## 5 Carotenoids
As can be seen in Table 4, most studies concerning astaxanthin accumulation by _H. pluvialis_ have an induction stage from green- to red-phase. This distinction is associated with _H. pluvialis_ cellular morphologies. Therefore, macrozooids (zoospores), microzooids, and palmella cellular morphologies are usually called \"green-phase cells\", while hematocysts \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Induction Stage**} \\ \hline
**Strain** & **Medium** & **Green-Phase Cells** & **Red-Phase Cells** & **Factors Studied** & **Observations** & **Ref.** \\ \hline _H. pluvialis_ & & N-replete & N-free & High light intensity time & Astaxanthin dominated \\ Flotow 184 dem. & mBG-11 & 75 umDI photons & 350 umDI photons & 48 h\(\to\)0, 6, 12, 24, 36, 48 h & Car and 97.7\% Car, after 24 and 48 n respectively \\ Wille K-0084 & & & m\({}^{-2}\) s\({}^{-1}\) & m\({}^{-2}\) s\({}^{-1}\) & 96 h\(\to\)0, 24, 48, 72, 96 h & 48 h respectively \\  & & N-replete & & Green-phase cells: CO\({}_{2}\) & \\ _H. pluvialis_ IPPAS & & 40 umDI photons & N-free & Concentrations (5, 10, 20\%), & 5\% CO\({}_{2}\) resulted in a higher \\ H-2018 (former BMI) & BG-11 & & m\({}^{-2}\) s\({}^{-1}\) & 480 umDI photons & Growth phase (exponential and stationary). Red-phase cells: CO\({}_{2}\) & \\  & & & m\({}^{-2}\) s\({}^{-1}\) & & Organic carbon: 0.5\% (\%/\%) of & \\  & & & & methanol, ethanol, glycol, & \\  & & & & acetaldehyde, isopropanol, & \\ _H. pluvialis_ NIES 14 & BG-11 & N-replete & N-free & Ethanol treatment: 1, 2, 3 and 5\% & \\  & & & & (\(v/\eta\)) & (\(v/\eta\)) & (\(v/\eta\)) ethanol \\  & & & & Light intensity: low (25 umDI photons m\({}^{-2}\) s\({}^{-1}\)) and & & \\  & & & & & (150 umDI photons m\({}^{-2}\) s\({}^{-1}\)) & \\  & & & & & Cultivation mode (batch and batch-) & Maximum values of \\ _H. pluvialis_ LUGU & & N-replete & N-free & batch-red) & astaxanthin (35.88 mg g\({}^{-1}\)) and & \\ (KM115647.1) & BG-11 & 30 umDI photons & 250 umDI photons & Concentration of SA (0.25, 0.5. 1.0, 2.0 and 4.0 mM) on red-batch & \\  & & & & & operation (7, 9, and 11 days) & \\  & & & & & \\ _H. pluvialis_ & & & & & & \\ NIES-144 & NIES-C & 50 umDI photons & 250 umDI photons & Cultivation mode (batch and semi-continuous & \\  & & & & Large-scale cultivation & \\  & & & & & \({}^{-2}\) s\({}^{-1}\)), Semi-continuous & \\  & & & & & process produced 700.4 mg L\({}^{-1}\) & \\  & & & & & of abstaxanthin over 60 days \\ _H. pluvialis_ LUGU & BG-11 & 30 umDI photons & N-free & N-rept & The highest astaxanthin \\  & & & & & (29.53 mg g\({}^{-1}\)) and lipid \\  & & & & & (51.75\%)) occurred with 15\% \\  & & & & & of WSE \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Strategies applied for enhancing astaxanthin production for _Haematococcus pluvialis_ strains. (aplanospores) are referred as \"red-phase cells\" [126]. The first predominate in favorable growth conditions, whereas the second occur under unfavorable environmental or culture conditions [126]. Since _H. pluvialis_ accumulate large amounts of carotenoids, especially astaxanthin, under the red-phase, stress inputs such as nitrogen depletion and high light intensity are often applied to _H. pluvialis_ to prompt astaxanthin production [126]. Several factors have already being studied for _H. pluvialis_ in both green- and red-phases with the aim to prompt astaxanthin production, namely the effect of high light intensity exposure time, growth phase, cultivation mode, and the use of alternative substrates such as succinic acid, ethanol, and walnut shell extracts, as detailed in Table 4. Nonetheless, commercial production of astaxanthin from _H. pluvialis_ still faces challenges such as the high operating costs of the mass cultivation of microalgae [128]. Besides _H.