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Industrial activities have increased the supply of iron to the ocean, but the magnitude of anthropogenic input and its ecological consequences are not well-constrained by observations. Across four expeditions to the North Pacific transition zone, we document a repeated supply of isotopically light iron from an atmospheric source in spring, reflecting an estimated 39 ± 9 % anthropogenic contribution to the surface ocean iron budget. Expression of iron-stress genes in metatranscriptomes, and evidence for colimitation of ecosystem productivity by iron and nitrogen, indicates that enhanced iron supply should spur spring phytoplankton blooms, accelerating the seasonal drawdown of nitrate delivered by winter mixing. This effect is consistent with regional trends in satellite ocean color, which show a shorter, more intense spring bloom period, followed by an earlier arrival of oligotrophic conditions in summer. Continued iron emissions may contribute to poleward shifts in transitional marine ecosystems, compounding the anticipated impacts from ocean warming and stratification. 

In many oceanic regions, anthropogenic warming will coincide with iron (Fe) limitation. Interactive effects between warming and Fe limitation on phytoplankton physiology and biochemical function are likely, as temperature and Fe availability affect many of the same essential cellular pathways. However, we lack a clear understanding of how globally significant phytoplankton such as the picocyanobacteriaSynechococcuswill respond to these co-occurring stressors, and what underlying molecular mechanisms will drive this response. Moreover, ecotype-specific adaptations can lead to nuanced differences in responses between strains. In this study,Synechococcusisolates YX04-1 (oceanic) and XM-24 (coastal) from the South China Sea were acclimated to Fe limitation at two temperatures, and their physiological and proteomic responses were compared. Both strains exhibited reduced growth due to warming and Fe limitation. However, coastal XM-24 maintained relatively higher growth rates in response to warming under replete Fe, while its growth was notably more compromised under Fe limitation at both temperatures compared with YX04-1. In response to concurrent heat and Fe stress, oceanic YX04-1 was better able to adjust its photosynthetic proteins and minimize the generation of reactive oxygen species while reducing proteome Fe demand. Its intricate proteomic response likely enabled oceanic YX04-1 to mitigate some of the negative impact of warming on its growth during Fe limitation. Our study highlights how ecologically-shaped adaptations inSynechococcusstrains even from proximate oceanic regions can lead to differing physiological and proteomic responses to these climate stressors. 

Abstract The chemistry of copper (Cu) in seawater is well known to be dominated by complexation with organic ligands. The prevailing paradigm is that Cu forms strong but labile complexes. Recently, a novel procedure revealed that only a small fraction of dissolved Cu exists as labile complexes. The majority is present as a fraction that is relatively inert on timescales of weeks or more and probably does not participate in coordination exchange reactions, including biologically mediated processes. Samples collected from the 2018 GEOTRACES GP15 cruise show that throughout the interior of the Pacific Ocean, this inert fraction comprises about 90% of the dissolved Cu. Labile Cu accumulates in surface waters, probably arising from photochemical decomposition of the inert fraction. There is also a modest accumulation of labile Cu near deep sea sediments and along the Alaskan shelf and slope. The results have important implications for Cu transport and biological availability. Inert Cu may influence Cu transport throughout the water column and contribute to the linear increase in Cu with depth, a distribution which is hard to explain for a biologically active trace metal. The origins of inert Cu are unknown. It may be produced slowly within the water column on the timescale of meridional overturning circulation. In the Columbia River, between 92% and 98% of the dissolved Cu is in the inert fraction, suggesting a possible terrestrial source of inert Cu to the ocean.