add Supplementary Figure 2 caption

assays/GC-MSMeasurementsForTheQuantificationOfSqualene/README.md

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 ### Figure 5 source
 
-https://www.frontiersin.org/files/Articles/1024981/fpls-14-1024981-HTML/image_m/fpls-14-1024981-g005.jpg
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+https://www.frontiersin.org/files/Articles/1024981/fpls-14-1024981-HTML/image_m/fpls-14-1024981-g005.jpg
+
+### Supplementary Figure 2 caption
+
+**Supplementary Figure 2.** GC-MS calibration curve for squalene after extraction of 50, 25, 12.5,
+6.25 and 3.125 µM of squalene using the method for squalene extraction from Synechocystis cells.
+Relative response is in relation to the 25 µM β-sitosterol standard, which was solved in the acetone
+used for extraction.
+
+### Supplementary Figure 2 source
+
+https://www.frontiersin.org/articles/10.3389/fpls.2023.1024981/full#supplementary-material
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