| Harmful algal blooms(HABs)have brought huge damage to marine ecosystem around the word.The inhibitory effects of seaweeds on the HAB-forming species have has attracted much attention,but the effects of HABs on macrophytes have been rarely studied.Prorocentrum donghaiense is the main HAB-forming species in the East China Sea,which has brought disaster to the aquaculture industry.In addition,the outbreak period of P.donghaiense blooms is coincident with the sexual reproduction period of Sargassum fusiformis.zygoteszygotes In this study,to explore the influence of P.donghaiense allelopathy on the breeding of S.fusiformis seedlings,and to provide theoretical basis and data support for evaluating the impact of offshore HABs on S.fusiformis industry,the effects of live cell suspension,the cell-free supernatant,ruptured cell suspension as well as lipophilic extracts of P.donghaiensis on the growth,development and photosynthetic activities of the S.fusiformis zygotes were studied.The main findings are as follows:(1)Under semi-continuous culture conditions,different concentration gradients of P.donghaiensis inhibited the growth and photosynthetic activity of S.fusiformis zygotes,and the inhibitory effects increased with enhanced cell densities.After 10 days of co-cultivation with P.donghaiense at 0,0.50'105,0.75'105,1.00'105and 1.50'105cell ml-1,the volumes of S.fusiformis zygotes were 31.23'10-3,28.54'10-3,25.80'10-3,22.36'10-3and 12.05'10-3 mm3,and the average relative growth rates calculated according to the changes in zygotes'volume were 0.26,0.25,0.24,0.23and 0.16 d-1,respectively.The the chlorophyll a(Chl a)fluorescence transient analysis(JIP-test)showed that the allelopathy of P.donghaiensis mainly hindered the electron transfer from the PSII receptor side and PSII to PSI,and inactivated some of the photochemical reaction centers in the S.fusiformis zygotes.(2)Under semi-continuous culture conditions,the live cell suspension(LC),the ruptured cell suspension(RC)and the cell-free supernatant(FC)of 1.00×105 cells ml-1 all inhibited the growth and chlorophyll a synthesis and photosynthesis of S.fusiformis zygotes.On the 10th day post-inoculation,the volumes of the zygotes in the control group,RC,LC,and FC treatments were 31.83'10-3,22.55'10-3,and 19.94'10-3 mm3,respectively.The corresponding chlorophyll a concentrations were579.60,501.82,562.91 and 262.15?g g-1,respectively.In addition,the chlorophyll a(Chl a)fluorescence transient analysis(JIP-test)results showed that the maximum quantum yield(?Po),the performance index(PI),the photon trapping performance(PTR),the performance of photon-excited electron transfer(PET),and PSII reaction center density(RC/ABS)of S.fusiformis zygotes were further inhibited under LC and FC treatments than those under RC treatment.These results suggested that the inhibitive effects of P.donghaiense on the growth and photosynthetic activity of S. fusiformis zygotes were mainly caused by releasing allelochemicals to the cultures.In addition,the ruptured cell suspension may contain some substances that can stimulate the growth and photosynthesis of S.fusiformis zygotes,and offsetting some of the inhibitory effects of the allelochemicals to a certain extent.(3)Under semi-continuous culture conditions,the growth and photosynthesis of S.fusiformis were inhibited byzygotes by P.donghaiense lipophilic extracts.After 2 weeks'cultivation,the maximum relative electron transport rate(r ETRmax),the apparent photosynthetic efficiency(?)and the maximum quantum yield(?Po)of the zygoteszygotes were significantly inhibited by the lipophilic extracts.Furthermore,the chlorophyll a(Chl a)fluorescence transient analysis(JIP-test)showed that the P.donghaiense extracts inhibited the electron transport in PSII of the zygotes,and also inactivated some reaction centers zygotes of PSII which led to a large amount of excitation energy dissipated as heat energy. |
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