## 4 Changing the Cultivation Conditions for Optimizing the Production of Bioactive Compounds
### Lipids
#### 4.2.3 Nutrients  
Other factors such as nutrients are involved in lipid and fatty acid synthesis. A well-documented description has been reviewed by [PERSON] [269]. The effect of phosphorus-limitation has been studied in different species of marine algae [270]. It has been reported that phosphorus-limitation led to an increased lipid content in several species such _Phaeodactylum tricornutum_, _Chaetoceros_ sp. and _Pavlova lutheri_, while a decreased lipid content in the green flagellates, _Nannochloris atomus_ and _Tetraselmis_ sp. was observed. The effect of nitrogen-limitation has been studied in the microcalga _Phaeodactylum tricornutum_[271]. A decrease of the nitrogen concentration caused the decrease of the galactolipid fraction from 21% to 12%. In contrast, both neutral lipids and phospholipids increased from about 73% to 79% and from 6% to 8%, respectively in nitrogen starved cells.  
The effect of CO\({}_{2}\) on the content and composition of lipid fatty acids and chloroplast lipids has been studied in _Dunaliella salina_[272]. An increase in the CO\({}_{2}\) concentration from 2% to 10% was shown to increase the total fatty acid levels. Differences in the fatty acid content and composition indicated increased fatty acid synthesis _de novo_ but an inhibition of their elongation and desaturation steps. This led to an increase in the relative content of saturated fatty acids at the highest levels of CO\({}_{2}\). After CO\({}_{2}\) stress, the MGDG/DGDG ratio was increased while the ratio of omega-3/omega-6 fatty acids, was clearly increased in the phosphatidylglycerol class.  
_Dunaliella salina_ is also tolerant to high salt. It has been shown that the expression of \(\beta\)-ketoacyl-coenzyme A synthase (which catalyzes the first and rate-limiting step in fatty acid elongation) increased in _Dunaliella salina_ cells transferred from 0.5 to 3.5 M NaCl [273]. It has been suggested that the salt adaptation in this microcalga induced modifications of the fatty acid composition of algal membranes, and lipid analysis indicated that microsomes, but not plasma membranes or thylakoids, from cells grown in 3.5 M NaCl contained a considerably higher ratio of 18C (mostly unsaturated) to 16C (mostly saturated) fatty acids compared with cells grown in 0.5 M NaCl. The salt-induced ketoacyl-CoA synthase, jointly with fatty aciddesaturases, was thought to play a role in adapting intracellular membrane compartments to function in the high internal glycerol concentrations used to balance the external osmotic pressure created by high salt [273].  
Role of Chloroplast and Reticulum Compartments in Lipid and Fatty Acid Synthesis in Marine Microalgae  
Marine microalgae can be considered as good models in lipid and fatty acid synthesis studies. Indeed, different works have described that in microalgae, the lipid and fatty acid metabolic pathways were distributed according to the location of enzymatic activities involved in their synthesis. The main compartments involved in lipid and fatty acid synthesis are chloroplast and endoplasmic reticulum in which the glycerolipid and phospholipid classes are recovered respectively. Concerning the phospholipid classes, their presence depends on the microalgal species. Indeed, phospholipids can be replaced by betaine lipids and thus gives some specificity in the concerned species. However, as for the phospholipids, the betaine lipids are also recovered outside the chloroplast, in the reticulum.  
In this section, we will describe the location of the different metabolic pathways by the use or data reported on microalgae. Moreover, and specifically in the marine microalgae, the influence of environmental factors on omega-3 fatty acid metabolism will be described.