Effects Of The Massive Cultivation Of Gracilaria Lemaneiformis On Plankton Community Structure And Its Inhibition On Microalgal Bloom

Plankton,as vital components of marine ecosystems,fundamentally supports global biological and geochemical processes.Plankton diversity reflects water quality,functions as the base of aquatic ecosystem,and thus should be closely monitored and assessed as a critical feature of marine ecosystems.Along with the rapid development of Gracilaria lemaneiformis cultivation in Nan’ao Island during the past decade and the growth inhibitory effects of G.lemaneiformis on phytoplankton,particularly harmful algal bloom-forming microalgae,both from laboratory and field experiments,we raised a hypothesis about the possible effect of massive cultivation of Gracilaria lemaneiformis on plankton biodiversity,which was tested by this study.By applying a pair of pariticularly designed 28 S r DNA-specific primers and PCR-based pyrosequencing for a large number of samples taken from the cultivation area and adjacent area in Nan’ao Island,our hypothesis that massive cultivation of Gracilaria lemaneiformis can maintain and even enhance the biodiversity of plankton community was perfectly proved tenable.A further investigation on the mechanisms for the inhibitory effects of G.lemaneiformis on the growth of harmful algal blooms（HABs）-forming microalgae was conducted as well.The major results are summerized as follows:1.51 samples were sequenced,and a total of 1 million effective sequences for the partial 28 S r RNA gene including the highly variable D2 domain were obtained and clustered into 2,221 operational taxonomic units（OTUs）,including phytoplankton: 821,protist: 494,other zooplankton: 273,fungi: 351 and unclassified: 282.2.Prior to seaweed inoculation（December 2014）,there was no significant difference in the OTU richness,communinity evenness and dominance in total plankton,phytoplankton,protist,other zooplankton,and fungi among sampling sites.The OTU richness was much higher in G and Cg areas than that in Co area（579 and 597 vs.299）when G.lemaneiformis cultivated and reached the maximal biomass in May 2015,3.In May 2015,The Shannon-Wiener Indice（community evenness）in G and Cg areas were much higher than that in Co area（One-way ANOVA,P<0.01）,while,understandably,the Simpson Indice（community dominance）in G and Cg areas were much lower than that in Co area（One-way ANOVA,P<0.01）.4.PCA and PCoA analyses showed that,while the plankton community structures were similar among all sampling sites before G.lemaneiformis cultivation（December 2014）,the plankton community structure in the seaweed cultivation area was markedly different from the control area and reached to its highest biomass in May with the cultivation and growth of G.lemaneiformis.5.Using High-throughput sequencing,a number of new（unclassified yet）,or newly recorded,or highly small-sized（pico-）species were found from the study area as the first time.6.Hierarchical Clustering Tree Analysis for the surface samples of May 2015 indicated that all the samples were divided into two main branches: one for the Co and the other for G and Cg,which was similar to the results in PCoA.7.The results of RDA showed that the eukaryote plankton communities of different areas were regulated by multiple environmental variables.Compared to other factors（e.g.temperature,p H,TN,NO₃^--N）,G.lemaneiformis biomass,TP,and Chl.a,were three significant environmental factors affecting the eukaryotic plankton community.G.lemeneiformis biomass was negatively correlated to Chl.a,TN,TP,and NO₃^--N,which indicated that G.lemeneiformis biomass played a decisive role in plankton community and water nutrients.8.It was further confirmed that G.lemeneiformis could inhibit growth of Scrippsiella trochoidea and Akashiwo sanguinea microalgae via allelopathy as a major contributor.The fresh thalli（FT）,extraction of fresh thalli（EFT）and extraction of dried and powdered thalli（EDP）were capable of lysing or strongly inhibiting the growth of all these two HAB species in a dosedependent manner within controlled laboratory experiments during which high nutrient levels,low bacterial levels,and common p H levels among treatments and controls.The EFT showed the strongest inhibition effects,then the EDP,while the FT showed the weakest relatively.A.sanguinea reacted to G.lemeneiformis as a vengeance to the allelopathy via promoting the growth of bacteria,which proved the complex interaction of G.lemeneiformis,microalgae and bacteria.All the results together explicitly demonstrated that cultivation of the seaweed G.lemaneiformis could enhance the biodiversity of eukaryotic plankton community,which provides further support for using the massive cultivation of G.lemaneiformis in coastal regions as an effective approach for environmental bioremediation,controlling harmful microalgal blooms,and promoting ecosystem stability.