Study On The Relationship Between Endosymbiont Symbiodiniaceae Sterol Metabolism And Coral Bleaching Under Environmental Stresses

Coral reefs are in significant decline globally due to climate change and environmental pollution.Functional Coral-Symbiodiniaceae symbiosis is the most important feature of coral reefs.However,the cellular mechanisms of the symbiotic interaction between cnidarians,such as corals and sea anemones,and the Symbiodiniaceae are poorly understood.Sterols are vital components of all eukaryotic cells and play an important role in many aspects of plants and animals.However,it remains uncertain how the sterols biosynthesis may change under bleaching-inducing stresses and whether and to what extent the sterol change in endosymbionts contribute to the dysfunction of symbioses.Moreover,the ocean is becoming more acidic due to rising atmospheric p CO₂,and ocean acidification is considered a major threat to coral reefs.However,little is known about the exact mechanism by which acidification impacts coral symbiosis.As an important component of the symbiotic association,to explore the responses of symbionts could greatly enhance our understanding of this issue.We checked the isoprenoid and sterol biosynthetic genes in the published cnidarians genomes and Symbiodiniaceae genomes,respectively.In addition,we analyzed the sterol profiles of Heteractis crispa and explored the relationship between sterol homeostasis and cnidarians bleaching.We next surveyed the sterol composition and biosynthetic pathways in Symbiodiniaceae.Moreover,we examined the dynamics of sterol profiles and their biosynthetic genes in different growth stage and to various bleaching inductive stimuli.At last,we identified metabolomic changes of Breviolum minutum in response to acidification.The main results of this research are showed as followed:1.Several genes of the mevalonate(MVA)pathway for synthesizing IPP(isopentenyl pyrophosphate),the building block for isoprenoid biosynthesis,have been positively identified in cnidarians genomes.In contrary,the genes from the 1-deoxyxylulose5-phosphate(MEP)pathway,an alternative route for IPP biosynthesis,were mostly missing.Consistent with the previous studies,our genome-based metabolic reconstruction supports an absence of sterol biosynthesis pathway in cnidarians.However,we identified several homologues to downstream sterol metabolic genes.Cnidarian cannot make sterol but may metabolize the acquired Symbiodiniaceae sterol for its own benefits.2.Total 18 sterols was identified in H.crispa.Among them,cholesterol is 9.63%of total sterols,other sterols are mostly side chain or/and nucleus alkylated sterols.Sterol homeostasis was associated with H.crispa healthy;inhibition of Symbiodiniaceae sterol biosynthesis induces cnidarian beaching,the decrease of total sterol,particularly,the drastic reduction of cholesterol,at least,one of the causative reasons for H.crispa beaching.3.Symbiodiniaceae exclusively express enzymes of the MEP pathway,and use that pathway for isoprenoid biosynthesis.Symbiodiniaceae resembles in the core sterol biosynthetic pathway but has different pathway architecture and substrate specificity.In total,in B.minutum and Fugacium kawagutii,17 sterols were identified,but with different dominant sterol species and biosynthetic intermediates,potentially due to host-specific symbiotic relationship.The propose sterol biosynthetic pathway in Symbiodiniaceae exhibits both common and distinct features from those in fungi,animals,green plants,and documented microalgae.4.Sterol biosynthesis and accumulation appears to be a feature of Symbiodiniaceae growth.Temporal tracking of sterol profiles and sterol-biosynthetic transcripts under different environmental cues reveals similarities and distinctions of sterol synthesis in the dinoflagellate in response to individual stresses.A common regulatory node was revealed in Symbiodiniaceae in response to different stresses,as that the total sterols decreased paralleled with the downregulation of squalene synthase gene under all studied stresses.This suggests that bleaching-inducing stresses on Symbiodiniaceae perturb their sterol biosynthesis.5.Marked alterations in metabolite pools were observed in response to acidification.Increase saturation of cell membrane and abundance of oligosaccharides(trehalose and stachyose)are important strategy in adaptation to acidification stress.Acidification may affect the biosynthesis of amino acids and proteins,and thereby inhibit the growth of B.minutum.