Effects Of Infection With Karenia Mikimotoi On Growth,Development And Photosynthesis Of Sargassum Fusiformis (Phaeophyta) At Different Life Stages

Seaweed aquaculture can mitigate eutrophication,provide shelters for various marine organisms and sustain the biodiversity,suppress growth of harmful algal bloom?HAB?-forming species through allelopathic effects thus reduce the risk of HABs.At present,the allelopathic effects of seaweed on HAB causative species have been widely studied,but the impacts of HABs or HAB causative species on macroalgae are largely uncharacterized,especially the effects on the development,photosynthesis and molecular basis of their embryos and seedlings.In China,the coastal waters of Dongtou District,part of Wenzhou City in the Zhejiang Province,are a major Sargassum fusiforme-producing region.In addition,during the period of harvest and propagation of S.fusiforme,HABs occur frequently.To explore the impacts of dinoflagellate blooms on maricultured S.fusiformis,the growth,development and photosynthesis of the embryos and seedlings of S.fusiformis infected with Karenia mikimotoi was investigated in the semi-cutinuous culture and batch culture individualy.In addition,we performed a transcriptomic comparison between the cultures of S.fusiforme seedling that was cultured alone and that was co-cultivated with HABs-forming dinoflagellate K.mikimotoi using Illumina RNA-Seq,to characterize on the molecular basis for the changes in photosynthesis of S.fusiforme to the stress of dense dinoflagellate blooms.The results are as follows:?1?In semi-continuous culture?the media were renewed each day?,co-cultivation with K.mikimotoi of 500?g Chl a l^-1and higher cell densities significantly?P<0.05?inhibited the development,growth,photosynthetic pigments contents and photosynthetic activities of the embryos.In addition,inhibitory effects increased with increased cell densities and prolonged infection time.On the 12^thday post-inoculation,the sizes of the mono-cultured embryos and those co-cultivated with K.mikimotoi at 500,750,1000 and 1500?g Chl a l^-1were 0.0205,0.0131,0.0125,0.0114 and 0.0102 mm³,respectively.The average relative growth rates?RGR?of the embryos during the culture period were0.1130,0.0785,0.0770,0.0677and 0.0615 d^-1,respectively.Furthermore,compared with the monocultured ones?Control?,infection with K.mikimotoi suspension at 500,750,1000 and 1500?g Chl a l^-1decreased the maximum relative electron transport rate(r ETR_max)by 64.21%,69.76%,76.39%and 79.73%,respectively.The corresponding apparent photosynthetic efficiency???was inhibited by 27.50%,53.75%,63.13%and 64.57%.?2?In batch culture,K.mikimotoi did not inhibit the eggs'fertilization but significantly inhibited the relative growth rate?RGR?,chlorophyll a?Chl a?content and the maximum relative electron transport rate(r ETR_max)of the embryos.On the fifth day post-inoculation,the size of the embryos co-cultivated with K.mikimotoi at 1000?mol Chl a l^-1began to decline significantly compared with the mono-cultured embryos and the difference increased with prolonged exposure.After 10 days'co-cultivation,the RGR,Chl a content and r ETR_maxof the embryos infected with K.mikimotoi at 1000?g Chl a l^-1decreased by 28.44%,4.99%and 63.99%,respectively,as compared with the ones cultivated without K.mikimotoi.?3?In semi-continuous culture?the media were renewed each day?,the embryos were cultivated with F/2 medium?Control?,live cell suspension?LC?,ruptured cell suspension?RC?and cell-free supernatant?FC?of K.mikimotoi at 1000?g Chl a l^-1,and the morphological changes of the embryos were recorded every other day.On the 6^thday post-inoculation and forward,the size of the embryos cultured with LC,RC and FC began to obviously smaller than those just cultivated with F/2medium although the morphologies of the embryos remained uniformed.On the 14^thday post-inoculation,the sizes of the embryos cultivated with F/2 medium,LC,RC and FC were 0.0407,0.0142,0.0254 and 0.0185mm³,respectively.The corresponding relative growth rates?RGR?of the embryos under above mentioned treatments were 0.1540,0.0935,0.1338and 0.1077 d^-1,respectively.In addition,compared the control?embryos cultured with F/2 medium?,the infection with LC,RC and FC inhibited the r ETR_maxof the embryos by 69.91%,28.52%and 56.36%,and decrased the?by70.00%,17.35%and 56.12%,repectively.The results indicated that inhibitory effects of K.mikimotoi on the embryos were mediated mainly through secretion of intracellular allelopathic substances and the cell-to-cell contacting infection.?4?In batch culture,the fresh weight?FW?,Chl a contents,apparent photosynthetic efficiency???,and relative electron transfer rate(r ETR_max)of S.fusiforme seedlings cultivated for ten days with K.mikimotoi decreased by 53.60%,13.48%,6.30%,and 36.51%,respectively,as compared with seedlings cultivated without K.mikimotoi.The leaves of the S.fusiforme seedlings began to rot and detach from the stems after a week of cultivation with K.mikimotoi,and additional epidermic cells decomposed with prolonged exposure to the dinoflagellate.In addition,transcripts of the genes related to photosystem reaction center subunits and photosynthesis antenna proteins were expressed differentially during the cultivation with K.mikimotoi,consistent with changes in the photosynthesis of S.fusiforme seedlings.These combined results indicate that the main mechanism for resistance to HABs is the formation of epidermis by a layer of tightly arranged cylindrical cells combined with differential expression of the photosynthesis-related genes.