# Trepenoid Metabolic Engineering in Photosynthetic Microorganisms
## 3 Photosynthetic Microorganisms as Terpenoid Production Hosts  
The heterologous production of terpenes requires the expression of foreign enzymes and often entire metabolic pathways. _E. coli_ and _S. cerevisiae_ are currently the main hosts for terpenoid production chosen for their high growth rates, relative ease of engineering, scalability, bioprocess technology, and--in particular--for the advanced knowledge available on their metabolism and genetic resources. However, these two organisms have not evolved to produce a wide variety of complex terpenoid metabolites.  
Photosynthetic organisms produce a much greater diversity of isoprenoids and use them for many more metabolic processes than yeast and _E. coli._ These isoprenoids are also required in higher amounts to satisfy metabolic demands for the production of photosynthetic pigments, sterols, isoprene, and secondary metabolites. Extensive engineering is required to get reasonable flux to isoprenoids in both yeast and _E. coli_, which suggests that isoprenoid metabolism is relatively inefficient in these organisms [16; 69]. Conversely, photosynthetic organisms dedicate a significant--albeit small when compared to the reactions of the central carbon metabolism--amount of metabolic resources to terpenoids to provide the production requirements for chlorophyll, carotenoids, and other photosynthetic pigments (Figure 2). While this means that the potential for interference of native metabolism is higher, it also indicates a much greater flux potential for isoprenoid pathways in photosynthetic organisms. The production of massive amounts of isoprene by numerous photosynthetic land plants and microalgae also points to a higher native flux capacity [70]. Moreover, photosynthetic organisms have an increased NADPH content, use of CO\({}_{2}\) as a feedstock (rather than secrete it in the atmosphere), and offer the ability to directly link photosynthesis with heterologous metabolite production via electron transfer proteins [54; 71]. Cyanobacteria and eukaryotic microalgae combine photosynthetic growth with simple cellular organization and, in model species, straightforward genetic manipulation methodologies. In the following sections, we review terpenoid engineering in these photosynthetic micro-organisms (Table 1).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Terpenoid** & **Production Organism** & **Titer** & **Reference** \\ \hline isoprene & _Synechocystis_ sp. PCC 6803 & 12.30 mg g\({}^{-1}\) & [72] \\ isoprene & _Synechococcus elongatus_ PCC 7942 & 1.26 g L\({}^{-1}\) & [73] \\ beta-phellandrene & _Synechocystis_ sp. PCC 6803 & 3.20 mg g\({}^{-1}\) & [74] \\ limonene & _Synechococcus_ sp. PCC 7002 & 4 mg L\({}^{-1}\) & [75] \\ limonene & _Synechocystis_ sp. PCC 6803 & 6.70 mg L\({}^{-1}\) & [76] \\ bisabolene & _Synechococcus_ sp. PCC 7002 & 0.60 mg L\({}^{-1}\) & [75] \\ farnesene & _Anabaena_ sp. PCC 7120 & 0.08 mg g\({}^{-1}\) & [77] \\ amorphadiene & _Synechococcus elongatus_ PCC 7942 & 19.80 mg L\({}^{-1}\) & [78] \\ squalene & _Synechocystis_ sp. PCC 6803 & 1.20 mg g\({}^{-1}\) & [79] \\ farnesene & _Synechococcus elongatus_ PCC 7942 & 4.60 mg L\({}^{-1}\) & [80] \\ geranyllinalool & _Synechocystis_ sp. PCC 6803 & 0.36 mg g\({}^{-1}\) & [81] \\ manoyl oxide & _Synechocystis_ sp. PCC 6803 & 2 mg L\({}^{-1}\) & [82] \\ (E)-alpha-bisabolene & _Chlamydomonas reinhardtii_ & 11 mg L\({}^{-1}\) & [48] \\ patchoul & _Chlamydomonas reinhardtii_ & 0.47 mg L\({}^{-1}\) & [83] \\ manoyl oxide & _Chlamydomonas reinhardtii_ & 50 mg L\({}^{-1}\) & [84] \\ lupeol & _Phaeodactylum tricornutum_ & 0.1 mg L\({}^{-1}\) & [85] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of terpenoid metabolic engineering works in photosynthetic microbes.