Nitrogen fixation mechanisms and distribution patterns of UCYN-A in the global ocean

Biological N2 fixation is fundamental for supporting life on our planet1. UCYN-A, previously classified as unicellular cyanobacterium, shows molecular characteristics of an early-stage N2-fixing organelle, or nitroplast2, inside the haptophyte cell Braarudosphaera bigelowii and appears to be one of the most abundant3 and important4,5 N2-fixers in the global ocean. However, the mechanisms underpinning N2 fixation in B. bigelowii/UCYN-A and its geographic distribution remain unclear. Here we use new experimental observations and literature data in combination with a novel cell-based metabolic model to explain the intracellular pathways governing N2 fixation of B. bigelowii/UCYN-A and how these pathways determine its global biogeography. Our model shows that B. bigelowii/UCYN-A can fix N2 at low light and over a broad range of temperatures, explaining the ubiquitous distribution of this organism also in relatively deep waters and at high latitudes. We find high surface N2 fixation rates almost everywhere in the Atlantic and Indian Oceans but not in the Pacific Ocean, where N2 fixation is limited to specific regions. Our results indicate that B. bigelowii/UCYN-A is a key player in the oceanic nitrogen cycle, especially in nutrient-limited regions of the oceans, and shed new light on the role this microorganism may play in marine ecosystems.