Ammonium sulfate (coking byproduct) downregulates assimilatory nitrate pathway to save energy for carbon sequestration of Nannochloropsis oceanica with at stable pH using HEPES-NaOH

How to utilize low-cost easily-acidified ammonium sulfate, a byproduct of coking industry, to farm microalgae for CO2 fixation is a difficult problem. Ammonium sulfate was mixed with sodium nitrate to cultivate Nannochloropsis oceanica at stable pH using HEPES-NaOH for solving acidification problem, and downregulate assimilatory nitrate reduction to save energy for improving carbon sequestration. Transcriptomic sequencing showed that nitrate assimilation consumed much ATP and reductivity derived from photosystem I, and competed with Calvin cycle for energy to limit CO2 fixation rate of microalgae in conventional nitrate medium. However, pure ammonium culture attenuated GS/GOGAT cycle (a main pathway for ammonium assimilation and amino acids synthesis) to limit nitrogen utilization rate of microalgal cells. In the mixed ammonium and nitrate supply modes, nitrogen metabolic pathway was down-regulated, while the Calvin cycle pathway was up-regulated to improve CO2 fixation. Nitrate reduced the extent of up-regulation of the ammonium transporter to prolong effective utilization time of ammonium. Asparagine synthetase activity in accessorial pathway of nitrogen assimilation substantially increased, while asparaginase was down-regulated to avoid harmful overaccumulation of ammonium in the cell. Down-regulation of tricarboxylic acid cycle attenuated intermediate 2-oxoglutarate shift to ammonium assimilation and improved efficiency of nitrogen utilization. The optimum condition for microalgal growth was at total nitrogen concentration of 5.28 mM and a nitrate to ammonium molar ratio of 2:3. Compared with the control (2.64 mM nitrate), the maximum photosynthetic electron transfer efficiency rETR increased by 34.9%, chlorophyll a concentration increased by 110% and average biomass growth rate increased by 33.7%.