## 1 Introduction  
Sterols are essential triterpenoids that function as regulators of cell membrane dynamics in all eukaryotic organisms [1]. In animals and higher plants, sterols participate in the synthesis of secondary metabolites involved in defense mechanisms, and steroid hormones that regulate growth and development [2]. Due to their presence in ancient sediments, sterol compounds are used as durable biomarkers to track important evolutionary events [3]. Sterols of plant origin, known as phytosterols, are used as nutraceuticalids for their cholesterol-lowering effects [4]. Other therapeutic applications such as anti-inflammatory [5] and anti-diabetic activities [6] are currently under research. In order to meet increasing demands in the global phytosterols market, about 7-9% per annum [7], diatoms have been proposed as an alternative source of sterols [8]. Diatoms are primary constituents of phytoplankton communities and principal players in the global carbon cycle. These photosynthetic microorganisms are an important ecological group of microalgae present in a great diversity of aquatic environments [9]. Diatoms exhibit higher photosynthetic efficiencies than plants and are adaptable to environmental challenges encountered in dynamic and competitive marine environments [10], which are also characteristics suited to the microbial production of bioproducts. Diatoms are emerging as alternative and sustainable hosts for terpenoids production [11; 12]. As complex organisms with a particular evolution history, diatoms possess a unique metabolism [13; 14; 15; 16] that can represent an advantage for production of terpenoid such as sterols [17]. Diatoms produce high proportions of a large variety of sterol compounds [18]. Sterol sulfates appear to be important regulators of diatom bloom dynamics, as they were shown to trigger programmed cell death in the marine diatom _Skeletonema marinoi_[19]. Recent studies suggest that sterol biosynthesis is tightly regulated. Levels of intermediate compounds in sterol synthesis change in response to different environmental conditions [20] and to the addition of chemical inhibitors [15]. However, end-point sterol levels remained unchanged under the same treatments. Deeper understanding of diatom sterol metabolism will provide ecological insights as well as enable future metabolic engineering efforts for biotechnological applications. In particular, the regulation of the sterol biosynthesis in diatoms is not yet well understood. Isoprenoid sterol precursors can be synthesized through either the cytosolic mevalonate (MVA) pathway or the plastitial methylerithriol phosphate (MEP) pathway. In most eukaryotic organisms, only one of the two pathways are present [21]. However, in plants, both pathways are functional but the MVA provides the substrates for sterol biosynthesis [22]. In diatoms, including the model diatoms _Thalassiosira pseudonana_ and _Phaeodactylum tricornutum_, both pathways are functional [15]. Similarly, in the bloom-forming species _Skeletonema marinoi_ and _Cyclotella cryptica_, sterols derive from mevalonate through cyclization of squalene [23]. However, there is no evidence for MVA presence in some diatoms such as _Haslea ostrearia_ and _Chaetoceros muelleri_[15; 24; 25]. In these diatoms, synthesis of isoprenoids may rely solely on the MEP pathway, as is the case for some green and red algae [21]. The presence of both the MVA and MEP pathways is an advantage for engineering efforts, as it potentially provides a higher pool of intermediates for isoprenoid production [15; 26]. It has been recently demonstrated that _P. tricornutum_ products from MVA pathway accumulated in the cytoplasm can be used for the production of non-endogenous terpenoids such as geraniol, indicating presence of free geranyl diphosphate, GPP, pool [12]. In the MVA pathway, three molecules of acetyl-CoA are transformed into isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) (Figure 1). In plants, fungi, and animals, the 3-hydroxy-3-methylglutaryl-co-enzyme-A reductase (HMGR, E.C.