FIGURE 4 The unsaturation of MGDG is highest at the lowest salinity. (a) The contribution of second monogalactosyldiacylglycerols (sMGDG) to cell lipids (sMGDG (%)) along the salinity gradient and (b) the relationship between first monogalactosyldiacylglycerols (fMGDG) and sMGDG in the Krka River Estuary (triangles) and the Wenchang River Estuary (squares). The lines represent linear fits without included salinity 0 of river waters. sMGDG at salinity 13.9 for the Krka River Estuary is an outlier and was excluded from the linear fit.
Although the primary source of MGDG was unconfirmed, we considered that it originates from phytoplankton. This assumption was supported by the fact that the main lipid producer in the oceans is autotrophic plankton (Gašparović et al., 2014) and MGDG is not a characteristic lipid for heterotrophic bacteria (e.g. Sebastián et al., 2016). Therefore, we analyzed in detail the MGDG species with HPLC/MS/MS that have FA with an even number of carbon atoms. The complete list of identified MGDGs for both estuaries can be found in the Table S3a and b. We found a linear correlation between the injected MGDG amount as calculated from the data obtained with TLC/FID and the cumulative MS/MS MGDG intensity (Figure S1).
MGDG fatty acid remodeling between fresh and estuarine waters is recorded (Table S3). Estuarine phytoplankton synthesize more unsaturated FA in MGDG than freshwater phytoplankton (Figure 5). Although there was a trend towards decreasing MGDG unsaturation with increasing salinity, the relationship between sMGDG (%) and average MGDG unsaturation was not statistically confirmed. This can be explained by the different ionization efficiency of the various MGDG from the different samples.