## 3 Results and Discussions
### Mass Balance  
Figures 4-7 depict the contribution of the different removal mechanisms (abiotic, biosorption, bioaccumulation, and biodegradation) for the studied contaminants within 7 days. The results were calculated using Equation (1) and expressed as the amount of contaminant removed concerning its initial concentration for each experiment. The primary removal mechanism for parabens in both species was biodegradation (Figure 4a-f). However, _Scenedesmus_ sp. also showed a low percentage of biosorption and bioaccumulation of MeP (Figure 4a) and PrP (Figure 4c). The adsorbed fraction exhibited a consistent trend for these compounds, initially increasing from day 1 to day 3 (from 0.43 to 1.71% for MeP and from 0.57 to 1.44% for PrP), and subsequently decreasing until day 7, reaching less than 0.04% for MeP and 0.64% for PrP. On the other hand, the bioaccumulated fraction of MeP increased during the initial days from 0.04-0.11% to 0.88% but showed an almost complete reduction (<0.04%) by day 7. Conversely, the bioaccumulated fraction of PrP showed an upward trend from day 1 to day 7, increasing from less than 0.005% to 0.58%. These results suggest that MeP and PrP presented a slow diffusion into the cells, and MeP probably underwent biodegradation by intracellular enzymes. The adsorbed and bioaccumulated fractions for BuP (Figure 4e) were negligible, accounting for less than 0.08% of the total contaminant concentration removed. The concentrations detected in both measurements were below the LOD. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**k\({}_{\text{O}}\) (h\({}^{-1}\)) (R\({}^{2}\))**} & \multirow{2}{*}{**k\({}_{\text{LO}}\) (h\({}^{-1}\)) (R\({}^{2}\))**} & \multicolumn{2}{c}{**k\({}_{\text{LO}}\) (h\({}^{-1}\)) (R\({}^{2}\))**} \\ \cline{4-5}  & & & _Scenedesmus_ sp. & _C. vulgaris_ \\ \hline MeP & \(1.3\times 10^{-3}\) (0.868) & \(1.1\times 10^{-3}\) (0.841) & \(9.0\times 10^{-2}\) (0.983) & \(3.8\times 10^{-1}\) (0.928) \\ PrP & \(7.0\times 10^{-4}\) (0.831) & \(1.8\times 10^{-3}\) (0.852) & \(5.7\times 10^{-2}\) (0.968) & \(7.4\times 10^{-2}\) (0.972) \\ BuP & \(2.9\times 10^{-3}\) (0.951) & \(2.8\times 10^{-3}\) (0.961) & \(5.4\times 10^{-2}\) (0.982) & \(7.5\times 10^{-2}\) (0.957) \\ BP & \(1.3\times 10^{-2}\) (0.936) & \(1.6\times 10^{-2}\) (0.958) & \(1.1\times 10^{-2}\) (0.924) & \(6.0\times 10^{-3}\) (0.901) \\ BPA & \(2.0\times 10^{-3}\) (0.940) & \(1.1\times 10^{-3}\) (0.956) & \(2.1\times 10^{-2}\) (0.976) & \(1.3\times 10^{-2}\) (0.815) \\ E & \(5.9\times 10^{-3}\) (0.966) & \(3.1\times 10^{-3}\) (0.954) & \(5.2\times 10^{-3}\) (0.819) & \(9.9\times 10^{-3}\) (0.805) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Removal rate constants (k) estimated for abiotic EDC degradation and biodegradation using first-order kinetics. The absence of parabens adsorbed by _C. vulgaris_ can be observed by analysing the results for this species (Figure 4b,d,f). In this sense, previous studies have reported that MeP biosorption by this species is negligible due to the hydrophobicity of its cell wall [59]. Figure 4: Contribution (%) of biosorption (red), bioaccumulation (blue), and biodegradation (green _Scenedesmus_ sp. and orange _C. vulgaris_) in MeP, PrP, and BuP removal by _Scenedesmus_ sp. (**a**,**c**,**e**) and _C. vulgaris_ (**b**,**d**,**f**). Error bars represent \(\pm\) standard error of the mean (\(n=3\)). The contribution of abiotic degradation is not plotted because it is negligible. This fact agrees with our results, which showed adsorbed fractions lower than 0.05% for MeP and PrP and ranging between 0.53 and 0.11% for BuP. Furthermore, the contribution of the bioaccumulation pathway to the removal of the studied parabens was also negligible. When EDCs are present inside microalgae cells, an oxidative stress defence mechanism may be triggered, leading to the formation of ROS in their organelles. According to the literature, no excess ROS formation was observed during the growth of _C.