## 3 Results and Discussion
### Biochemical Composition  
#### 3.2.1 Proteins  
As it was mentioned in the introduction, _A. platensis_ is considered as one of the most important microalgal species that is produced in commercial level because of its nutritional value, especially for its content in proteins (up to 70%), carbohydrates (10-20%), phycocyanin (5-15%), minerals and vitamins. Table 2 shows the protein content, while protein productivity of _A. platensis_ grown with different glycerol concentration under the three examined light intensities is shown in Figure 2. The most apparent result is that the addition of glycerol improved the protein content of the cultures in all light intensities compared to the control (only under 5 Klux the differences were not statististical different). The increase in protein content was more intense as light intensity decreased. However, since glycerol addition had negative effect on growth, the calculation of protein productivity (mg/L/d) gives a combined result on the protein content and biomass production. As it can be seen, the best results were obtained with 1, 1.5 and 0.5 g/L of glycerol for 5 and 10 Klux, for 15 Klux and for NH\({}_{3}\) 15 Klux, respectively. These findings agree with the study of [PERSON] et al. [8], where it was as well observed that glycerol addition had a positive effect on protein content.  
However, given that glycerol did not result in increased growth of _A. platensis_ it is not clear if the increase in the protein content was due to a somehow metabolic assimilation of glycerol, which resulted in the facilitation of protein synthesis and hence the increase of the protein content. Glycerol is known to be freely diffused inside cells, however till today only few species are demonstrated to utilize glycerol either as carbon or as energy source, while there is still limited knowledge about the metabolic mechanisms of its assimilation by microalgae/cyanobacteria [28; 29; 30]. On the other hand the increased protein content could be partially explained due to the presence of the bacteria, as _A. platensis_ may activated a kind of defense mechanism to cope the competition by bacteria for resources, such as light, space and nutrients by synthesizing secondary metabolites of proteinaceous nature [31].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Glycerol Concentration g/L** & **5 Klux** & **10 Klux** & **15 Klux** & **NH\({}_{3}\)-15 Klux** \\ \hline
0 & 39.84 \(\pm\) 5.44 & 44.81 \(\pm\) 3.85 & 41.96 \(\pm\) 3.52 & 53.25 \(\pm\) 1.95 \\
0.5 & 48.01 \(\pm\) 9.43 & 51.65 \(\pm\) 6.26 & 49.50 \(\pm\) 3.40 & 59.66 \(\pm\) 2.25 \\
1 & 58.14 \(\pm\) 6.55 & 55.01 \(\pm\) 6.77 & 53.20 \(\pm\) 1.60 & 57.86 \(\pm\) 3.66 \\
1.5 & 54.80 \(\pm\) 12.77 & 50.58 \(\pm\) 2.96 & 50.96 \(\pm\) 1.73 & 56.70 \(\pm\) 4.30 \\
3 & & & 60.70 \(\pm\) 3.62 & 57.82 \(\pm\) 4.85 \\
6 & & & 64.30 \(\pm\) 6.85 & 38.36 \(\pm\) 3.88 \\
9 & & & 15.93 \(\pm\) 2.33 & 52.85 \(\pm\) 5.31 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Protein content of _A. platensis_ cultivated under different glycerol concentrations and light intensities.  
Figure 2: Protein productivity of _A. platensis_ cultivated under different glycerol concentrations and light intensities.