## 4 Changing the Cultivation Conditions for Optimizing the Production of Bioactive Compounds
### Pigments
#### 4.1.2 Carotenoids  
In microalgae, photosynthesis is efficiently performed if carotenoids are bound to peptides to form pigment-protein complexes in the thylakoid membrane. In algae, carotenoids function essentially as accessory light harvesting pigments during the light phase of photosynthesis and as agents to photoprotect the photosynthetic apparatus from excess light dissipation by scavenging ROS such as singlet oxygen and free radicals [218, 219, 220]. Carotenoids in the majority of green algae are synthesized and accumulate within plastids, while secondary xanthophyls (e.g., Asta in _Haematococcus pluvialis_) accumulate in the cytosol. However, these xanthophyls are synthesized in the chloroplast, but they accumulate in the cytosol (Figure 1). Thus xanthophyls can be found in all cellular compartments [221]). NADPH is required in the early steps of carotenoid biosynthesis [222]; consequently, for an efficient carotenoid biosynthesis, efficient light-dependent reactions of photosynthesis are needed (NADP\({}^{+}\) must be photoreduced) (Figure 1).  
Most of the carotenoids of cyanobacteria are embedded in the photosynthetic membrane which is both, the site of their synthesis and the site of their major functions of light harvesting and photoprotection. Some carotenoids are exported to the periphery of the cell, especially during high intensity light exposure, where it may provide additional photoprotection [223, 224]. Other carotenoid proteins may be involved in the transport of carotenoid from the photosynthetic membrane to the exterior.  
Control of photosynthetic gene expression and enzyme activity is coordinated by a number of molecular triggers including photosynthetic metabolites, hormones, Chl and carotenoid biosynthesis precursors, photoreceptors, stress-generated ROS, and soluble redox active compounds [225, 226]. All known enzymes involved in carotenoid synthesis are nuclear encoded, but the carotenoid synthesis itself takes place in chloroplasts [222, 227, 228] (Figure 1). Most of xanthophylls are structural and functional components of the cellular photosynthetic apparatus and for this reason are considered as primary carotenoids essential for survival [93] whereas it is not the case for secondary carotenoids produced in large amounts [93, 229, 230]. Caroenoid content in some microalgae can be increased up to 140 mg g\({}^{-1}\) biomass by manipulating environmental conditions [149].  
Figure 1: Scheme of the biochemical pathways leading to carotenoids and other terpenoids. The DOXP/MEP pathway produces the isoprenoid precursors DMAPP and IPP, that are required for the production of carotenoids and other chloroplast-produced terpenoids. Some of the intermediates of the DOXP/MEP pathway can be exported to the cytoplasm and used in the mevalonate pathway of terpenoid synthesis. The cytoplasmic mevalonate pathway does not provide IPP to the chloroplastic DOPX/MEP pathway neither in chloroplast nor in chromoplasts [231]. In contrast, the chloroplastic pathway delivers IPP to the cytoplasmic pathway, allowing the incorporation of photosynthetically fixed C into sterols [231]. Dashed arrows indicate the Asta biosynthetic pathway from \(\beta\)-carotene.