## 4 Changing the Cultivation Conditions for Optimizing the Production of Bioactive Compounds
### Lipid and Fatty Acid Plastid Synthesis in Microalgae  
Main reports have been conducted in plant models from which lipid trafficking pathways have been proposed with a difference in lipid location. Even if the lipid composition of plant cell membranes can vary according to environmental factors, cell type culture or development state, it is recognized that glycolipids such as MGDG, DGDG or sulfoquinovosyldiacylglycerolleyond are exclusively lipid elements of plastitial membranes while the phospholipids have a less typical distribution. Among the phospholipid classes, the PC, absent from the thylakoids, are only present in the outer membrane of plants. Concerning the PE and phosphatidylserine, these classes are totally absent of the plants [120]. From these results, it can be suggested that the enzyme complex involved in the lipid synthesis are located in specific cellular compartments or that lipid transport can occur in cell to transfer lipids from one compartment to another one. It is well-recognized that the major lipid class in the chloroplast compartment is the glycolipid class even if phosphatidylglycerol is the main phospholipid in this subcellular fraction. Fatty acids incorporated into glycolipids are related with their metabolism. Indeed, according to the chain length, fatty acids are synthesized during a _de novo_ plastid metabolism or are imported from the endoplasmic reticulum before being incorporated into the glycolipids [274]. According to the identification of the enzymes involved in the glycolipid synthesis in the chloroplast, and their resemblance with the bacterial enzymes, the plast metabolic pathways were named prokaryotic pathway while the reactions involved in the endoplasmic reticulum were named the eukaryotic pathway. The prokaryotic pathway is characterized by properties to incoporate C16 fatty acids at the _sn-1_ position. Some fatty acids synthesized in the plast compartment are exported to the endoplasmic reticulum and these are involved in the synthesis of lipids of non chloroplastic membranes. These fatty acids, and specifically the C18 ones, are more incorporated into the _sn-2_ position, and are used to synthesize longer and more unsaturated fatty acids through the eukaryotic pathway [274]. Another example of fatty acid and lipid traffic between the endoplasmic reticulum and the chloroplast is that in some algal species, fatty acids incorporated into diacylglycerollendoplasmic reticulum compartment are metabolized into the chloroplast one to produce in situ diacylglycerols for the formation of chloroplast galactolipids. Then, once these fatty acids are incorporated into structural lipids of chloroplasts, they can undergo desaturation without the need for deacylation-reacylation [275]. These results confirmed previous studies in which it was shown that MGDG-bound palmitate was desaturated into palmitoleate in the cyanobacteria _Anabaena variabilis_, without any detectable deacylation and reacylation [276]. Experiments conducted in _Dunaliella salina_, using radioisotope labeling experiments, showed that MGDG and DGDG were more associated with desaturation of galactolipid-bound fatty acids, in relation with desaturase enzymes located in the chloroplast envelope [277]. In the marine diatom _Phaeodactylum tricornutum_, it has been reported the characterization of two cDNAs coding for a plastidal and microsomal delta-12 desaturase. According to these results a compartmentalization in the synthesis of the different fatty acids have been proposed with an specific incorporation of EPA into the galactolipid fraction of the plastidic compartment [277]. In _Pavlova lutheri_ species, two unusual compounds, diacylglycerolservl carboxyhydroxymethylcholine (DGCC) and diacylglycerolservl glucuronide (DGGA), are identified in the lipid fraction. DGCC is enriched in palmitate and EPA, and DGGA contained predominantly oleate, docosapentaenoic and docosahexaenoic (DHA) acids.