Optimizing Phosphorus and Iron Concentrations to Maximize Cyanobacterial Growth and Nitrogen Fixation.

    Cyanobacteria are oxygenic, photosynthetic bacteria and have the ability to fix atmospheric nitrogen. The abundance of cyanobacteria may be limited by the availability of nutrients, such as iron (Fe) and phosphorus (P), both of which are required for nitrogenase synthesis.  Cyanobacterial growth may also be limited by low P availability because P is important in energy transfer.  Iron is a very important compound that can bind proteins which affect the light response of cyanobacteria. The objective of this study was to evaluate effects of different P and Fe concentrations on cyanobacterial growth and N fixation. This study was conducted from May 2014 until July 2015 under laboratory conditions. The treatments were arranged in a Complete Randomized Design with two replications. In the first experiment, four P concentrations [Control (7.6*10^-4 M P), 5% (3.8*10^-5 M P), 25% (1.9*10^-4 M P), and 50% (3.8*10^-4 M P)] were evaluated for three weeks.  Control was based on nutrient concentrations in Allen-Arnon media, and other treatments were percentages of the Allen-Arnon concentrations. In the second experiment, cyanobacteria were grown in media containing varying combinations of P and Fe concentrations [5% and 50%P/Fe1 (1.8*10^-5 M Fe based on Allen-Arnon media), 5 and 50%P/Fe0.1 (1.8*10^-6 M Fe), and 5 and 50%P/Fe0.01 (1.8*10^-7 M Fe)]. The optical density (OD) and Total Kjeldahl Nitrogen (TKN) concentrations were significantly lower in the  5%P treatment in the first experiment. In the second experiment, the OD and TKN were lowest in the treatment with the highest Fe concentration. In conclusion, the 0.1 Fe treatment exhibited the best growth.