## 3 Results and Discussion
### Cultivable Bacterial Abundance  
Microbial abundance was determined for all PBRs in terms of _E. coli_, other coliforms except _E. coli_, and total heterotrophic bacteria (Figure 7). As expected, total coliforms were not detected in the C+ PBR. Both C. _vulgaris_ and AMS had high similarity in their effects on these cultivable bacterial groups after 9 d, reducing coliforms to values lower than the limit of quantification (5 CFU mL\({}^{-1}\)). _E. coli_ reduction was faster (after 3 d) than other coliforms (after 6 d), and no statistical differences were observed in these parameters between the CV and C\(-\) PBRs (\(p\) > 0.05). According to the Pearson correlation for the CV PBR (Figure S1), aside from a strong negative correlation and significance between _C. vulgaris_ MCC and _E. coli_ concentration (r > 0.9 and \(p\) < 0.05), it was not possible to establish other clear and significant correlations (Figure S1). However, the pH increase seems to have a role, as observed in the study of [PERSON] [53], in which a pH above 8.5 induced the inactivation of _E. coli_. While optimal pH conditions for faecal coliform growth are between 6.5 and 7.5, pH levels above 8.5 are very effective in their removal [22]. The pH has also been reported as playing an important role in reducing the proliferation of pathogens in wastewater stabilization ponds, in which microalgae had a crucial role in water alkalization as a result of photosynthesis [54]. Moreover, other studies report a reduction of four orders of magnitude in the concentration of total coliforms and _E. coli_ after a pH increase from 8.4 to 10.5, and a total coliform reduction rate of 92% in high retention ponds for UWWT at a pH of 9.4 [55; 56]. Indeed, the decrease of coliforms (except _E. coli_) abundance over time was highly negatively correlated with pH increase in the CV and C\(-\) PBRs (r > 0.9 and \(p\) < 0.05), as well as with microalgal growth and HA production in the CV (r > 0.9 and \(p\) < 0.05) (Figure S1). Moreover, HAs have adverse effects on pathogenic bacteria in the presence of light due to photo-oxidation, since HAs act as exogenous photosensizers [18]. In photo-oxidation, the photosensizers absorb light and transfer i energy to other molecules, leading to the development of reactive oxygen species that damage the cell membranes of microorganisms [18]. Therefore, the results from the present study suggest that the decrease in the abundance of coliforms is related to _C. vulgaris_ growth, indicating that this is an efficient biological system to promote UWW disinfection. Regarding total heterotrophic bacteria, the initial abundance in C+ PBR was lower than in CV and C\(-\) PBRs, due to the sterilization process of UWW, which also reduced total coliforms to non-detectable levels. Furthermore, the total heterotrophs content (6.9 logs CFU 100 mL\({}^{-1}\)) in the C+ PBR confirmed that the _C. vulgaris_ pre-inoculum was a microalgae-bacteria consortium. Additionally, the increase of the total heterotrophs in the C+ PBR over time, when compared to the CV PBR suggests that bacteria from the _C. vulgaris_ consortium competed with bacteria from UWW in the CV PBR. In the C+ PBR, a decrease in the total heterotrophs content was observed from days 3 to 9, probably due to nutrient starvation caused by the rapid microalgae growth or the production of any antibacterial compound. More constant values of total heterotrophs were detected in the CV PBR,suggesting that a microalgae-bacteria consortium was well established [57]. Microalgae can transform solar energy into chemical energy, producing O\({}_{2}\) and organic matter. In turn, bacteria may use the O\({}_{2}\) for respiration, decompose the organic compounds, and supply CO\({}_{2}\) and growth-promoting factors and vitamins (B12, B1 and B7) that boost microalgal growth [58; 59; 60]. It should not be neglected that other operational characteristics related to microalgal growth may have had a role in the bacterial abundance during the experiments.