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1. Warming Iron-Limited Oceans Enhance Nitrogen Fixation and Drive Biogeographic Specialization of the Globally Important Cyanobacterium Crocosphaera

   https://doi.org/10.3389/fmars.2021.628363  

   Yang, Nina; Merkel, Carlin A.; Lin, Yu-An; Levine, Naomi M.; Hawco, Nicholas J.; Jiang, Hai-Bo; Qu, Ping-Ping; DeMers, Michelle A.; Webb, Eric A.; Fu, Fei-Xue; et al (February 2021, Frontiers in Marine Science)

   null (Ed.)

   Primary productivity in the nutrient-poor subtropical ocean gyres depends on new nitrogen inputs from nitrogen fixers that convert inert dinitrogen gas into bioavailable forms. Temperature and iron (Fe) availability constrain marine nitrogen fixation, and both are changing due to anthropogenic ocean warming. We examined the physiological responses of the globally important marine nitrogen fixer, Crocosphaera watsonii across its full thermal range as a function of iron availability. At the lower end of its thermal range, from 22 to 27°C, Crocosphaera growth, nitrogen fixation, and Nitrogen-specific Iron Use Efficiencies (N-IUEs, mol N fixed hour –1 mol Fe –1 ) increased with temperature. At an optimal growth temperature of 27°C, N-IUEs were 66% higher under iron-limited conditions than iron-replete conditions, indicating that low-iron availability increases metabolic efficiency. However, Crocosphaera growth and function decrease from 27 to 32°C, temperatures that are predicted for an increasing fraction of tropical oceans in the future. Altogether, this suggests that Crocosphaera are well adapted to iron-limited, warm waters, within prescribed limits. A model incorporating these results under the IPCC RCP 8.5 warming scenario predicts that Crocosphaera N-IUEs could increase by a net 47% by 2100, particularly in higher-latitude waters. These results contrast with published responses of another dominant nitrogen fixer ( Trichodesmium ), with predicted N-IUEs that increase most in low-latitude, tropical waters. These models project that differing responses of Crocosphaera and Trichodesmium N-IUEs to future warming of iron-limited oceans could enhance their current contributions to global marine nitrogen fixation with rates increasing by ∼91 and ∼22%, respectively, thereby shifting their relative importance to marine new production and also intensifying their regional divergence. Thus, interactive temperature and iron effects may profoundly transform existing paradigms of nitrogen biogeochemistry and primary productivity in open ocean regimes. 

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2. Phosphorus limitation heightens vulnerability of Crocosphaera watsonii to ocean warming compared with iron limitation

   https://doi.org/10.3389/fmicb.2025.1718897  

   Mansour, Amjad A; Gholmieh, Laura M; Gholmieh, Ellya C; Duan, Ran; Bian, Xiaopeng; Chen, Yutong; John, Seth G; Hutchins, David A; Fu, Fei-Xue (January 2026, Frontiers in Microbiology)

   IntroductionPhytoplankton rely on diazotrophs like the globally importantCrocosphaera watsoniito provide bioavailable nitrogen through nitrogen fixation. Given predicted global warming and oligotrophic scenarios, we investigated how nutrient limitation modulatesC. watsonii’s response to ocean warming, to better understand future marine ecosystem health. MethodsWe performed complete temperature curves (20–34 °C) under three nutrient conditions [iron (Fe)-limited, phosphorus (P)-limited, and Fe/P-replete] to compare physiological responses and test previously hypothesized temperature-nutrient interactions. ResultsAcross the viable temperature range, replete culture growth and fixation rates showed narrower, unimodal-like curves, contrasting the greater plateauing witnessed in nutrient-limited thermal curves. Under both limitations, nitrogen fixation was more impacted than carbon fixation. Fe-limited cultures performed better at higher temperatures (survival range: 22–34 °C) and shared thermal optima with replete cultures for growth rates (28–32 °C), while P-limited cultures performed better at lower temperatures (survival range: 20–32 °C) with a narrower optimum range (28–30 °C). Furthermore, extreme temperatures appear to outweigh and override nutrient limitation effects at the warmer end for Fe-limited cultures (34 °C) and the cooler end for P-limited cultures (20 °C), as no significant differences were observed between limited and replete cultures for both growth and fixation rates at these temperatures. DiscussionGiven predicted sea temperature increases and the rising frequency and intensity of oceanic heat waves, our results suggest thatC. watsoniiin P-limited regimes like the North Atlantic may be more vulnerable to warming than in Fe-limited regimes like the North Pacific. P-limitation may forceC. watsoniito migrate to higher, cooler latitudes for survival, possibly leaving lower-latitude phytoplankton and ecosystems more vulnerable to nitrogen limitation. 

