Glycerol Is an Osmoprotectant in Two Antarctic Chlamydomonas Species From an Ice-Covered Saline Lake and Is Synthesized by an Unusual Bidomain Enzyme

Abstract

Glycerol, a compatible solute, has previously been found to act as an osmoprotectant in some marine Chlamydomonas species and several species of Dunaliella from hypersaline ponds. Recently, Chlamydomonas reinhardtii and Dunaliella salina were shown to make glycerol with an unusual bidomain enzyme, which appears to be unique to algae, that contains a phosphoserine phosphatase and glycerol-3-phosphate dehydrogenase. Here we report that two psychrophilic species of Chlamydomonas (C. spp. UWO241 and ICE-MDV) from Lake Bonney, Antarctica also produce high levels of glycerol to survive in the lake's saline waters. Glycerol concentration increased linearly with salinity and at 1.3 M NaCl, exceeded 400 mM in C. sp. UWO241, the more salt-tolerant strain. We also show that both species expressed several isoforms of the bidomain enzyme. An analysis of one of the isoforms of C. sp. UWO241 showed that it was strongly upregulated by NaCl and is thus the likely source of glycerol. These results reveal another adaptation of the Lake Bonney Chlamydomonas species that allow them to survive in an extreme polar environment.

Keywords: Antarctica; Chlamydomonas; Lake Bonney; glycerol synthesis; glycerol-3-phosphate dehydrogenase; phosphoserine phosphatase.

Figure 1

Responses of Lake Bonney Chlamydomonas species to increasing salinity. (A), Glycerol production in Chlamy-UWO and Chlamy-ICE. Two independent measurements were made for Chlamy-UWO. For UWO-2, three measurements were made for each sample. (B), Expression of PSP/GPDH isoform 2 of Chlamy-UWO using different reference genes.

Figure 2

Structure of Chlamy-UWO PSP/GPDH isoform 1. (A), Domain structure, consisting of an N-terminal chloroplast-targeting signal, a phosphoserine phosphatase domain and a glycerol-3-phosphate dehydrogenase domain. (B), Stereoview of the molecule generated by Swiss-Model using the PSP/GPDH of D. salina as template. The lower portion, beginning at Thr60, shows the PSP domain with an embedded magnesium ion (olive). The upper portion, ending at Phe639, shows the GPDH domain. Color codes: red, beta sheet; blue, alpha helix; cyan, coil; green, turn; yellow, 3₁₀ helix.

Figure 3

Neighbor-joining phylogenetic tree of the ~600 a.a. region containing the PSP and GPDH domains of PSP/GPDH bidomain enzymes of 10 chlorophytes. Bootstrap values less than 65% are not shown.