Award: OCE-1559165

Nitrogen (N) is essential for all life on Earth. Along with other elements such as carbon, oxygen, hydrogen, phosphorus and sulfur, N is required for the biological synthesis of cellular components including DNA, proteins, and lipids. The availability of these essential elements ultimately limits biomass production in both terrestrial and aquatic ecosystems and is regulated by complex cycles mediated by biological, non-biological and anthropogenic forces. The N cycle is unique in that it is abundant as dinitrogen (N2) gas in the atmosphere and ocean but is largely unavailable to organisms in this form. Only specialized single-celled organisms, called diazotrophs, can utilize the gaseous N2. Diazotrophs are important components of the marine ecosystem, as they act as a biological ?fertilizer? by converting N2 into ammonia (NH3), the N form used by all organisms for growth. In the oceans, many diazotrophs live in symbiosis with phytoplankton. The small, unicellular diazotroph, UCYN-A, which lives in symbiosis with a small unicellular Prymnesiophyte (haptophyte) called Braarudosphaera bigelowii, is of particular interest because they have broad distributions and a unique biology. The UCYN-A symbiosis is found throughout much of the world?s oceans previously considered unimportant regions of diazotrophic activity, including waters that already contain readily available forms of N for use by the microbial communities (e.g. coastal and upwelling regions), and in polar waters. However, prior to this study, the potential importance of the UCYN-A symbiosis in coastal and continental shelf regions could only be inferred based on reports of their presence. To evaluate their activity in coastal waters, we applied a suite of cutting-edge cell staining techniques (Catalyzed Reporter Deposition Fluorescently Labeled In-Situ Hybridization, CARD-FISH) to visualize these tiny microbes and then measured the uptake of N2 into individual cells using stable isotope tracers and a specialized instrument for measuring isotopic compositions at nanoscales (nanoscale Secondary Ion Mass Spectrometry, nanoSIMS). We now know that N2 fixation is widespread along the continental shelf between the Southern California Bight and Sebasti?n Vizca?no Bay (Baja California Sur), and in adjacent offshore waters, and that the UCYN-A symbioses can account for the majority of the N2 fixation in this system. Furthermore, this region has two closely related strains of the UCYN-A symbiosis, and our results show their activity is likely influenced by different environmental factors. These measurements of UCYN-A cell-specific N2 fixation in temperate coastal environments provide the link between their presence in coastal environments and their potential to make a significant contribution to coastal N2 fixation. Another significant outcome from this study is the finding that the prymnesiophyte host does not (or cannot) use the nitrogen source nitrate (NO3-), which is quite unusual for a marine phytoplankton. We have known for some time that the genome of the UCYN-A cyanobacterial symbiont is greatly reduced compared to non-symbiotic cyanobacteria. The lack of host NO3- utilization implies metabolic modifications have also occurred in the host, though the underlying mechanism(s) remain an open question. The UCYN-A symbiosis is of great interest to the study of the evolution of cellular organelles (organellogenesis), as the symbiont is hypothesized to be on the evolutionary trajectory of becoming a specialized organelle for N2 fixation. The insights we have gained about the biology and ecology of this symbiosis will fuel future research, with the potential to influence our understanding of the evolution of organelles. Last Modified: 06/29/2022 Submitted by: Jonathan P Zehr