# Effects of Caprolactam Wastewater on Algal Growth and Nutrients Removal by _Arthrospira platensis_
## 3 Results and Discussion  
### Effects of WCP on the Growth and Photosynthetic Performance of A. platensis  
In this experiment, all the culture media of the treatment and control had consistent initial concentrations of N (3.30 g/L of NaNO\({}_{3}\)) and P (0.13 g/L of KH\({}_{2}\)PO\({}_{4}\)) to avoid the influence of the nutrient concentration on algal growth. To our knowledge, this is the first study evaluating the effects of real WCP on _A. platensis_.  
Figure 2A shows that _Arthospira_ grew linearly over time in the control and all three treatments, but a clear decreasing trend in the growth rate was shown as the WCP concentration increased. After 6 d of cultivation, the final biomass concentrations varied between different WCP concentrations (Figure 2B), with the highest DW of 2.57 g/L observed in the culture without WCP addition and the lowest DW of 1.86 g/L observed in the culture with 50% WCP addition. Compared with the control, the growth of _A. platensis_ was inhibited by 7.13%, 14.80%, and 29.57% at WCP concentrations of 12.5%, 25.0%, and 50.0% (\(p/v\)), respectively (Figure 2C). These results suggest a dose-dependent toxic effect of WCP on the growth of _A. platensis_. Many studies have demonstrated that wastewater needs to be diluted before use in cultivation because of the presence of toxic substances. [PERSON] et al. (2015) reported that the growth of _Spirulina subsalsa_ decreased as the wastewater increased from a monosodium glutamate factory [37]. [PERSON] et al. (2012) reported that blade wastewater had inhibitory effects on _Desmodesmus subspicatus_ at the lowest concentration of 3.125% (\(v/v\)) [38]. [PERSON] et al. (2018) found that 9% (\(v/v\)) olive mill wastewater had a significant inhibitory effect on the growth of _Scenedesmus_ sp. [39]. [PERSON] et al. (2013) reported that a municipal wastewater concentration of less than 40% had stimulatory effects on _A. platensis_ growth, whereas an inhibitory effect was observed at concentrations greater than 40% [40]. [PERSON] et al. (2021) found that _Spirulina_ NCU-Sm grew well in 100% raw soy sauce wastewater [6]. These results indicate that wastewater composition is an important factor affecting algal growth. In this study, metal toxicity was neglected because the concentration of heavy metal ions in the WCP was very low (data not published). Therefore, it is important to investigate other possible inhibitory factors to improve the performance of N and P removal from WCP by _A. platensis_.  
The maximum quantum yield of photosystem II (PSII) (Fv/Fm) is a sensitive indicator of stress in oxygenic photosynthetic organisms [41]. To investigate the effect of WCP on the photosynthetic activity of _A. platensis_, variations in the maximum photochemical yield (Fv/Fm) were recorded. On the second day of culture, the Fv/Fm value of _A. platensis_ in the 50.0% WCP treatment was significantly lower than that in the control (\(p<0.05\)), whereas the 12.5% and 25.0% WCP treatments had no significant effect. These results indicated that compared with lower concentrations of WCP, the harmful effect of higher concentrations of WCP on the activity of PSII was observed early. On day 6, the photosynthetic activity of _A. platensis_ significantly decreased in all cultivations with WCP addition (Figure 2D), thereby indicating that the low WCP concentration (12.5%) also had a significant inhibitory effect on the activity of PSII in _A. platensis_.  
Figure 3 shows that the inhibition rate of _A. platensis_ exposed to different concentrations of WCP changed over time. The inhibition rates of all three treatments increased as the culture time increased. Thus, inhibition was not alleviated during cultivation.