Physiological responses of Enteromorpha linza and Enteromorpha prolifera to seawater salinity stress

To investigate the physiological responses and adaptation mechanisms of Enteromorpha to seawater salinity stress, a laboratory experiment with Enteromorpha linza and E. prolifera was conducted to study their fresh mass(FM), relative growth rate(RGR), relative electrical conductivity (REC), chlorophyll(Chl) and carotenoid(Car) contents, Chl a/ Chl b, Chl/ Car, chlorophyll fluorescence parameters, and osmotic adjustment ability(OAA) under the stress of different salinity levels of diluted and concentrated seawater for 10 days. Compared with the control, 10%-200% salinity seawater increased the FM and RGR of the two Enteromorpha species obviously, 100% and 50% salinity seawater made the FM and RGR of E. linza and E. prolifera peaked, respectively, while 300% salinity seawater decreased the FM and RGR of E. linza and E. prolifera significantly, with the decrement being larger for E. linza. The biomass of E. linza and E. prolifer only had an increase in 50% and 100% sanity seawater and in 10%, 50%, 100%, and 200% salinity seawater, respectively. The Chl and Car contents and Chl a/ Chl b of E. linza and E. prolifera had a significant increase in 10% salinity seawater, but decreased after an initial increase with the increasing salinity level of seawater. The Chl and Car contents and Chl a/ Chl b of E. linza and E. prolifera peaked in 100% and 50% salinity seawater, respectively. With increasing salinity of seawater, the light use efficiency(α), maximal photochemical efficiency of PSII(F_v/F_m), actual photochemical efficiency of PSII in the light(Yield), maximal relative electron transport rate(rETR_max, and half-saturation light intensity(I_k) of E. linza and E. prolifera all showed the same variation trend as Chl. 10%-300% salinity seawater enabled E. linza and E. prolifera to express certain osmotic adjustment ability(OAA), and the OAA of E. linza and E. prolifer peaked in 100% and 50% salinity seawater, respectively. The growth of Enteromorpha had no correlation with Chl/Car, but was significantly negatively correlated with REC and positively correlated with Chl, Car, Chl a/Chl b, F_v/F_m, Yield, rETR_max, α, I_k, and OAA. To sum up, 100% salinity was the optimal salt concentration for the growth of E. linza, and 50% salinity was optimal for E. prolifera. E. prolifera could adapt to a wider range of salinity than E. linza. The parameters REC, Chl, Car, Chl a/Chl b, F_v/F_m, Yield, rETR_max, α, I_k, and OAA could be used to evaluate the salt adaptation of Enteromorpha.