FIGURE 5 Average MGDG unsaturation (number of double bonds
(DB)) in the Krka River Estuary (triangles) and the Wenchang River
Estuary (squares).
Our aim was to investigate the influential parameters responsible for
the sMGDG accumulation, which MGDG species increase under salinity
stress and which phytoplankton group is possibly responsible for their
synthesis. To this end, we first performed PCA to determine the
environmental variables (salinity (S), temperature (T), DIN and
PO43¯) responsible for sMGDG
accumulation (Figure 6a and b, Tables S4 and S5). For this analysis, we
have taken data from the estuaries and not from the rivers. The
SiO44- concentrations were not
considered in this analysis as diatoms did not appear to be important
for sMGDG accumulation. First two principal components in the principal
component analysis between five variables for the Krka River Estuary and
the Wenchang River Estuary explained 90.95% and 92.37% of the total
variability, respectively. The position of the salinity and
PO43¯ variables indicates that
salinity changes and the availability of DIN contribute the most to the
observed results. Temperature shows opposite trends in the PCA with
salinity for both estuaries. The reason for this temperature
distribution in the Krka River Estuary is the mixing of cold river water
with warmer seawater in September, while the opposite occurred in the
Wenchang River Estuary in May, when warmer river water mixed with colder
seawater.
Second, we performed a PCA considering sMGDG (%), salinity, DIN,
pigments and 28 variables of MGDG with fatty acid double bond
combinations for the Krka River Estuary and Wenchang River Estuary,
respectively. After a preliminary PCA, the MGDG species that correlated
significantly with other variables were selected for further PCA
(Figures. 6c and d, Table S6 and S7). First two principal components in
the PCA for the Krka River Estuary explained 58.80% of the total
variability among 34 variables, while for the Wenchang River Estuary
they explained 53.81% of the total variability between 28 variables.
In the Krka River Estuary, in addition to sMGDG and DIN,peridinin and violaxanthin together with MGDG species with
the fatty acid double bond combinations 6+6, 5+6, 1+4 and 0+5 have the
largest negative PC1 loadings. These results indicate that
dinoflagellates (pigment peridinin ) and chlorophytes and
prasinophytes (pigment viola ) were probably the main contributors
for the accumulation of sMGDG at low salinity.
PCA for the Wenchang River Estuary variables shows that the greatest
positive PC1 loadings next to sMGDG (%) and DIN, have lutein(chlorophytes and prasinophytes), and 4+5, 3+4 and 1+3 MGDG species
(Figure 6d). This indicates that chlorophytes and prasinophytes with the
MGDG double bond combinations 4+5, 3+4 and 1+3 probably contributed to
the accumulation of sMGDG at low salinity.
The main MGDG fatty acids that contributed to increased MGDG
unsaturation (MGDG (%)) in the Krka River Estuary were fatty acids with
the combination of double bonds 6+6, 5+6, 1+4 and 0+5. In the Wenchang
River Estuary fatty acids with a combination of double bonds 4+5, 3+4
and 1+3.