## 4 Microbial Fermentation for Commercial Production of C\({}_{30}\) Caroenoids  
Only a few of the known carotenoids are commercially available, with most being C\({}_{40}\) carotenoids such as astaxanthin and lycopene [11]. Chemical synthesis is the most common method used for industrial carotenoid production; however, it is associated with risks such as the generation of hazardous waste, and possible adverse effects include allergic reactions in humans [62]. Caroenoids obtained from plant extractions have environmental and geographic limitations and are relatively costly to purchase [63]. Microbial synthesis of carotenoids is increasingly preferable because of their sustainable nature, viable options for low substrate costs, simplicity of production, and monitoring of practices and applications [10; 64]. A recent report on the global market for carotenoids stated that it was expected to reach USD 2.7 billion in 2027, increasing from USD 2.0 billion in 2022. This is consistent with their previous report on predicted growth. However, the majority of carotenoids are still supplied through chemical synthesis and are predominantly composed of C\({}_{40}\) carotenoids such as \(\beta\)-carotene (C\({}_{40}\)H\({}_{56}\)), lutein (C\({}_{40}\)H\({}_{56}\)O\({}_{2}\)), astaxanthin(C\({}_{40}\)H\({}_{52}\)O\({}_{4}\)), and lycopene (C\({}_{40}\)H\({}_{56}\)). The C\({}_{30}\) apocarotenoid, \(\beta\)-apo-8-carotenal (C\({}_{30}\)H\({}_{40}\)O), does have a part in the market value of carotenoids and can be sourced from the cyanobacterium _Arthospira platensis_ (or Spirulina), a microalgae species that can also synthesize various C\({}_{40}\) carotenoids [65]. Carotenoid prices are dependent on origin and purity. Astaxanthin prices range from 2500 to 7000 USD/kg from microalgae _Haematococcus pluvialis_, but synthetic astaxanthin prices are typically less than 2000 USD/kg, with a production cost of only 1000 USD/kg, making them much more competitive and leaving the natural supply for more consicentous consumers who desire natural products [66; 67]. \(\beta\)-carotene is priced between 300 and 3000 USD/kg when sourced from microalgae _Dunaliella salina_ and has biomass production costs of about 17 USD/kg dry weight calculated from a process model [68; 69; 70]. Microalgae carotenoid production is highly costly, hindered by high capital, labor, and processing costs. Astaxanthin from _Haematococcus_ has an estimated production cost of 718 USD/kg based on a conceptually designed facility, compared to costs of over 3000 USD/kg incurred by established firms producing the carotenoid from the microalgae [71]. Adaptive and recombinant microbial strains can reduce production costs in theory, but there is a lack of detailed economic assessment studies on carotenoid production by source that cover production, capital, labor, and processing costs. In addition, the studies that do tackle financial feasibility and requirements also focus on C\({}_{40}\) carotenoids rather than other types. Employing microorganisms as natural sources of carotenoids is beneficial to meet the growing demand for natural products. The short life cycles of microorganisms and lower land requirements for their production make them advantageous over plants for carotenoid extraction. In addition, many microorganisms can grow in stressful environments and use agricultural waste as nutrient sources, showcasing their environmental benefits. Microbes can be exploited to produce multiple desired products. The ease of genetic manipulation for enhanced microbial biosynthesis further highlights the potential of microbes for carotenoid production over chemical synthesis and plant extraction. Carotenoids are lipid-soluble compounds typically extracted using organic solvents, with nonpolar solvents utilized for nonpolar compounds and polar solvents for polar carotenoids. Bacterial carotenoids can also be extracted by relatively costly enzymatic action. This process includes cell membrane hydrolysis in a greener, more sustainable process, although yield remains a concern.