## 4 Discussion  
The first point to be discussed is the biological behavior of the culture. Indeed, to obtain a reliable estimate of carbon capture, microalgae growth must be optimal. The first comment is that the protocol aimed at obtaining light-limited linear growth of properly functioning cells. As shown in Figure 2, the culture proliferation can reasonably be deemed linear. This observation is also supported by the quality of the linear regression over the curve. The second comment is on the functioning of the photosynthetic apparatus. The three indicators of PSII functioning are stable over the culture. This is a token of the fact that the cells were put in an adequate environment as they did not undergo acclimation. Furthermore, the absolute values of these indicators are relatively close to the ones observed for the same strain under optimal nutrient, pH, temperature, and light conditions [17]: 1.31 \(\pm\) 0.04 for ABS/RC, 0.97 \(\pm\) 0.02 for TR\({}_{0}\)/RC, and 0.53 \(\pm\) 0.02 for ET\({}_{0}\)/RC. The only minor difference is the value of the ABS/RC indicator (circa 0.3 lower in this work). This may point to higher light availability in our case. It can nevertheless be concluded that the culture growth unfolded under conditions perfectly valid for the study.  
Therefore, it was possible to obtain the culture photosynthetic efficiency from these data by combining the biomass production rate with the input light energy. The photo-conversion efficiency value (4.34 \(\pm\) 0.68%) correlates well with values reported by other scholars (e.g., 5.01% for _Chlamydomonas reinhardtii_ under white light [29] and 5.65% at maximum for _Phaeodactylum tricornutum_[30]). However, it lies below the theoretical maximum of 23.8% [31] as well as the highly engineered laboratory value (e.g., 15% under extremely low light--25 umolPhotonPAR/m\({}^{2}\)/s--of low industrial relevance [29]). Nevertheless, these figures must be manipulated cautiously as they are not derived from identical setups. In this work, white LEDs were used. In this configuration, the microalgae do not capture part of the light energy (mainly within the green range of the spectrum). Therefore, it can considerably lower the photosynthetic efficiency. The theoretical maximum was derived, assuming only red photons were supplied to the culture. As a rule of thumb, one can state that using the proper combination of red/blue light doubles the culture photosynthetic efficiency with respect to white light [32]. Furthermore, increasing the CO\({}_{2}\)/O\({}_{2}\) ratio in the sparged gas could lower the extent of photorespiration and further increase the effective photosynthetic yield [1].  
Our results show that, as expected, the carbon fixation through the growth of our biomass is far from reaching the equivalent carbon produced for the energy used in the process. If ever some process would reach carbon neutrality to produce biomass, it is important to keep in mind that the use of this biomass itself will then tip the scale in the opposite direction. First of all, both the upstream and downstream processes should be taken into account in the total assessment. In particular, harvesting the algae is one of the most energy-demanding steps [33]. The actual use of biomass is also to be considered. In the case of biofuel, firing the accumulated biomass will unavoidably release into the atmosphere all the carbon that has been captured. While obvious, this fact is to be remembered when considering the impact of the process as a whole. On the contrary, the carbon actually captured by the algae still participates in purifying the local air or water [34].  
In addition, reaching a carbon-neutral process of biomass accumulation, energetically speaking, would not mean a totally green solution, as land, water, and nutrients would still need to be provided to the culture [20].