Abstract
With the abnormal rise in ocean temperatures globally in recent years, coral bleaching is becoming common and serious. However, the response mechanisms and processes of coral symbionts to bleaching are not well understood. In this study, metagenomics and metatranscriptomics were used to explore the composition of coral symbionts and their functions in response to coral bleaching. All four bleaching coral species displayed a significant reduction of the abundance and function of Dinophyceae-like eukaryotes at the DNA and RNA levels. However, different species of bleaching coral have their own characteristic symbiotic components. Bleaching Acropora tenuis and Goniastrea minuta corals exhibited a very high abundance of prokaryotes and associated gene functions, especially for opportunistic bacteria. In contrast, algae and fungi were identified as the main microbial associate components and had relatively high RNA abundance in bleaching Pocillopora verrucosa and Pocillopora meandrina. Different coral species, whether unbleached or bleaching, have the same symbiotic taxa that perform the same biological functions in vivo. Different stages of bleaching, or transitional states, were identified by different genome content and functional gene abundance among bleaching corals. These stages should be considered in future coral bleaching studies to accurately determine symbiont structure and function. An implicit hypothesis is that there is a causal relationship between the stability of eukaryotic communities and coral bleaching.
Highlights
El Nino events have a significant impact on global climate, most notably causing warming events, which affect the stability of marine ecosystems (Heron et al, 2016; Hughes et al, 2018a)
Cofactor and vitamin biosynthesis, and ATP synthesis. Both the metagenomic and metatranscriptome results of the current study indicated that the main contributing prokaryotes for these functions were Proteobacteria, Bacteroidetes, Chlorobi and Actinobacteria (Supplementary Figure S1), and the abundance of bacterial orders was significantly correlated with function, especially at the RNA level (t-test, R2 > 0.8, p < 0.01)
Previous studies demonstrated that the metabolism of the microbial community could shift from autotrophy to heterotrophy under stress, resulting in an increase in the abundance of microbial genes involved in sulfur and nitrogen metabolism, and secondary metabolism (Thurber et al, 2009; Littman et al, 2011)
Summary
El Nino events have a significant impact on global climate, most notably causing warming events, which affect the stability of marine ecosystems (Heron et al, 2016; Hughes et al, 2018a). Metagenomics has been used to investigate the taxonomic diversity and metabolic capabilities of coral-associated microbes under thermal stress or bleaching (Thurber et al, 2009; Littman et al, 2011; Lee et al, 2015; Ziegler et al, 2017) These studies suggested that microbes can undergo major shifts, from symbionts to opportunistic microbes or potential disease-causing bacteria, during heat stress or bleaching (Bourne et al, 2007; Thurber et al, 2009; Littman et al, 2011). The metabolism of the microbial community can shift from autotrophy to heterotrophy (Littman et al, 2011), which involves sulfur and nitrogen metabolism, fatty acid and lipid utilization, and secondary metabolism
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