NITROGEN FIXATION FACILITATED BY HOPANOID PRODUCTION IN MARINE CYANOBACTERIAL DIAZOTROPHS

The bioavailability of nitrogen (N) is critical for phytoplankton growth, affecting global primary production and CO2 sequestration. Biological dinitrogen (N2) fixation, catalysed by the enzyme nitrogenase, converts N2 into bioavailable forms like ammonia. This process is strictly anaerobic, as nitrogenase is inactivated by oxygen (O2). Cyanobacteria, the primary oceanic N2 fixers, face the challenge of conducting both oxygenic photosynthesis and N2 fixation in an oxic environment. To overcome this, marine cyanobacterial diazotrophs employ various strategies, such as forming specialised cells or segregating nitrogen fixation and photosynthesis temporally and spatially. However, these methods do not address how non-heterocyst-forming cyanobacteria prevent O2 diffusion through cell membranes. Recent studies suggest that hopanoids, lipids that may act as O2 barriers, are present in non-heterocyst-forming cyanobacteria, potentially aiding N2 fixation in oxygen-rich waters. Hopanoids help maintain membrane stability and reduce permeability to O2. Despite the documentation of hopanoid production in nitrogen-fixing cyanobacteria, its role in facilitating N2 fixation remains unclear. To shed light on this, the NITROHOP project aims to investigate whether hopanoid production supports N2 fixation in marine cyanobacterial diazotrophs. By using quantitative PCR and atomic force microscopy NITROHOP will explore the temporal relationship between hopanoid synthesis and nitrogen fixation, examining gene expression and lipid membrane characteristics. This will be tested in three different cyanobacteria diazotrophs: UCYN-A, Crocosphaera, and Trichodesmium, differentiating their ecological strategies to perform both processes over day-night cycles. This research will will answer a long-standing question about the mechanisms enabling the process of oxygen diffusion and their role on marine nitrogen fixation in oxygenated environments.