Nitrogen metabolism in the picoalgae Pelagomonas calceolata: disentangling cyanate lyase function under different nutrient conditions.

Among nitrogen sources compounds in the environment, cyanate (OCN-) is a potential important actor given the activity and prevalence of cyanate lyase genes in microalgae. However, the conditions in which this gene is activated and the actual capacities of microalgae to assimilate cyanate remain underexplored. Here, we studied the nitrogen metabolism of the abundant and cosmopolite picoalgae Pelagomonas calceolata (Ochrophyta/Pelagophyceae) with environmental metatranscriptomes and culture experiments under different nitrogen sources (nitrate, ammonium, urea and cyanate) and concentrations. We observed that in nitrate-poor oceanic regions, the cyanate lyase gene is one of the most differentially expressed gene, suggesting that cyanate is an important molecule for P. calceolata persistence in oligotrophic environments. In the lab, we confirmed that this gene is overexpressed in low-nitrate medium together with several genes involved in nitrate recycling from endogenous molecules (purines and amino acids). P. calceolata is capable of growth on various nitrogen sources including nitrate, urea and cyanate but not ammonium. RNA sequencing of these cultures revealed that the cyanate lyase gene is surprisingly underexpressed in cyanate conditions indicating that this gene in not involved in extracellular cyanate catabolism to ammonia. Taken together, environmental datasets and lab experiments show that if the cyanate lyase is important in nitrate-poor environments it is probably to reduce the toxicity of cyanate as a consequence of endogenous nitrogenous compound recycling rather than the use extracellular cyanate to produce ammonium. ### Competing Interest Statement The authors have declared no competing interest.