Zooxanthellate Lineage-specific Dynamics Of Endosymbiotic Microbial Communities In Response To Thermal Stress In The Hermatypic Coral Pocillopora Damicornis

Corals live in symbiosis with a highly diverse and abundant community of microbes,which are proposed to contribute immensely to the coral health and nutrient cycling in hospite.Diazotrophs,dinitrogen(N2)-fixing microorganisms,can supplement coral host and its symbiotic zooxanthellae with fixed nitrogen in the form of ammonia(NH3).In recent decades,increase in sea surface temperatures caused by global warming has severely threatened the coral reef ecosystem.While long-term thermal stress can lead to coral bleaching and disease outbreaks,coral symbiotic bacteria have the capacity for rapid response to environmental change.There is increasing evidence that coral symbiotic bacteria are crucial for the resilience of the entire coral holobiont.Yet the functional roles of bacteria in increasing coral fitness to thermal stress has not been adequately examined.In this study,I investigated the dynamics of coral symbiotic bacterial communities in general and diazotrophic communities specifically under elevated temperature in the hermatypic coral Pocillopora damicornis with distinct Symbiodiniaceae clades(Cladocopium spp.vs.Durusdinium spp.)using high throughput sequencing of 16S rDNA and nifH.The main findings of this study are as follows:1.The most dominant bacterial groups associated with P.damicornis belonged to the phyla Cyanobacteria,Proteobacteria,Bacteroidetes and Firmicutes,among which Proteobacteria species in the classes Alphaproteobacteria and Gammaproteobacteria,Bacteroidetes species in the order Cytophagales,and Firmicutes species in the order Clostridiales were the most abundant.The phyla Proteobacteria and Cyanobacteria dominated the diazotrophic communities associated with the coral host.In particular,Alphaproteobacteria species in the orders Rhizobiales,Rhodospirillales,Sphingomonadales and Rhodobacterales were the most abundant,and other relatively abundant diazotrophs included Gammaproteobacteria species in the order Chromatiales,Betaproteobacteria species in the order Burkholderiales and Deltaproteobacteria species in the order Desulfarculales.2.The relative abundance of major bacterial taxa changed over the course of thermal stress.Cyanobacterial genera Aerosakkonema and Oscillatoria,Ochrobactrum of the order Rhizobiales were sensitive to elevated temperature and significantly reduced during the thermal stress experiment.Conversely,the relative abundance of Firmicutes species in the genus Oceanirhabdus was significantly increased under thermal stress.Similar pattern of community change was also found in the coral symbiotic diazotrophs.Rhodospirillales species in the genus Azospirillum and Chromatiales species in the genus Marichromatium were significantly increased under thermal stress.In contrast,the relative abundances of Rhodovulum,Methyloceanibacter,Novosphingohium,and Azoarcus significantly decreased.3.The changes in coral symbiotic bacterial communities under thermal stress appear to be correlated with the different clades of Symbiodiniaceae.The bacterial communities of P.damicorni.s with dominant clade C symbiont were more negatively affected by the thermal stress,with induced indicator species previously recorded in diseased or stressed coral.Whereas P.damicornis with dominant clade D symbiont maintained relatively more stable bacterial community structure and having more bacteria similar to those found in extreme environments.These bacterial indicator species may contribute to the adaptation of coral to environmental change and can be recognized as bioindicators to predict the coral host response to heat stress.This study investigated the dynamics of coral symbiotic bacterial and diazotrophic communities under elevated temperature in the stony coral P.damicornis with distinct Symbiodiniaceae clades.The results highlight the importance of coral symbiotic bacteria,when coupled with zooxanthellae,in mediating coral responses to environmental change,which has implications for coral reef protection and restoration under global climate change.