**4. Conclusions**

The glyceroglycolipid composition of *S. elongatus* PCC 7942 changes to adapt to phosphate starvation. Glyceroglycolipid composition is regulated post-transcriptionally or post-translationally, allowing for more efficient adaptation to phosphate stress conditions. However, after 12 days of cultivation, the glyceroglycolipid composition is mainly regulated based on the MGDG/DGDG ratio in order to maintain the glyceroglycolipid homeostasis, which is beneficial in maintaining resilience when exposed to the preferred culture conditions again.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/18/7/360/s1, Figure S1: Total glyceroglycolipids content under phosphate starvation. Values are the means ± standard deviations from the three separately grown cultures. Figure S2: Glycerolglycolipids relative content under phosphate starvation given by TLC-scanner. (a) MGDG. (b) DGDG. (c) SQDG. Values are the means ± standard deviations from the three separately grown cultures. Figure S3: PPI analysis of differentially expressed genes in *Synechococcus elongatus* PCC 7942 under different phosphate concentrations (0, 0.04g/L) after being cultivated for 12 days (Genes with annotation in String database are represented by abbreviation. Genes without annotation in String database are represented by gene number. Names of genes with degree less than 5 are not shown). Figure S4: Standard curve of OD730 and Dry Cell Weight of *Synechococcus elongatus* PCC 7942. Table S1: Differentially expressed genes under phosphate starvation in *Synechococcus elongatus* PCC 7942. Table S2: Enriched GO terms under phosphate starvation in *Synechococcus elongatus* PCC 7942. Table S3: Expression of genes in GO term GTP binding in *Synechococcus elongatus* PCC 7942 under phosphate starvation. Table S4: Enriched KEGG pathways under phosphate starvation in *Synechococcus elongatus* PCC 7942. Table S5: Expression of genes participating in glyceroglycolipid synthesis in *Synechococcus elongatus* PCC 7942 under phosphate starvation. Table S6: Expression of genes related to photosynthesis in *Synechococcus elongatus* PCC 7942 under phosphate starvation. Table S7: Primers used in qRT-PCR.

**Author Contributions:** Conceptualization, X.X. and X.M.; methodology, X.X. and X.M.; software, X.X.; validation, X.X. and X.M.; formal analysis, X.X.; investigation, X.X.; resources, X.X. and X.M.; data curation, X.X.; writing—original draft preparation, X.X.; writing—review and editing, X.X. and X.M.; visualization, X.X.; supervision, X.M.; project administration, X.M.; funding acquisition, X.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Natural Science Foundation of China (No.41476122). It was also supported by the National High Technology Research and Development Program (863 Program) of China (No. 2013AA065805).

**Conflicts of Interest:** The authors declare no conflict of interest.
