Nature Geoscience最新文献

Mesoscale oceanic eddies contribute to the redistribution of resources needed for plankton to thrive. However, due to their fluid-trapping capacity, they can also isolate plankton communities, subjecting them to rapidly changing environmental conditions. Diazotrophs, which fix dinitrogen (N₂), are key members of the plankton community, providing reactive nitrogen, particularly in large nutrient-depleted regions such as subtropical gyres. However, there is still limited knowledge about how mesoscale structures characterized by specific local environmental conditions can affect the distribution and metabolic response of diazotrophs when compared with the large-scale dynamics of an oceanic region. Here we investigated genetic diazotroph diversity and N₂ fixation rates in a transect across the Gulf Stream and two associated eddies, a region with intense mesoscale activity known for its important role in nutrient transport into the North Atlantic Gyre. We show that eddy edges are hotspots for diazotroph activity with potential community connectivity between eddies. Using a long-term mesoscale eddy database, we quantified N₂ fixation rates as up to 17 times higher within eddies than in ambient waters, overall providing ~21 µmol N m⁻² yr⁻¹ to the region. Our results indicate that mesoscale eddies are hotspots of reactive nitrogen production within the broader marine nitrogen cycle.

Anoxygenic phototrophic Fe(II) oxidizers (photoferrotrophs) are thought to have thrived in Earth’s ancient ferruginous oceans and played a primary role in the precipitation of Archaean and Palaeoproterozoic (3.8–1.85-billion-year-old) banded iron formations (BIFs). The end of BIF deposition by photoferrotrophs has been interpreted as the result of a deepening of water-column oxygenation below the photic zone, concomitant with the proliferation of cyanobacteria. However, photoferrotrophs may have experienced competition from other anaerobic Fe(II)-oxidizing microorganisms, altering the formation mechanism of BIFs. Here we utilize microbial incubations to show that nitrate-reducing Fe(II) oxidizers metabolically outcompete photoferrotrophs for dissolved Fe(II). Moreover, both experiments and numerical modelling show that the nitrate-reducing Fe(II) oxidizers inhibit photoferrotrophy via the production of toxic intermediates. Four different photoferrotrophs, representing both green sulfur and purple non-sulfur bacteria, are susceptible to this toxic effect despite having genomic capabilities for nitric oxide detoxification. Indeed, despite nitric oxide detoxification mechanisms being ubiquitous in some groups of phototrophs at the genomic level (for example, Chlorobi and Cyanobacteria) it is likely that they would still be affected. We suggest that the production of reactive nitrogen species during nitrate-reducing Fe(II) oxidation in ferruginous environments may have inhibited the activity of photoferrotrophs in the ancient oceans and thus impeded their role in the precipitation of BIFs.

The lower limb of the Atlantic Meridional Overturning Circulation is an important feature of Earth’s climate system as it returns recently ventilated water to the deep ocean and is a major sink for anthropogenic carbon. The Deep Western Boundary Current—the primary component of the lower limb—flows southwards along the eastern flank of Greenland transporting dense water formed in the Nordic seas. Since 2014, the Deep Western Boundary Current has been continuously monitored at this location from a mooring array to observe the current’s velocity and hydrographic structure close to its source. Here we find that the Deep Western Boundary Current transport has decreased by 26% over the first six years of observations, due to (1) a thinning of the traditionally defined Deep Western Boundary Current layer (σ_θ > 27.8 kg m⁻³) from a known freshening signal propagating through the subpolar region (56%), and (2) weakening velocities (44%). Despite this decrease, the Atlantic Meridional Overturning Circulation has remained relatively steady over the same period. Ultimately, this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports.