Regulation of the Coral-Associated Bacteria and Symbiodiniaceae in Acropora valida Under Ocean Acidification.

Ruiqi Ge,Jiayuan Liang,Kefu Yu,Liangyun Qin,Chuanqi Deng,Biao Chen,Jinni Chen,Yongqian Xu,Xiaopeng Yu

doi:10.3389/fmicb.2021.767174

Abstract

Ocean acidification is one of many stressors that coral reef ecosystems are currently contending with. Thus, understanding the response of key symbiotic microbes to ocean acidification is of great significance for understanding the adaptation mechanism and development trend of coral holobionts. Here, high-throughput sequencing technology was employed to investigate the coral-associated bacteria and Symbiodiniaceae of the ecologically important coral Acropora valida exposed to different pH gradients. After 30 days of acclimatization, we set four acidification gradients (pH 8.2, 7.8, 7.4, and 7.2, respectively), and each pH condition was applied for 10 days, with the whole experiment lasting for 70 days. Although the Symbiodiniaceae density decreased significantly, the coral did not appear to be bleached, and the real-time photosynthetic rate did not change significantly, indicating that A. valida has strong tolerance to acidification. Moreover, the Symbiodiniaceae community composition was hardly affected by ocean acidification, with the C1 subclade (Cladocopium goreaui) being dominant among the Symbiodiniaceae dominant types. The relative abundance of the Symbiodiniaceae background types was significantly higher at pH 7.2, indicating that ocean acidification might increase the stability of the community composition by regulating the Symbiodiniaceae rare biosphere. Furthermore, the stable symbiosis between the C1 subclade and coral host may contribute to the stability of the real-time photosynthetic efficiency. Finally, concerning the coral-associated bacteria, the stable symbiosis between Endozoicomonas and coral host is likely to help them adapt to ocean acidification. The significant increase in the relative abundance of Cyanobacteria at pH 7.2 may also compensate for the photosynthesis efficiency of a coral holobiont. In summary, this study suggests that the combined response of key symbiotic microbes helps the whole coral host resist the threats of ocean acidification.

Highlights

Since the Industrial Revolution, the concentration of atmospheric CO2 has continuously increased
In the principal coordinates analysis (PCoA) based on the Operational taxonomic units (OTU) level, wherein PC1 accounted for 15.94% of the overall variance variation, the results showed the community composition of the coral-associated bacteria under the different pH conditions (Figure 4C)
The response of A. valida to ocean acidification is likely a complex biological process, which includes the dynamic changes in the coral hosts, coral-associated bacteria, and Symbiodiniaceae

Summary

Introduction

Since the Industrial Revolution, the concentration of atmospheric CO2 has continuously increased. As the most significant carbon pool on the earth, the ocean has continuously absorbed CO2 from the atmosphere, causing the pH of the surface seawater to drop and promoting ocean acidification, which is one of the biggest threats to the global coral reef ecosystem (Hoegh-Guldberg et al, 2007). Ocean acidification affects the photosynthesis, calcification, growth, and survival of coral reef (Albright and Langdon, 2011). The fourth report of the Intergovernmental Panel on Climate Change (IPCC) in 2007 focused on the adverse effects of ocean acidification on corals and other calcareous organisms (Mingle, 2020). The report titled “The ocean and cryosphere in a changing climate,” released by the IPCC in 2019, further pointed out that ocean acidification and other environmental pressures have impacted the distribution and abundance of marine organisms (Mingle, 2020). If CO2 emissions continue to increase, the existing coral reef ecosystems will face survival problems by the end of the 21st century (Ricke et al, 2013)

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