Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N 2 ). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (∼0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp northsouth biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N 2 fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial-temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales.nitrogen fixation | atmospheric iron deposition

Ammonia-oxidising archaea (AOA) mediate the rate-limiting step of nitrification, the central component of the marine nitrogen cycle that converts ammonia to nitrite then nitrate. Competition with phytoplankton for ammonium and light inhibition are considered to restrict AOA activity to below the photic zone, but observations of surface nitrification now demand a further understanding of the factors driving AOA distribution and activity. Pico- to nanomolar concentrations of iron (Fe) limit the growth of microorganisms in a significant portion of the world’s surface oceans, yet there is no examination of the role of Fe in AOA growth despite the process of ammonia oxidation being considered to rely on the micronutrient. Here we investigate the Fe requirements and Fe uptake strategies of the Nitrosopumilus maritimus strain SCM1, a strain representative of globally abundant marine AOA. Using trace metal clean culturing techniques, we found that N. maritimus growth is determined by Fe availability, displaying a free inorganic Fe (Fe′) half saturation constant 1–2 orders of magnitude greater for cell growth than numerous marine phytoplankton and heterotrophic bacterial species driven by a reduced affinity for Fe′. In addition, we discovered that whilst unable to produce siderophores to enhance access to Fe, N. maritimus is able to use the exogenous siderophore desferrioxamine B (DFB), likely through a reductive uptake pathway analogous to that demonstrated in phytoplankton. Our work suggests AOA growth in surface waters may be Fe limited and advances our understanding of AOA physiology on the cellular and mechanistic levels with implications for ecosystem dynamics and the biogeochemical N-cycle.

The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual "new" nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO 2 , and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesizing data from seven cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including two previously unreported studies crossing the largely undersampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N 2 fixation rates and the distribution of Trichodesmium. Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N 2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource-ratio framework, illustrated using an idealized numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource-ratio framework are argued to be consistent with both the previously described North-South Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non-Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation-based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed.