# Trepenoid Metabolic Engineering in Photosynthetic Microorganisms
## 4 Perspectives  
Even though there has been considerable progress in their metabolic engineering, heterologous production of terpenoids is far from straightforward in photosynthetic microorganisms. However, with the expansion of available hosts, engineering tools, and understanding of terpenoid metabolism regulation, increased product titre and diversity will become available. Synthetic and systems biology are expected to contribute greatly to this effort by providing more options and helping in more holistic--taking into account not just the heterologous pathway and the connected metabolic branches, but the whole metabolism--engineering.  
Presently, cyanobacteria and algae are limited by small genetic engineering toolboxes compared to model heterotrophs, have a lack of knowledge about regulatory mechanisms, and have relatively slow growth. They also require non-standard equipment (light growth cabinets and photobioreactors). Production under different growth conditions (hetero/mixo/autotrophic) and light regimes (continuous light/photoperiod) has shown some promise but needs to be further evaluated. As toolboxes for manipulation develop further and our knowledge of regulatory systems expands, engineering will become easier in these organisms. Moreover, expanding our engineering efforts to more algal and cyanobacterial species such as the cyanobacterium _Synechococcus_ UTEX 2973, algae members of the genus _Nammochloropsis_ (Eustigmatophyceaeae), or more non-model species may reveal strains more suitable for high terpenoid productivities. Ultimately, photosynthetic terpenoid \"chassis cells\" that deliver reasonable base rates, titers, and yields [130] similar to those currently used for industrial production in _E. coli_ and yeast will become available.  
A better understanding of flux regulatory mechanisms will improve our ability to successfully redirect carbon to heterologous pathways without impinging unduly on essential metabolic processes. These mechanisms are bound to be different than the ones in heterotrophic mechanisms, given the different origin of metabolic precursors (photosynthetic) and the more complex redox regulation. Historically, terpenoid engineering in photosynthetic microbes has only just begun. The first report was just eight years ago [97] and relatively few groups work in these organisms when compared to several decades and likely hundreds of labs worldwide working in terpenoid engineering in yeast and _E. coli_. It is probable that increasing the numbers of groups will engage with these microorganisms as tools and understanding improves. This is drawn by the benefits of CO\({}_{2}\) fixation as a carbon feedstock and the potential for improved production of specific target terpenoids especially decorated plant terpenoids. Photosynthetic microorganisms may provide more production options including for the expression of more complex plant pathways.  
All authors wrote the manuscript. All authors have read and approved the final version of the manuscript.  
K.V. is supported by a CSIRO Synthetic Biology Future Science Fellowship co-funded by CSIRO and the University of Queensland, MF is supported by a CSIRO Synthetic Biology Future Science Fellowship co-funded by CSIRO and the University of Technology Sydney. This work is supported under the CSIRO-UQ Synthetic Biology Alliance.  
The authors declare no conflict of interest.