# _Arthrospira platensis_ Cultivation in a Bench-Scale Helical Tubular Photobioreactor
## 3 Results and Discussion  
Five cultivations of _Arthrospira platensis_ were performed in duplicate at different photosynthetic photon flux densities (23 \(\leq\) PPFD \(\leq\) 225 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) by fed-batch pulse-feeding pure carbon dioxide from a cylinder into the helicoidal photobioreactor. In this way, it was possible to verify the influence of this variable on both the start-up profile and the pseudo-steady state biomass concentration (\(X_{\rm m}\)), cell productivity (\(P_{\rm X}\)), and photosynthetic efficiency (PE) of this system. The start-up results of these cultivations in terms of biomass concentration and photosynthetic efficiency are illustrated in Figures 2 and 3, while those of parameters calculated at the beginning of pseudo-steady state conditions are summarized in Table 1.  
In all the cultivations, biomass grew following the typical sigmoid behavior of batch growth curves and reached a threshold cell concentration under steady-state conditions, which may have been the result, depending on the light intensity conditions, either of lack of a limiting substrate or of possible photoinhibition occurrence. The maximum value of this growth parameter (\(X_{\rm m}\) = 11.0 g\({}_{\rm DW}\) L\({}^{-1}\)) was observed at PPFD = 190 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\), which suggests that no photoinhibition occurred under these conditions, confirming the results of previous studies on this cyanobacterium [43; 52]. The specific growth rate (\(\mu_{\rm X}\)) also increased with PPFD, but it reached a maximum value (0.62 day\({}^{-1}\)) at quite lower PPFD (61 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\)), beyond which a quick decline was observed. This result suggests that the lack of a carbon source rather than photoinhibition may have been the growth limiting factor up to PPFD = 190 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\). However, by further increasing the light intensity (PPFD = 225 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\)), the photoinhibition likely became the predominant factor because of the excess photon flux on both photosystems of the cyanobacterium with their consequent damage [43]. The volumetric cell productivity appeared to be influenced by PPFD, according to the following empirical second-degree polynomial equation:  
\[P_{\rm X}=6.5\times 10^{-3}\ {\rm PPFD}-2.0\times 10^{-5}\ {\rm PPFD}^{2}\ ({\rm R}^{2}=0.8064) \tag{4}\]while the photosynthetic efficiency clearly decreased with increasing PPFD. However, despite the highest value of PE occurred at the lowest light intensity, these conditions led to the lowest \(X_{\text{m}}\) value. Figure 2: Time behaviors of biomass concentration (\(X\), \(\bullet\)) and photosynthetic efficiency (PE, \(\square\)) along fed-batch pulse-feeding cultivation of _Arthrospira platensis_ in a helical photobioreactor at a PPFD of (**a**) 23 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\) and (**b**) 61 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\). Figure 3: Time behaviors of biomass concentration (\(X_{\rm s}\), \(\bullet\)) and photosynthetic efficiency (PE, \(\square\)) along fed-batch pulse-feeding cultivation of _Arthospira platensis_ in a helical photobioreactor at PPFD of (**a**) 82 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\), (**b**) 190 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\), and (**c**) 225 \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\). Considering that, on an industrial scale, free sunlight is used as an energy source, to optimize the system for stable long term-production of biomass, the best conditions to ensure the maximum cell growth in the absence of any photoinhibition should be searched. As mentioned earlier, these conditions were ensured in the proposed photobioreactor configuration at PPFD = 190 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\), under which we obtained \(X_{\rm m}\) = 11.0 g\({}_{\rm DW}\) L\({}^{-1}\), whereas the maximum photosynthetic efficiency (PE = 10.4%) was obtained at the lowest light intensity (PPFD = 23 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)).