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

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.

Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities

“…Corals associated with Cladocopium goreaui (C1) have previously shown elevated photosynthetic efficiency (Cantin et al, 2009;Morgans et al, 2020). It is therefore possible that elevated photosynthesis is a common trait within some taxa from the C1 radiation and may explain the higher photosynthetic efficiency mechanism of C1 to face OA as observed by Ge et al (2021) inAcropora valida . The elevated photosynthetic capacity of A. tenuis in Bouraké, coupled with the dominance of major ITS2 sequence C1, could explain their significantly higher growth rates, as previously observed in A. tenuis juveniles (Cantin et al, 2009;Little et al, 2004).…”

Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities

“…Interestingly, however, Bouraké corals exhibited more consistent major ITS2‐type profiles between replicate colonies at low and variable pH than corals from the reference site. Microbiome stability (both for symbionts and/or bacteria) was linked to greater physiological resilience to OA (Ge et al, 2021 ; Grottoli et al, 2018 ; Quigley et al, 2017 , 2019 ; Ros et al, 2021 ), suggesting that consistent major ITS2‐type profiles for Bouraké corals under pH treatments could facilitate their success; however, further work will be needed to verify this hypothesis.…”

“…Corals associated with Cladocopium goreaui (C1) have previously shown elevated photosynthetic efficiency with regard to other Symbiodiniaceae species (Cantin et al, 2009;Morgans et al, 2020;Stat, Morris, & Gates, 2008;Wall et al, 2020). It is therefore possible that elevated photosynthesis is a common trait within some taxa from the C1 radiation and may explain the higher photosynthetic efficiency mechanism of C1 to face OA as observed by Ge et al (2021) in Acropora valida. The elevated photosynthetic capacity of A. tenuis in Bouraké, coupled with the dominance of major ITS2 sequence C1, could explain their significantly higher growth rates, as previously observed in A. tenuis juveniles (Cantin et al, 2009;Little et al, 2004).…”

Coral bleaching from a nutrient perspective is understudied: A bibliometric survey

“…In addition, the circle size of ocean acidification is larger than that of nutrient and most other keywords in Figure 4. However, recent indoor studies demonstrated that Symbiodiniaceae community composition was hardly affected by ocean acidification, suggesting that more field studies on Symbiodiniaceae responses to ocean acidification are required while considering the experimental period and acidification intensity (Ge et al, 2021;Zhou et al, 2021). It is worth mentioning that a field with relatively limited research is not necessarily an unpopular field but can be the latest research field.…”