Biological nitrogen fixation is an important part of the marine nitrogen cycle, supporting carbon export and sequestration. In this Review, Sohm, Webb and Capone describe the nutrients that limit nitrogen fixation and the distribution of diazotrophic species in the world's oceans.

/pdf/emerging-patterns-of-marine-nitrogen-fixation-130si46rvp.pdf

Emerging patterns of marine nitrogen fixation

https://www.cell.com/trends/microbiology/pdf/S0966-842X(10)00215-5.pdf

Nitrogen fixation by marine cyanobacteria

The macronutrient phosphorus is critical to the physiological ecology of eukaryotic microalgae and cyanobacteria. What are the forms of phosphorus produced and used by these groups? This chapter reviews the distribution and processing of phosphorus in eukaryotic microalgae and cyanobacteria with a focus on how new so-called “omics” methods, advances in chemical analyses, and cell-specific approaches have driven new discoveries.

Nutrients and Their Acquisition: Phosphorus Physiology in Microalgae

The globally distributed colonial cyanobacterium Trichodesmium and unicellular diazotrophs including Crocosphaera together carry out the majority of marine biological nitrogen (N2) fixation. Future sea surface warming is predicted to influence their abundance and distribution, but temperature reaction norms have been determined for very few representatives of each genus. We compared thermal responses within and between the 2 genera Trichodesmium and Crocosphaera by measuring reaction norms for growth, N2 fixation, carbon fixation, and elemental ratios in 7 strains from a global culture collection. Temperature reaction norms of Trichodesmium and Crocosphaera were remarkably similar for all isolates within each genus, regardless of their geographic origin. Thermal limits of Trichodesmium and Crocosphaera ranged from 18 to 32°C and 24 to 32°C, and optimum growth temperatures were ~26 and ~30°C, respectively. The highest cellular ratios of nitrogen to phosphorus and carbon to nitrogen were found at optimum growth temperatures, and the lowest ratios near their thermal limits. In a mixed competition experiment, Trichodesmium growth rates were ~25% higher than those of Crocosphaera at 24°C, while those of Crocosphaera were ~50% higher at 28°C. Comparison of these results to current and projected seasonal temperature regimes in the subtropical Atlantic and Pacific Oceans suggests that predicted warmer temperatures may favor Crocosphaera over Trichodesmium, but that both genera may be excluded where future temperatures consistently exceed 32°C. Sea surface warming could profoundly alter the community structure and stoichiometry of marine N2-fixing cyanobacteria, thus fundamentally changing the biogeochemical cycling of this globally significant source of new nitrogen.

/pdf/differing-responses-of-marine-n2-fixers-to-warming-and-232fod8hsl.pdf

Differing responses of marine N2 fixers to warming and consequences for future diazotroph community structure

Growth limitation of phytoplankton and unicellular nitrogen (N2) fixers (diazotrophs) were investigated in the oligotrophic Western South Pacific Ocean Based on change in abundances of nifH or 23S rRNA gene copies during nutrient-enrichment experiments, the factors limiting net growth of the unicellular diazotrophs UCYN-A (Group A), Crocosphaera watsonii, γ-Proteobacterium 24774A11, and the non-diazotrophic picocyanobacterium Prochlorococcus, varied within the region At the westernmost stations, numbers were enhanced by organic carbon added as simple sugars, a combination of iron and an organic chelator, or iron added with phosphate At stations nearest the equator, the nutrient-limiting growth was not apparent Maximum net growth rates for UCYN-A, C watsonii and γ-24774A11 were 019, 061 and 052 d−1, respectively, which are the first known empirical growth rates reported for the uncultivated UCYN-A and the γ-24774A11 The addition of N enhanced total phytoplankton biomass up to 5-fold, and the non-N2-fixing Synechococcus was among the groups that responded favorably to N addition Nitrogen was the major nutrient-limiting phytoplankton biomass in the Western South Pacific Ocean, while availability of organic carbon or iron and organic chelator appear to limit abundances of unicellular diazotrophs Lack of phytoplankton response to nutrient additions in the Pacific warm pool waters suggests diazotroph growth in this area is controlled by different factors than in the higher latitudes, which may partially explain previously observed variability in community composition in the region

/pdf/analogous-nutrient-limitations-in-unicellular-diazotrophs-30tke8h7w9.pdf

Analogous nutrient limitations in unicellular diazotrophs and Prochlorococcus in the South Pacific Ocean

We investigated the phosphorus (P) status and N2 fixation rates of Trichodesmium populations from the North Pacific, western South Pacific, and western North Atlantic. Colonies of Trichodesmium were collected and analyzed for endogenous alkaline phosphatase (AP) activity using enzyme-labeled fluorescence (ELF) and for nitrogenase activity using acetylene reduction. AP hydrolyzes dissolved inorganic phosphate (DIP) from dissolved organic phosphorus and is active in Trichodesmium colonies experiencing P stress. Across multiple stations in the subtropical North and South Pacific, there was low to moderate ELF labelin gi nTrichodesmium, although labeling was present in other taxa. In contrast, Trichodesmium ELF labeling in the North Atlantic ranged from low to high. Low ELF labeling corresponded with high DIP concentrations while high ELF labeling occurred only at North Atlantic stations with DIP concentrations #40 nmol L21, indicating that Trichodesmium was not experiencing dramatic P stress in the Pacific Ocean while P stress was evident in the western North Atlantic. However, nitrogenase activity was significantly higher in the P-stressed western North Atlantic than in the Pacific Ocean (0.40–1.30 compared to 0.01–0.46 nmol C2H4 h21 colony21). These data underscore the differential basinlevel importance of P availability to Trichodesmium and suggest that factors other than P are constraining their N2 fixation rates in the Pacific.

/pdf/cross-basin-comparison-of-phosphorus-stress-and-nitrogen-3rqqblrtyc.pdf

Cross-Basin Comparison of Phosphorus Stress and Nitrogen Fixation in Trichodesmium

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Cross-Basin Comparison of Phosphorus Stress and Nitrogen Fixation in Trichodesmium