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 + NO2-) 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 NOrN 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 Q(A) was not able to rcoxidize and could not transfer electrons to the next electron acceptor, Q(B), accumulating P-680(+) (excited PS II reaction center) and limiting oxygen production. A semi-continuous reactor containing a Scenedesmus culture was monitored by photo-respiromeiry 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 (K-L) for OPRNET, NPQ, maximum photosynthetic rate (EERMAX) and the performance index PIABS s was 23.7 +/- 1.2, 26.36 +/- 1.10, 39 +/- 2 and 26.5 +/- 0.4, respectively.

Automated summary

Total nitrite accumulation due to the activity of ammonia-oxidizing bacteria was monitored in microalgae-bacteria consortia, with a focus on the inhibitory effect of nitrite/free nitrous acid on microalgae photosynthesis and the mechanism of inhibition. The results showed that nitrite accumulation had a negative impact on net oxygen production rate and hindered the electron transport chain between Photosystems II and I at certain concentrations. The inhibitory effects were correctly modeled by Hill-type models, with specific half inhibition constants for different parameters.