## 1 Introduction  
Terpenoids have a wide range of applications as pharmaceuticals due to their anticancer (paclitaxel), antimalarial (artemisinin), antiviral (betulinic acid) and antibiotic (fumigalcavine A) properties [1]. They are also a versatile source of commercial compounds, including fragrances (limonene), biofuels (bisabolane), pigments (carotenoids), flavorings (guaid), biopolymers (rubber) and pesticides (pyrethrin I) [2]. All naturally produced terpenoids are synthesized from just two five-carbon (C5) hamiterpene diphosphates, isopentenyl diphosphate (IPP) and its highly electrophilic isomer, dimethylallyl diphosphate (DMAPP), by a sequential head-to-tail condensation [3]. Two natural pathways for the biosynthesis of IPP and DMAPP are the cytosolic mvelanot (MVA) and chloroplastic 2-C-methyl-D-erythritol 4-phosphate (MEP) (also known as 1-deoxy-D-xylulose 5-phosphate (DOX)) pathways [2, 4]. The MVA pathway, which is utilized in archaea, eukaryotes, and some bacteria, consists of six enzymatic steps that start with acetyl-CoA in the cytosol. IPP, synthesized via the MVA pathway, is isomerized by isopentenyl diphosphate isomerase (IDI) to form DMAPP [4, 5]. Within most bacteria, green algae and cyanobacteria, the MEP pathway consists of seven enzymatic steps that start with the condensation of pyruvate and D-glyceraldehyde 3-phosphate (GAP) to produce IPP and DMAPP at a ratio of \(51\) in the plastid [4, 5]. Eukaryotic green microalgae are especially promising as innovative green cell-factory biocatalysts for the sustainable light-driven production of heterologous high-value terpenoids. Their fast and inexpensive phototrophic growth combined with the availability of mature genetic engineering tools means they are increasingly being viewed as alternatives to established heterotrophic hosts [6; 7]. _Chlamydomonas reinhardtii_ is the prototypical microalgal model species since it is fast-growing and photoautotrophic, i.e., it uses sunlight and carbon dioxide to produce organic materials, and is readily cultivatable in inexpensive water-based media. It is thus a potentially excellent natural route for the efficient production of high-value terpenoids, such as the sesquiterpenes (E)-\(\alpha\)-bisabolene and patchoul, and the diterpenoid \(13R(+)\) manoyl oxide [6; 8]. Indeed, several recent reports have confirmed that _C. reinhardtii_ is capable of heterologous terpenoid production [9; 10; 11]. _C. reinhardtii_ relies solely on the MEP pathway in the chloroplast to supply C5 terpenoid precursors [9; 10; 11; 4]. However, the weak carbon fluxes through the native MEP pathway limit the IPP and DMAPP titers, which may impede the production efficiency of high-value terpenoid products via this route [12]. To increase the isoprenoid production, the titers of IPP and DMAPP need to be enhanced by increasing the carbon flux towards the MEP pathway [13]. Numerous rational design studies have previously targeted bottleneck steps within the MEP pathway, but these have had little positive impact on target production [10] or have negatively affected product levels, e.g., in the case of amorpha-4,11-diene [14]. The MEP pathway is highly interconnected with other elements of the cellular metabolism, which may be the reason for the multiple bottlenecks that impose constraints on rational engineering [15]. Low transgene expression rates and difficulties in expressing multiple genes in algal strains are further limiting factors [10; 11]. As a result, a simpler, more effective approach to the terpenoid precursor metabolic engineering to enhance the high-value terpenoid production is badly needed. The recent development of novel alcohol-dependent hemiterpene (ADH) biosynthetic pathways to produce terpenoid precursors, which circumvent the native metabolism, offers an attractive opportunity [3]. These ADH pathways are much simpler than the native MVA and MEP pathways and comprise just two sequential phosphorylation steps.