Photosynthetic Influencing Mechanism Of Macroalgae Gracilaria Lemaneiformis And Ulva Fasciata Powders On The Two HABs

Macroalgae,the indigenous species widely distributed in marine areas,as its advantage of economically valuable and environmental friendly,is an ideal material to be used to control Hamful Algal Blooms?HABs?.In this experiment,the dried powder of two common macroalgae,Gracilaria lemaneiformis?G.lemaneiformis?and Ulva fasciata?U.fasciata?,were selected to co-cultured with Heterosigma akashiwo?H.akashiwo?and Skeletonema costatum?S.costatum?to explore the photosynthetic influencing mechanism of the two seaweeds dried powder against red tide microalgae.Chlorophyll a content,chlorophyll fluorescence parameters,photosynthesis-related gene expression and transcriptome were determined.The main results of the study are as follows:1)When the dried G.lemaneiformis powder was co-cultured with S.costatum,the low-concentration-stimulating and the high-concentration-inhibiting pattern on the growth of S.costatum was observed.Under the co-culture system of 1 g/L dried powder,the content of chlorophyll a increased,the fluorescence intensity of each phase of OJIP curve went up,the energy flux of electron transfer and the number of PSII active reaction centers per excited cross-section?ETo/CSo and RC/CSo?increased,which indicated that the functional structure of PSII was intact and the physiological state of microalga was good.Conversely,higher concentrations?2 g/L and 3 g/L?of G.lemaneiformis powder had a strong dose-dependent inhibitory effect on the S.costatum.After 24 hours‘co-culture,the inhibitory effect was strongest.The significant decrease of chlorophyll a content,proportion of oxygen-evolving complex and the number of active reaction centers,coupling with OJIP curve tending to be flat,electron transport chain blocking-up,energy absorbed by PSII used to drive electron transfer reducing,energy dissipation increasing,and decline of the efficiency of the primary photochemistry,was observed,all of which suggested that PSII,including the donor side,acceptor side,and the reaction center,were seriously damaged.In addition,after 24 hours‘co-culture with 2 g/L G.lemaneiformis dried powder,the expression of the related genes,psb A,psb C and rbcL of S.costatum were significantly down-regulated.2)The dried G.lemaneiformis powder showed an inhibitory effect on H.akashiwo,which was positively related to the biomass of dried G.lemaneiformis.1 g/L G.lemaneiformis powder damaged the PSII reaction center of H.akashiwo but showed little effect on the donor side and the receptor side.Chlorophyll a content and the number of active reaction centers decreased;the light energy absorbed by chlorophyll antenna per excited cross-section and the energy used for electron transfer were reduced under the treatment of low-concentration G.lemaneiformis powder.Higher concentration treatments of 2 g/L and 3 g/L exerted the strongest inhibition in the treatment of 24 h.The apparent growth inhibition rate was 84.9%in the 2 g/L dried powder group.The damage of oxygen-evolving complex in the donor side of PSII,severe blocking-up of electron transport chain,the decrement in number of active reaction centers and efficiency of the primary photochemistry was detected.Photosynthesis-related genes,psb A,psb C and rbcL of H.akashiwo displayed specific expression and regulation response to the stress of 12 hours‘G.lemaneiformis powder treatment.3)When the dried U.fasciata powder was co-cultured with S.costatum,even a low concentration treatment showed a strong inhibited effect on S.costatum.Similarly,the inhibitory effect was positively correlated with the concentration of dried powder.The growth inhibition of the low concentration treatment reached the strongest at 24 h,then weakened after 48 h.The allelochemicals of the dried powder of U.fasciata affected the expression of photosynthesis-related genes of S.costatum,the expression of psbA,psb C and rbc L gene was declined significantly after 24 h,which might cause serious damage to the light-harvesting protein complex,the donor side,receptor side and reaction center of PSII.Decrease of Chlorophyll a content and the effect to the intensity of the chlorophyll fluorescence parameters was basically consistent with the above-mentioned high-concentrated dried G.lemaneiformis powder group.4)Dried U.fasciata powder also showed an inhibitory effect on H.akashiwo with a dose-effect.When 1 g/L biomass of dried U.fasciata was added,the damage site might be mainly in the PSII reaction center,which was similar to the low-concentration dried G.lemaneiformis above-mentioned.When the concentration was increased to 2 g/L,the PSII of H.akashiwo,including the donor side,receptor side and reaction center,were seriously damaged.The changing trend of chlorophyll fluorescence parameters was almost consistent with that of high concentration of dried G.lemaneiformis powder,but the inhibition effect of U.fasciata was more stronger.Photosynthesis-related gene,psb A,psb C,and rbc L responded specifically to the stress of the dried U.fasciata powder.From the result of transcriptome analysis,it was found that the dried U.fasciata powder disturbed the normal process of photosynthesis in H.akashiwo by destroying the functional structure of PSII and PSI core complex,light-harvesting chorophyll protein complex,Cyt b6/f complex and ATP synthase,furtherly affecting the absorption and transmission of light energy in the photosystem,electron transport chain,oxidative phosphorylation and CO₂ fixation process.In addition,DNA duplication and repair,ABC transporters were interfered,which further illustrated the death mechanism of H.akashiwo.The experimental results proved that dried G.lemaneiformis and U.fasciata powder can significantly inhibit the growth of S.costatum and H.akashiwo.In general,the allelopathic inhibition effect of the dried powder of G.lemaneiformis was greater than that of U.fasciata,and H.akashiwo was more susceptible to macroalgae powder compared to S.costatum.Dried macroalgae powder probably interfered with the normal expression of photosynthesis-related genes in red tide microalga,which resulted in the damage of the structural function of the microalgae photosynthetic apparatus,absorption of light energy,energy conversion,electron transfer,ATP synthesis and process of CO₂ fixation and DNA duplication and eventually led to the death of microalga.In summary,dried G.lemaneiformis and U.fasciata could be a potential agicidal candidate for controlling S.costatum and H.akashiwo blooms.