## 1 Introduction
Enzymes: GPD2, bidomain NAD(P)+-dependent glycerol-3-phosphate dehydrogenase isoform 2; GK, glycerol kinase. GPD gene was strongly upregulated by NaCl exposure, correlating with an increase in intracellular glycerol concentration [5]. In the halotolerant alga _Dunaliella salina_, bidomain GPDs are encoded by a small multigene family and _DsaGPD2_ and _DsaGPD4_ exhibited substantial induction upon high salt stress [18; 24]. In _Dunaliella viridis_, two bidomain GPD encoding genes were also transiently induced in response to hypersalinity shock [25]. However, at least in _Dunaliella tertioleta_, even though the intracellular glycerol concentration increased substantially after salt stress, no significant changes in the abundance of bidomain GPD proteins were detected in proteomic analyses [26]. The bidomain GPDs encoded by genes upregulated under salinity stress in _C. reinhardtii_ and _D. salina_ localize to the chloroplast [15; 17], suggesting that they are key components of a distinctive plastid pathway for fast glycerol synthesis during acclimation to hyperosmotic stress. However, in _C. reinhardtii_, _GPD2_ and _GPD3_ transcript abundance also increased substantially under nitrogen- or phosphorus-deprivation, conditions that trigger the accumulation of carbon storage compounds such as triacylglycerols (TAGs) [27; 28; 15; 20]. In fact, the G3P produced by GPD enzymes is also an intermediate in lipid biosynthesis (Figure 1), forming the backbone of membrane glycerolipids and TAGs [6; 15; 20], and it may act as a bridge for carbon transfer between carbohydrate and lipid metabolism [29]. Furthermore, the knockdown of _C. reinhardtii GPD2_ expression by RNA interference demonstrated its requirement for both glycerol accumulation under NaCl stress [15] and TAG accumulation under N- or P-deprivation [15; 20]. Thus, changes in the expression and/or the activity of bidomain GPDs seem to be an important part of the response of diverse microalgae to environmental stresses such as hypersalinity or nutrient deprivation. However, the regulatory mechanism(s) is poorly understood, and some observations appear to be incongruuous. For instance, several chlorophytes significantly upregulate bidomain GPD gene expression in response to salt stress, but the corresponding proteins are not detected or do not change in abundance. Discerning this regulatory mechanism(s) may enhance our understanding of algal stress responses, but it may also have biotechnological implications for the sustainable production of glycerol through direct CO\({}_{2}\) capture. Interestingly, for membrane glycerolipid and TAG synthesis, the production of G3P (rather than glycerol) as the final product of bidomain GPD activity would be energetically favored. Otherwise, glycerol would need to be converted back to G3P, with ATP expenditure, by a glycerol kinase (Figure 1). This reasoning prompted us to propose that the canonical GPD domains (with reductase activity) and the GPP domain (with phosphatase activity) of bidomain GPDs may be regulated independently by a post-translational mechanism(s), allowing these enzymes to synthesize primarily G3P or glycerol depending on environmental conditions or metabolic demands [15]. In the present study, we tested this hypothesis by examining the regulation of transgenic _C. reinhardtii_ GPD2 in cells exposed to salinity stress.