## 5 Conclusions  
Some microalgae are remarkable in their ability to thrive under a wide range of salinity conditions, which involves, among other responses [6, 66], the capacity to rapidly adjust their intracellular concentration of compatible solutes such as glycerol. As a probable adaptation to fluctuating osmolarity/salinity in their habitats, core chlorophytes have evolved unusual, chloroplast-localized bidomain GPDs capable of converting DHAP directly to glycerol. These enzymes possess canonical glycerol-3-phosphate dehydrogenase domains fused to a glycerol-3-phosphate phosphatase domain, and have been implicated in both glycerol and TAG synthesis. It is also becoming apparent that the expression and activity of these enzymes are tightly regulated at multiple levels. Extensive evidence suggests that the corresponding genes are transcriptionally controlled, and the work reported here supports the existence, at least for _C. reinhardtii_ GPD2, of intricate post-translational regulation. Indeed, the GPP domain of GPD2 appears to be activated, via a mechanism(s) involving a protein kinase(s), only upon exposure of cells to salt stress. Elucidating the molecular basis of these mechanisms may provide insights into the fast acclimation responses of microalgae to osmotic/salinity stress, but it may also have biotechnological implications. [PERSON] [76] proposed that glycerol may become one of the key platform compounds for the synthesis of biopolymers and other high-value chemicals in the upcoming bioeconomy. Microalgae are being studied as biofatories for glycerol production by direct CO\({}_{2}\) capture, and the introduction of bidomain GPDs into yeasts has been suggested as another way to improve glycerol bioproduction [26, 67, 77]. However, successful metabolic engineering for glycerol production employing bidomain GPDs, in either homologous or heterologous hosts, will likely require a greater understanding of their regulatory controls.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/phycology4020012/s1](https://www.mdpi.com/article/10.3390/phycology4020012/s1)]([https://www.mdpi.com/article/10.3390/phycology4020012/s1](https://www.mdpi.com/article/10.3390/phycology4020012/s1)), Figure S1. Analysis of GPD2 protein abundance in overexpression strains OX-9, OX-29, and OX-30; Figure S2. Effect of light and the ROS scavenger \(N\)-acetyl-L-cysteine (NAC) on glycerol accumulation by the wild type and OX-12 strains; Figure S3. Phosphorylation of the GPP domain of GPD2 in OX-12 cells exposed to salinity stress.  
Conceptualization: D.M.-S. and H.C.; Investigation: I.C.-P., B.S., Y.K. and D.M.-S.; Data Analyses: I.C.-P., B.S., Y.K., D.M.-S. and H.C.; Writing--Original Draft Preparation: I.C.-P. and H.C.; Writing--Review and Editing: I.C.-P., B.S., Y.K., D.M.-S. and H.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by a grant from the National Science Foundation (NSF) to H.C. (award number 2131783).  
Not applicable.  
Any further data related to this study are available on request from the corresponding author.  
The authors declare no conflicts of interest.