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3. Alternative strategies of nutrient acquisition and energy conservation map to the biogeography of marine ammonia-oxidizing archaea

   https://doi.org/10.1038/s41396-020-0710-7  

   Qin, Wei; Zheng, Yue; Zhao, Feng; Wang, Yulin; Urakawa, Hidetoshi; Martens-Habbena, Willm; Liu, Haodong; Huang, Xiaowu; Zhang, Xinxu; Nakagawa, Tatsunori; et al (October 2020, The ISME Journal)

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   Abstract Ammonia-oxidizing archaea (AOA) are among the most abundant and ubiquitous microorganisms in the ocean, exerting primary control on nitrification and nitrogen oxides emission. Although united by a common physiology of chemoautotrophic growth on ammonia, a corresponding high genomic and habitat variability suggests tremendous adaptive capacity. Here, we compared 44 diverse AOA genomes, 37 from species cultivated from samples collected across diverse geographic locations and seven assembled from metagenomic sequences from the mesopelagic to hadopelagic zones of the deep ocean. Comparative analysis identified seven major marine AOA genotypic groups having gene content correlated with their distinctive biogeographies. Phosphorus and ammonia availabilities as well as hydrostatic pressure were identified as selective forces driving marine AOA genotypic and gene content variability in different oceanic regions. Notably, AOA methylphosphonate biosynthetic genes span diverse oceanic provinces, reinforcing their importance for methane production in the ocean. Together, our combined comparative physiological, genomic, and metagenomic analyses provide a comprehensive view of the biogeography of globally abundant AOA and their adaptive radiation into a vast range of marine and terrestrial habitats. 

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4. Contrasting Responses of Oceanic and Coastal Synechococcus to Iron Limitation and Warming Interactions

   https://doi.org/10.1111/1758-2229.70158  

   Duan, Ran; Xu, Min; Bian, Xiaopeng; Kojima, Conner Y; Hou, Shengwei; Zheng, Qiang; John, Seth G; Hutchins, David A; Fu, Fei‐Xue (August 2025, Environmental Microbiology Reports)

   ABSTRACT This study explored the contrasting physiological and transcriptional responses to iron (Fe) and warming temperature interactions in two South China SeaSynechococcusisolates belonging to clade II from the open ocean and CB5 from the coastal ocean. The two picocyanobacterial strains utilised contrasting photosynthesis, Fe uptake, and nutrient acquisition strategies to cope with Fe limitation. In the oceanic strain, moderate warming under Fe limitation upregulated expression of photosynthesis and nutrient and Fe transport genes, increasing its growth and photosynthesis. In contrast, gene expression under low Fe in the coastal strain was less affected by warming. The oceanic isolate exhibited substrate regulation of Fe acquisition and preferred organic nutrient sources. The coastal strain had a much higher Fe quota, faster turnover of the D1 gene in photosystem II, and was optimised for inorganic nitrogen sources. Both strains showed multi‐tiered Fe uptake strategies and general stress responses to heat shock and oxidative stress. In general, gene regulation in the oceanic strain responded more effectively to both stressors than in the coastal isolate. Fe‐temperature interactions in both strains are complex and may lead to synergistic and antagonistic responses, potentially influencing global biogeochemical cycles in warmer oceans. 

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5. Feedbacks between phytoplankton and nutrient cycles in a warming ocean

   https://doi.org/10.1038/s41561-024-01454-w  

   Hutchins, David A; Tagliabue, Alessandro (June 2024, Nature Geoscience)

   Climate warming increasingly drives changes in large-scale ocean physics and biogeochemistry, and affects the kinetics of biological reactions. Together these factors govern phytoplankton productivity, thereby shaping the responses of ocean carbon and nutrient cycles to global change. Here we bring together results from experimental, observational and modelling studies to highlight how interactive feedbacks between warming and nutrient limitation can affect the responses of biogeochemically critical marine primary producers. The availability of many bioactive elements in seawater will be altered markedly in the future, thereby shifting resource deficiencies. These modifications to nutrient limitation when compounded by concurrent warming can change phytoplankton optimum growth temperatures and elemental use efficiencies in group-specific and nutrient-specific ways. The biogeochemical impacts of these nutrient and warming interactions reflect a distinction between the thermal reactivity of major cellular structural elements like nitrogen (N) and catalytic micronutrients like iron (Fe). Integrating the mechanistic feedbacks between warming, nutrient availability and primary productivity into Earth system models is necessary to improve confidence in projections of ocean biogeochemical cycle transformations in a changing climate. 

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