## 3 Results and Discussion
### Metabolic Response and Connections with Isotopic, Spectrophotometric, and Monitoring Data
From a content perspective, the biomass showed an average C and N concentration of 48.4% \(\pm\) 1.6% and 9.2% \(\pm\) 0.8%, respectively, with no significant differences observed between biomass grown in contact with W and that grown in contact with A. The numerical result appears to be consistent with the average values reported in the relevant literature for species of this algal family [33] and is slightly higher than the values reported by [15] for the same strain. Observing the temporal trends of %C and %N for both W and A (Figure 4), it appears that they proceed in a similar way. It is interesting to note that while the concentration of C remains relatively stable over the 15-day observation period (R\({}^{2}\) < 0.1), there is a percentage accumulation of N over time (from 8% to 10%, R\({}^{2}\) > 0.6), which might be explained by an intracellular accumulation of nitrogen-containing macromolecules. When we consider the environmental conditions of Exp3 during which the samples were collected (Table 2), it seems reasonable to hypothesise that microalgae exposed to a light intensity of approximately 32,000 lux may have undergone increased production over time of photosynthetic proteins, such as phycobilins. This hypothe \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\delta^{13}\)C (\%) vs. VPDB & \(\delta^{15}\)N (\%) vs. Air & \% C & \% N \\ \hline  & Mean \(\pm\) SD & Mean \(\pm\) SD & Mean \(\pm\) SD \\ \hline Biomass grown in A & \(-\)26.0 \(\pm\) 0.4 & \(-\)13.6 \(\pm\) 0.5 & 48.5 \(\pm\) 1.7 & 9.3 \(\pm\) 0.8 \\ \hline Biomass grown in W & \(-\)26.0 \(\pm\) 0.5 & \(-\)11.0 \(\pm\) 1.6 & 48.3 \(\pm\) 1.6 & 9.2 \(\pm\) 0.8 \\ \hline Allen medium (dry residue) & - & \(-\)0.9 \(\pm\) 0.2 & - & 21.5 \(\pm\) 0.3 \\ \hline Wastewater (dry residue) & \(-\)26.3 \(\pm\) 1.8 & 5.1 \(\pm\) 1.4 & 0.9 \(\pm\) 0.1 & 1.3 \(\pm\) 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of EA-IRMS analysis, i.e., measurements of stable isotope ratios \(\delta^{13}\)C and \(\delta^{15}\)N, and C and N content of the analysed matrices. Data are expressed in aggregate form and reported as mean \(\pm\) standard deviation (SD) versus their standards, whereas VPDB is the Vienna Pee Dee Bellemite. Allen medium and wastewater represent the initial compositional signals of each of the two culture media, respectively. sis is supported by several studies demonstrating a positive relationship between light intensity received by microalgal cultures--including those of the Galdieriaceae family--and their phycocyanin content [16; 34]. In general, most microalgae, including species of the genus _Galdieria_, are typically cultivated under light intensities ranging from approximately 2000 to 4000 lux (corresponding to \(\approx\)30-60 \(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)) [11; 20]. However, increasing the light intensity to values between 5000 and 6000 lux (approximately 100 \(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)) has been shown to enhance pigment production [35; 36; 37]. Although higher intensities such as those tested in the present study (32,000 lux) are not commonly adopted, [PERSON] et al. 2020 [34] demonstrated that light intensities approaching 600 \(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) (\(\approx\)32,000-36,000 lux) effectively amplified the photoinductive response, resulting in increased synthesis of photosynthetic pigments. Concerning the information derived from isotopic data, as mentioned in Section 2.4.1, whenever a system exchanges with external matrices, it is inevitably influenced by the isotopic signature of that matrix according to appropriate mixing models [28]. In this case, despite the two different growth media, the \(\delta^{13}\)C signal (Figure 5) showed a slight shift towards higher values for both A (R\({}^{2}\) = 0.6) and W (R\({}^{2}\) = 0.7). This suggests a common carbon-related metabolic pathway based on a source characterised by a higher \(\delta^{13}\)C signal.