Cell-specific Nitrogen Fixation Rates of the Symbiotic Cyanobacterium UCYN-A Vary with Depth and Photoperiod

Dinitrogen (N₂) fixation is an important source of new nitrogen to the tropical and subtropical oceans and is performed by a select group of microorganisms. Surveys of the nifH gene, which encodes the Fe protein component of the nitrogenase enzyme that catalyzes N₂ fixation, show that the uncultivated cyanobacterium UCYN-A is among the most globally abundant marine N₂-fixing microorganisms. UCYN-A lives associated with a haptophyte alga; the UCYN-A symbiont lacks carbon fixation genes and receives fixed carbon from the algal host in exchange for fixed nitrogen. Because UCYN-A remains uncultivated, the environmental factors controlling N₂ fixation rates by this organism are poorly understood. We investigated how the single-cell N₂ fixation rates and nifH transcription levels of UCYN-A vary as a function of depth and photoperiod during a research cruise to the North Pacific Subtropical Gyre. ¹⁵N₂ tracer experiments were conducted using 24 hour in situ arrays and 12 hour day/night deck-board incubations. UCYN-A cell-specific N₂ fixation rates were measured using catalyzed reporter deposition fluorescence in situ hybridization coupled with nanoscale secondary ion mass spectrometry (nanoSIMS). UCYN-A cell-specific N₂ fixation rates were highest at the surface and decreased with depth; however, low rates were still detectable down to 100 m. This was likely due to the light-dependence of photosystem I, which may provide ATP and reductants needed to fuel N₂ fixation by UCYN-A. There was also a strong diurnal pattern in cell-specific N₂ fixation rates, with fixation only occurring during the day period. We will compare the depth-dependence and diurnal pattern of UCYN-A N₂ fixation rates to patterns in nifH gene expression levels. These results offer new insights into the environmental drivers of UCYN-A and N₂ fixation dynamics in the North Pacific Subtropical Gyre.