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

Abstract

How to utilize low-cost easily-acidified ammonium sulfate, a byproduct of the coking industry, to farm microalgae for CO₂ fixation is a difficult problem. Ammonium sulfate was mixed with sodium nitrate to cultivate Nannochloropsis oceanica at a stable pH using HEPES-NaOH to solve the 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 the Calvin cycle for energy, thus limiting the CO₂ fixation rate of microalgae in conventional nitrate media. However, pure ammonium culture attenuated the GS/GOGAT cycle (a main pathway for ammonium assimilation and amino acid synthesis) to limit the nitrogen utilization rate of microalgal cells. In the mixed ammonium and nitrate supply modes, the 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 the effective utilization time of ammonium. The asparagine synthetase activity in the accessorial pathway of nitrogen assimilation substantially increased, while asparaginase was down-regulated to avoid harmful overaccumulation of ammonium in the cell. The down-regulation of the tricarboxylic acid cycle attenuated intermediate 2-oxoglutarate, causing a shift to ammonium assimilation and improved efficiency of nitrogen utilization. The optimum condition for microalgal growth was a 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%, the chlorophyll a concentration increased by 110% and the average biomass growth rate increased by 33.7%.