Ecological Process Of Symbiotic Microorganisms In Environmentally Sensitive Reef-building Corals In The South China Sea In Response To Temperature Changes

Global warming and increasing human activities lead to the rapid degradation of global coral reefs.How corals respond to future environmental changes has become a focus of scientific issues.In addition to the adaptive regulation of corals,the coordinated regulation of endosymbiotic microorganisms is also the key to the host's resistance to environmental warming.In this study,the environmentally sensitive corals Acropora spp.and Pocillopora spp.in the South China Sea were used as the research objects,and the physiology of coral symbiosis under different temperature conditions was analyzed by a combination of field investigation and simulation experiment.The characteristics and the dynamic changes of the colony are used to reveal in depth how the reef-building coral symbionts coordinately respond to environmental changes.The main research results are as follows:A total of 82 samples of Acropora spp.and Pocillopora spp.were collected in the field from the four main coral reef areas of the South China Sea Nansha Islands(NS),Xisha Islands(XS),Hainan Lingshui(LS)and Weizhou Island(WZ).Analyze the physiological characteristics and community composition of bacteria and Symbiodiniaceae.The results show that the content of symbiotic Symbiodiniaceae density and bacteria increase with the increase in latitude(decrease in annual average sea surface temperature).Corals of different genera in the same coral reef areas have significant differences in the community composition of Symbiodiniaceae.In NS,Acropora spp.are dominated by C91,Cspc and C3w;Pocillopora spp.are dominated by D1,C1d and C42.As the latitude increases,the dominant subclade composition of Acropora spp.change from C50 on the XS and LS to C1 on WZ;in the LS coral reef area,Pocillopora spp.are dominated by C1d and C42 symbiodinium subclade composition.The D1 subclade group almost disappeared in this area.In terms of the composition of coral-associated bacteria,both Acropora spp.and Pocillopora spp.are dominated by Proteobacteria at the phylum level.Nevertheless,the relative abundance decreases with the increase of latitude.(Acropora spp.:reduced from87.6%of NS to 52.0%of WZ;Pocillopora spp.:reduced from 51.6%of NS to42.5%of LS).However,the relative abundance of Cyanobacteria with photosynthesis function increases with the latitude(Acropora spp.:4.1%of WZ increased by 0.2%of NS;Pocillopora spp.:8.2%of LS increased by 0.9%of NS),which has a great relationship with balance of energy supply.Correlation analysis shows that SST,nutrients and light intensity are critical environmental factors that affect the ecological characteristics of coral symbiosis.Based on the results of field analysis,one of the representative coral species,Pocillopora damicornis,was selected for the indoor experiment of high-temperature stress(26?34°C)to explore the dynamic characteristics of coral symbiont physiology and community structure.The results showed that the photosynthetic efficiency,photosynthetic pigment content,Symbiodiniaceae density and bacterial content of coral holobiont all decreased rapidly with the increase of temperature(32?34°C).When the temperature was restored(26°C),photosynthetic efficiency and photosynthetic pigments were quickly restored,but the Symbiodiniaceae density remained at a very low level without any signs of recovery.It is worth noting that the content of photosynthetic bacteria increased significantly during the coral recovery period,from 0.25×10¹⁰ copies g^-1 at 34°C to 1.13×10¹⁰ copies g^-1 at 26°C.At the same time,the relative abundance of the C1c.C45,C1 and C1ca subclades increased significantly during the thermal stress.Therefore,the Symbiodiniaceae and photosynthetic bacteria ensure the energy supply of the entire coral symbiosis through a joint response mode,thereby improving the high-temperature tolerance and resilience of the coral.In this study,the combination of field data and indoor experiment analyzed the physiological characteristics and dynamic changes of colonies of environmentally sensitive corals in the South China Sea under different environmental temperatures.In the process of coral adapting to or resisting environmental changes,the microbial symbionts have an obvious synergistic response to balance the energy supply,thereby improving the coral holobionts ability to adapt to environmental changes.This research provides in-depth insights for corals to adapt to global warming,and also provides the theoretical basis for artificially cultivating environmentally tolerant coral seedlings.