the negative effect of acidification.

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.

Abstract. Dinitrogen fixation (NF) by marine cyanobacteria is an important pathway to replenish the oceanic bioavailable nitrogen inventory. Light is the key to modulating NF; however, field studies investigating the light response curve (NF-I curve) of NF rate and the effect of light on diazotrophderived nitrogen (DDN) net release are relatively sparse in the literature, hampering prediction using models. A dissolution method was applied using uncontaminated 15 N 2 gas to examine how the light changes may influence the NF intensity and DDN net release in the oligotrophic ocean. Experiments were conducted at stations with diazotrophs dominated by filamentous cyanobacterium Trichodesmium spp. in the western Pacific and the South China Sea. The effect of light on carbon fixation (CF) was measured in parallel using the 13 C tracer method specifically for a station characterized by Trichodesmium bloom. Both NF-I and CF-I curves showed a I k (light saturation coefficient) range of 193 to 315 µE m −2 s −1 , with light saturation at around 400 µE m −2 s −1 . The proportion of DDN net release ranged from ∼ 6 to ∼ 50 %, suggesting an increasing trend as the light intensity decreased. At the Trichodesmium bloom station, we found that the CF / NF ratio was light-dependent and the ratio started to increase as light was lower than the carbon compensation point of 200 µE m −2 s −1 . Under low-light stress, Trichodesmium physiologically preferred to allocate more energy for CF to alleviate the intensive carbon consumption by respiration; thus, there is a metabolism tradeoff between CF and NF pathways. Results showed that shortterm (< 24 h) light change modulates the physiological state, which subsequently determined the C / N metabolism and DDN net release by Trichodesmium. Reallocation of energy associated with the variation in light intensity would be helpful for prediction of the global biogeochemical cycle of N by models involving Trichodesmium blooms.

The N2 fixation and primary production rates were measured simultaneously using 15N2 and 13C incubation assays in the northern South China Sea influenced by the Kuroshio intrusion (KI) seasonally. The degree of KI (KI index, range from 0 to 1) was assessed by applying an isopycnal mixing model. The water column integrated N2 fixation and primary production for stations with KI index larger than 0.5 were 463 ± 260 μmol N·m−2·day−1 and 62 ± 19 mmol C·m−2·day−1, respectively, significantly higher than those for stations with KI index lower than 0.5 (50 ± 10 μmol N·m−2·day−1 and 28 ± 10 mmol C·m−2·day−1, respectively). Trichodesmium was the dominant diazotroph at stations with KI index larger than 0.5, with 2 orders of magnitude higher nifH gene abundance than that at stations with KI index lower than 0.5. However, the highest N2 fixation rates were found in waters with moderate KI index around 0.6, suggesting that frontal zone mixing might stimulate N2 fixation. Our results demonstrated that diazotrophs (mainly Trichodesmium) were tightly associated with the KI, which modulated the biogeographic distribution of N2 fixers. In summary, we found the transportation of Trichodesmium by KI, then, we quantified the fraction of KI and N2 fixation rates in the northern South China Sea. The results suggested that KI generated a new biogeographic regime which could significantly influence the carbon and nitrogen cycles far away from the main stream.

Constraining which nutrients limit phytoplankton growth is important for understanding ocean productivity, its response to climate change, and providing a benchmark on the accuracy of ocean biogeochemical models. The extensive subtropical Northwest Pacific is assumed to be nitrogen limited, but this is based mostly on indirect evidence. We conducted experiments along an~8000 km cruise track showing a geographic switch from sites that were nitrogen limited and those that were nitrogen-iron co-limited. Co-limited sites showed much larger responses to nutrient supply, due to blooming of diatoms that were initially undetectable. Because diatom growth leads to higher biomass accumulation and more efficient energy transfer up food chains, we hypothesize that this gradient could be important for regulating matching gradients in predatory tuna.