# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 2 Cultivation Conditions
### Light  
During photosynthesis, light energy is used to convert inorganic carbon into organic compounds. Light is either provided naturally by the sun or artificial sources, depending on the location and purpose. If the intensity is too low, growth is limited, but if it exceeds a certain threshold, photo-inhibition occurs due to excess energy [33]. The optimum light intensity varies, but usually lies between 60 and 2000 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)[34]. When a light beam travels through the liquid culture, there is a gradient from high light intensity at the outer layers to low intensity at the center. It is attenuated by the cultivation medium, which can especially be a problem if it is not transparent but of a dark color. This has to be kept in mind when digestate is used as nutrient source. Additionally, microalgal cells absorb light as they need it for oxygenic photosynthesis. Thus, special care has to be taken during the photobioreactor design (i.e., layer thickness, mixing) to make sure every cell in the culture is exposed to suitable light intensities [35].  
Furthermore, when choosing artificial light, not only the light intensity but also the spectral composition is of great importance [33]. For photosynthesis, the wavelength range from 400 to 700 nm is relevant [34]. The optimum light spectrum varies, as the pigment composition differs between species [34]. The pigment composition also depends on availability of nutrients in the cultivation medium [36; 37]. In microalgae cells, pigments are responsible for capturing the light and directing it through the light harvesting complex  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **Unit** & _Chlorella_ & _Arthrospira_ & _Dunaliella_ & _Haematococcus_ \\ \hline Temperature & [\({}^{\circ}\)C] & 25–30 [19; 20] & 30–38 [21] & 30–40 [19] & 18–22 [19] \\ pH & [-] & 6.5–7.5 [19] & 9–10 [21] & 9–11 [22] & 7 [23] \\ Max. salinity & [\% (\(w/v\)) NaCl] & 1 [19] & \textless{}3 [19] & 20–35 [19; 24] & 1 [19] \\ Typical cultivation & [-] & BBM b [19; 25]/ & & & Modified Johnson & BBM b [10; 19] \\ medium & & BG-11 c [26] & & [19; 22] & & \\ N a  & [mM] & 2.94/17.63 [6] & 29.40 [6] & 9.89 [27] & 2.94 [6] \\ P a  & [mM] & 1.72/0.23 [6] & 2.87 [6] & 0.26 [27] & 1.72 [6] \\ K a  & [mM] & 2.15/0.46 [6] & 17.2 [6] & 12.83 [27] & 2.15 [6] \\ N/P ratio a  & [-] & 1.71/76.78 [6] & 10.24 [6] & 38.46 [27] & 1.71 [6] \\ \hline \end{tabular}
\end{table}
Table 1: A coarse overview over typical cultivation variables for commercially popular genera such as _Chlorella_, _Arthrospin_, _Dunaliella_, and _Haematococcus_.  
to chlorophyll \(a\). Each pigment can only absorb light of certain spectra. Nevertheless, chlorophyll, which absorbs light mainly in the blue (420-470 nm) and red (600-700 nm) spectral range, is dominant in most species [34; 38]. Sources that emit light primarily in this range should be chosen in order to use the light energy efficiently in a production process. When using cultivation medium of darker color, the light will be absorbed to a certain degree [18]. This makes choosing the right light source and intensity vital.