## 4 Changing the Cultivation Conditions for Optimizing the Production of Bioactive Compounds
### Pigments
#### 4.1.2.3 Nutrients  
Nutrient starvation is generally efficient for the overproduction of carotenoids in cyanobacteria [243]. It is also the case for microalgae. For instance, nitrogen starvation triggers \(\beta\)-carotene in _Eustigmatos_ cf. _polyphem_ (Eustigmatophyceaeae) [244] and in _Dunaliella salina_[245]. \(\beta\)-carotene can accumulate in lipid globules until 14 mg per liter cell volume (_i.e._, 2.7% of ash-free dry weight).  
Actually, [PERSON] _et al._[245] applied a combined high light and nitrogen stress and observed a \(\beta\)-carotene accumulation which negatively correlated with the degree of insaturation of the total fatty acid pool. In a frame of an optimal production of biofuels by microalgae, nitrogen starvation is often applied and leads at the same time to an overproduction of \(\beta\)-carotene. Nitrogen starvation leads to a modification of the photosynthetic apparatus. The linear electron flow from water to NADP\({}^{+}\) was slowed down while the cyclic electron flow was increased [246]. An overproduction of \(\beta\)-carotene can also be obtained with high nitrate contents (1-3 mM) in _Dunaliella salina_[247, 248].  
Low nitrate level, coupled with high light intensity, was the key to high cellular accumulation of Asta in _Haematococcus_ sp [249, 250] or _Chlorella_ sp [251]. Chloroplasts of \(H\). _pluvialis_ degenerate in the transition from green coccoid cells to red cyst cells during encystment. Oil droplets containing Asta accounted for more than 50% of the cyst cell volume [252].  
Varying the salt concentration is an alternative to get more carotenoids. For instance, when grown under optimized conditions of salinity and light intensity, _Dunaliella_ sp produces up to 14% \(\beta\)-carotene [149, 253, 254]. The culture of the cyanobacterium _Synechocystis_ sp. PCC 6803 in high-salt conditions (0.7 M NaCl) triggers the up-regulation of protective carotenoid pigments, especially zeaxanthin and myxoxanthophyll [255]. The presence of externally added trehalose at the same time of the salt stress increased even harder the carotenoid to Chl \(a\) ratio in this strain. The high accumulation of carotenoids in the presence of trehalose could allow a best cell protection against salt induced production of ROS, and thus a best cell protection against oxidative damage [255].  
4.1.2.4. Other Stress  
As carotenoids are thought to play an important role in the protection of cellular membrane lipoproteins against oxidative damage by scavenging several active oxygen species, heavy metal stress may stimulate carotenogenesis in cyanobacteria. Actually, elevated levels of \(\beta\)-carotene and Asta were observed in the cyanobacteria _Hapalosiphon fontinalis_ in response to chromium (VI) [256].  
Cells of _Anabaena_ sp. PCC 7120, a low desiccation tolerant cyanobacterium, was subjected to prolonged desiccation and effect of loss of water was examined on antioxidant response as well as on overall viability in terms of photosynthetic activity. In desiccated state, Chl \(a\) and carotenoid contents increased after 1 h of incubation under desiccation. The oxygen-evolving activity declined in desiccated cyanobacterial biomass while rehydration led to instant recovery, indicating that cells protect the photosynthetic machinery against desiccation [257]. As desiccation leads to oxidative stress, an enhanced level of carotenoids most likely facilitates quenching of ROS.