Variable Nitrogen Fixation in Crocosphaera watsonii Experiencing Iron-Light Co-limitation

The conversion of dinitrogen gas into ammonia by marine diazotrophs accounts for a large portion of 'new production' in otherwise nitrogen limited ecosystems. Understanding the physiology and plasticity of nitrogen-fixing phytoplankton is paramount to accurately understanding the global nitrogen cycle. Of particular importance to this understanding is the role of iron in meeting the cellular requirements for biological dinitrogen fixation. In this work, we explore the impact of iron limitation on the well-studied C. watsonii 8501. By measuring dinitrogen fixation and fixed nitrogen uptake rates, we present the variability of nitrogen source utilization as a function of iron limitation. We then further investigate the relationship between nitrogen utilization and iron-limitation by repeating the work under high and low light intensities. The resulting matrix of treatments elucidates the role of iron and light co-limitation on nitrogen utilization ratios in a unicellular cyanobacteria.