Underlying Molecular Mechanism Of Karenia Mikimotoi Against Grazing Pressure From Zooplankton

Karenia mikimotoi is a toxin-producing dinoflagellate,which can form harmful algal blooms over the world,causing severe ecosystem damages and great economic losses.Many studies have been performed on its taxonomic identification,nutrient physiology and ecological processes of related blooms.It was found that the allelopathic effect and anti-grazing mechanism of K.mikimotoi might play important roles in the development of the K.mikimotoi population and blooms.To understand the anti-grazing mechanism of K.mikimotoi,using Pseudodiaptomus annandalei and Artemia salina as the zooplankton models in this paper,physiological,biochemical and molecular responses of the K.mikimotoi cells to the grazing pressure from zooplankton were analyzed by transcriptomics.In our study,direct exposure group(grazing group)and indirect exposure group(filtrate group)were designed.In the direct exposure group,K.mikimotoi cells were co-cultured with zooplankton,while the microalgal cells were exposed in the filtrates from the co-cultured medium of K.mikimotoi cells and zooplankton in the indirect exposure group.It was found that after direct exposure to the zooplankton,the growth of K.mikimotoi was significantly inhibited,and the photosynthetic rates was decreased.Meanwhile,the content of hemolytic toxin,carbohydrates and neutral lipids were increased,and the composition of fatty acid were changed.Similar alterations were observed in the indirect exposure group,suggesting that K.mikimotoi could sense the feeding pressure through the allelochemicals in the filtrate,and respond to it.Transcriptome analysis showed that grazing pressure significantly affected the gene expression in K.mikimotoi.The expression of 4907 gene in K.mikimotoi was significantly changed when it co-cultured with P.annandalei.After exposed in the co-cultured filtrate with P.annandalei,gene expression of 13,671 in K.mikimotoi was significantly changed.Among the differential expression genes in P.annandalei-exposed K.mikimotoi,genes related to photosynthesis were significantly down-regulated,while those related to energy metabolism,carbon metabolism,lipid metabolism and some ABC transporters were significantly up-regulated.These transcriptomic data were further corroborated by the biochemical analyses which revealed that glycolysis and TCA cycle of the K.mikimotoi cells was significantly increased,while photosynthetic efficiency was decreased.Taking together,we proposed that stronger grazing pressure could increase the energy metabolism and carbon metabolism,and induce electron transport in K.mikimotoi.K.mikimotoi might produce more energy and regulate its nutrient composition against grazing from zooplantkon.The up-regulation of the ABC transporters suggested that K.mikimotoi might transport secondary metabolites such as toxins to the cell membrane or the sourounding environment against grazers,whereas up-regulation of axon protein gene related to flagella indicated that K.mikimotoi could strengthen its its motility to avoid predators.It is worth noting that different from the direct feeding group,the photosynthetic related genes including Psb27 and PetH in the indirect feeding group were significantly up-regulated,and the oxidative phosphorylation related genes such as Ndufs6 and SDH1 were significantly down-regulated,suggesting that the response of K.mikimotoi to feeding pressure varied with the degree of pressure.Anyhow,under feeding pressure K.mikimotoi could increase prodution of hemolytic toxins,carbohydrates and neutral lipids,and make changes in fatty acid composition and expression of important genes related to photosynthesis,energy metabolism,carbon metabolism,lipid pathways and ABC transporters,etc.These results suggest that besides producing hemolytic toxins that directly affect the survival of the predator,K.mikimotoi could fight against grazing by altering energy metabolism,regulating its own nutrient composition,and even enhancing its motility.