Light and depth dependency of nitrogen fixation by the non-photosynthetic, symbiotic cyanobacterium UCYN-A

The symbiotic cyanobacterium UCYN-A is one of the most globally abundant marine dinitrogen (N₂ )-fixers, but cultures have not been available and its biology and ecology are poorly understood. We used cultivation-independent approaches to investigate how UCYN-A single-cell N₂ fixation rates (NFRs) and nifH gene expression vary as a function of depth and photoperiod. Twelve-hour day/night incubations showed that UCYN-A only fixed N₂ during the day. Experiments conducted using in situ arrays showed a light-dependence of NFRs by the UCYN-A symbiosis, with the highest rates in surface waters (5-45 m) and lower rates at depth (≥ 75 m). Analysis of NFRs versus in situ light intensity yielded a light saturation parameter (I_k ) for UCYN-A of 44 μmol quanta m^-2  s^-1 . This is low compared with other marine diazotrophs, suggesting an ecological advantage for the UCYN-A symbiosis under low-light conditions. In contrast to cell-specific NFRs, nifH gene-specific expression levels did not vary with depth, indicating that light regulates N₂ fixation by UCYN-A through processes other than transcription, likely including host-symbiont interactions. These results offer new insights into the physiology of the UCYN-A symbiosis in the subtropical North Pacific Ocean and provide clues to the environmental drivers of its global distributions.