Comparison Of Salinity Adaption Among Different Cell Types Of Phaeocystis Globosa

Phaeocystis globosa is a common marine red tide algae.Wide range of adaptability to salinity is one of the reasons for the widespread impact and frequent occurrence of P.globosa bloom.To date,although a large number of studies have been conducted to study the salinity adaption of P.globosa,the role of its life history in salinity adaptation was ignored.Its heterotypic life history is considered to be the important competition advantages of P.globosa and may help it blooms in competition with other phytoplankton.In order to understand the role of the special life cycle of P.globosa in salinity adaptation,this study conducted salinity gradient experiments within the salinity range of 0 to 50.The differences in salinity adaptation of different types of cells in the life cycle of P.globosa were compared from four aspects:cell growth,colony characteristics,photophysiology,and cell antioxidant capacity.The results of this study are as follows.（1）The salinity adaptation ranges of different cell types of P.globosa were different.The salinity ranges for maintaining positive growth of haploid solitary cells,diploid solitary cells and colonial cells were 10～45,15～45 and 0～50,respectively;correspondingly,they specific growth rates in these salinity range were 0.18 d^-1～0.31 d^-1,0.07 d^-1～0.19 d^-1 and 0.06 d^-1～0.64 d^-1,respectively.The salinities for optimal growth of these three cell types were 15～30,30～40and 30～45,respectively;correspondingly,they specific growth rates in these optimal salinities were 0.30 d^-1～0.31 d^-1,0.17 d^-1～0.19 d^-1 and 0.60 d^-1～0.64d^-1,respectively.Under low（salinity≤10）and high salinities（salinity=50）caused stresses,haploid and diploid solitary cells exhibited negative growth or death;however,diploid cells within colonies maintained a low positive growth as a results of colony protection(0.06 d^-1～0.08 d^-1).（2）The colonies formation from diploid solitary cells is the key for P.globosa bloom.Diploid solitary cells could not form colony at salinities of 0,5,and 50.Diploid solitary cells formed the highest number colonies at salinity of35.The number of colonies gradually decreased with the decrease of salinity from salinity of 35 to 10.However,diploid solitary cells could not maintain normal colony structure and formed"ghost colonies"at 10 salinity.Although diploid solitary cells could not form colonies under low salinities,once colonies were formed,colony membrane may protect colonial cells from low salinity stress.The colonies color became lighter and the colonial cells condensed together under low salinities stress,but colonial cells maintained positive growth in these salinity conditions.（3）The PSⅡdamage of colonial cells was less than that of diploid and haploid solitary cells under the same salinities stress.Meanwhile,PSⅡdamage of colonial cells were more easily recovered after the relief of salinities stress.The PSⅡof haploid and diploid solitary cells were damaged under low（salinity≤10）or high salinity（salinity=50）stress;and compared to initial values,their maximum light energy conversion efficiency（F_v/F_m）,actual light energy conversion efficiency(Φ_PSⅡ)and the fluorescence values of chlorophyll a（chl-a）decreased 6.98%～97.62%,17.86%～96.55%and 5.34%～99.82%,respectively.Extreme low salinity stress（salinity≤5）caused the most severe and irreversible damage to the PSⅡof haploid and diploid solitary cells,which result in a large number of cell deaths.The PSⅡof colonial cells was damaged only under 0 salinity stress.However,their maximum relative electron transfer rate(r ETR_max)and semi saturated illumination（I_k）increased 5.15±0.34 times and 4.06±0.12 times,respectively,playing a Photoprotection.（4）Hydrogen peroxide（H₂O₂）productions of haploid and diploid solitary cells of P.globosa significantly increased under extreme low salinity stress（salinity≤5）.In contrast,the H₂O₂ production of colonial cells did not significantly increase（p>0.05）at these salinities stress.In response to oxidative stress of H₂O₂,the activity of catalase（CAT）in haploid and diploid solitary cells of P.globosa significantly increased（p<0.05）.However,peroxidation damage to the lipid membrane of cells was unavoidable,and the propylene glycol（MDA）content of the haploid and diploid solitary cells were increased 110.84～185.28 times and 43.93～73.33 times.Colonies could protect the diploid cells（colonial cells）from oxidative damage.Diploid solitary cells promptly increased its polysaccharides secretion under low salinity stress（salinity≤10）,and the polysaccharides concentration of diploid solitary cells was 5.77±1.52 times of that produced by haploid cells at 10 salinity.This may help diploid cells to resist low salinity stress and corresponding oxidative damage.