Assessing and modeling nitrite inhibition in microalgae-bacteria consortia for wastewater treatment by means of photo-respirometric and chlorophyll fluorescence techniques

Total nitrite (TNO2 = HNO2 + NO-2) accumulation due to the activity of ammonia-oxidizing bacteria (AOB) was monitored in microalgae-bacteria consortia, and the inhibitory effect of nitrite/free nitrous acid (NO2-N/FNA) on microalgae photosynthesis and inhibition mechanism was studied. A culture of Scenedesmus was used to run two sets of batch reactors at different pH and TNO2 concentrations to evaluate the toxic potential of NO2-N and FNA. Photo-respirometric tests showed that NO2-N accumulation has a negative impact on net oxygen production rate (OPRNET). Chlorophyll a fluorescence analysis was used to examine the biochemical effects of NO2-N stress and the mechanism of NO2-N inhibition. The electron transport rate (ETR), non-photochemical quenching (NPQ), and JIP-test revealed that the electron transport chain between Photosystems II and I (PS II and PS I) was hindered at NO2-N concentrations above 25 g N m-3. Electron acceptor QA was not able to reoxidize and could not transfer electrons to the next electron acceptor, QB, accumulating P680+ (excited PS II reaction center) and limiting oxygen production. A semi-continuous reactor containing a Scenedesmus culture was monitored by photo-respirometry tests and Chlorophyll a fluorescence to calibrate NO2-N inhibition (5-35 g N m-3). Non-competitive inhibition and Hill-type models were compared to select the best-fitting inhibition equations. Inhibition was correctly modeled by the Hill-type model and a half inhibition constant (KI) for OPRNET, NPQ, maximum photosynthetic rate (ETRMAX) and the performance index PIABS was 23.7 ± 1.2, 26.36 ± 1.10, 39 ± 2 and 26.5 ± 0.4, respectively.