Proton stoichiometric imbalance during algae photosynthetic growth on various nitrogen sources: toward metabolic pH control

The use of algae as a potential platform for fuels or biochemical production requires process design and control that can be implemented at agronomic scales. Toward achieving pH control in large unmixed systems, we present a rigorous set of direct measurements of non-buffered proton uptake and efflux during growth on ammonium and nitrate, observing nearly unit molar proton imbalance H⁺/OH^? respectively for these nitrogen sources. This proton imbalance can be shown to be consistent with the initial assimilation steps of nitrogen from glutamate to peptide bonds which indicates that the remainder of metabolism is largely net proton balanced. These results are refined by demonstrating pH balance for growth with incrementally fed nitric acid and ammonium hydroxide. In contrast to the typical assumption of simple charge balance, each displays a slight proton uptake (around 10 % excess) that is considerably lower than urea, which displayed a molar H⁺ uptake per N assimilated of up to 33 %. This work illustrates details of proton imbalance that have been largely obscured in laboratory work due to use of elevated CO₂ and its associated carbonate equilibrium. Combined with the recent demonstration of preferential, mutually exclusive assimilation of ammonium over nitrate in Chlorella and Chlamydomonas, these results provide the stoichiometry and dynamics of photosynthetic algae growth needed to implement large-scale pH control in the absence of buffering.