## 3 Results and Discussion  
### Effect of Light and Glycerol Concentration on the Growth Performance of _A. platensis_  
Figure 1a illustrates the biomass concentration of _A. platensis_ cultivated with different glycerol concentrations under three light intensities. The results shown are the averages of three subsequent days of cultivation (after 3 weeks of stabilization) and reflect the semi-steady state biomass concentration of _A. platensis_ (see Section 2.3). In all cases studied, there was a decrease on biomass concentration with the increase of glycerol concentration, except of the series of the 1 st phase with ammonia (0.5, 1 and 1.5 g/L glycerol), where biomass concentration was almost the same for all the three glycerol concentrations. The experimental series with ammonia added performed in general better compared to those where only nitrate was used. This was not surprising because it has been frequently observed that ammonia at low concentrations can boost growth given that ammoniacal-nitrogen is preferred by microalgae/cyanobacteria compared to nitrates since its uptake and assimilation consumes less energy resulting in higher productivities [19; 20; 21]. Figure 1: (**a**) Biomass concentration of _A. platensis_ cultivated in semi-continuous mode in modified Zarrouk medium with the supplementation of glycerol (0.5, 1, 1.5, 3, 6 and 9 g/L) under three different light intensities (5, 10 and 15 Klux) with nitrate as the nitrogen source or with ammonia as nitrogen source (NH\({}_{3}\) of 20 mg-N/L/d at 15 Klux). (**b**) Optical density (OD\({}_{750\mathrm{nm}}\)) of the fraction of the bacteria grown on the _A. platensis_ cultures with different glycerol concentrations (0.5–9 g/L). Since it has been demonstrated that assimilation of organic carbon is restricted as light intensity increases [10; 11], it was hypothesized that under low light intensities glycerol could be assimilated more efficiently. However, as it is shown in Figure 1a there was a positive correlation between biomass concentration and light intensity, i.e., biomass concentration increased with the enhancement of light intensity from 5 to 15 Klux. It was observed that the decreasing trend of biomass concentration for each light intensity in relation to glycerol concentration was steeper at lower light intensities. The opposite trend was observed for the corresponding optical density of the bacteria present in the cultures. As it is shown in Figure 1b there was an increase of the OD\({}_{750\text{nm}}\) related to the bacteria as glycerol concentration increased, reflecting that by increasing glycerol concentration growth of bacteria was enhanced. More specifically, the cultures were contaminated by populations of heterotrophic gram-negative bacilli (see background picture in the Figure 1b). It is though worth noting that increase of OD\({}_{750\text{nm}}\) of the bacteria was independent to the light intensity, showing that the most important factor for the bacterial growth was glycerol concentration. Therefore, the data shown in the Figure 1b are the average of all three light intensities at the given glycerol concentration. In previous works, the bacteria isolated from the culture of _A. platensis_ SAG 21.99 were identified as _Halomonas_ spp., _Leucobacter_ sp., _Aeromicrobium_ sp., and _Staphylococcus_ spp. [22]. The dominant bacterial species that typically contaminate _A. platensis_ cultivation ponds, include _Alkalimonas delamerensis_, _Cecembia lonaresis_, _Pseudomonas mendocina_, _Paenibacillus__camelliae_, and also members of the largely halophiles _Halomonas_, _Mesorhizobium_, _Idiomarina_ and _Microoccus_[23]. There is a subsequent question raised: Was the decreased growth capacity of _A. platensis_ at increasing glycerol concentration due to any inhibitory effect of glycerol on _A. platensis_ photosynthetic machinery or was it an outcome of the competition between _A. platensis_ and the grown bacteria?