Abstract
With the increasing anthropogenic CO2 concentration, ocean acidification (OA) can have dramatic effects on coral reefs. However, the effects of OA on coral physiology and the associated microbes remain largely unknown. In the present study, reef-building coral Acropora gemmifera collected from a reef flat with highly fluctuating environmental condition in the South China Sea were exposed to three levels of partial pressure of carbon dioxide (pCO2) (i.e., 421, 923, and 2070 μatm) for four weeks. The microbial community structures associated with A. gemmifera under these treatments were analyzed using 16S rRNA gene barcode sequencing. The results revealed that the microbial community associated with A. gemmifera was highly diverse at the genus level and dominated by Alphaproteobacteria. More importantly, the microbial community structure remained rather stable under different pCO2 treatments. Photosynthesis and calcification in A. gemmifera, as indicated by enrichment of δ18O and increased depletion of δ13C in the coral skeleton, were significantly impaired only at the high pCO2 (2070 μatm). These results suggest that A. gemmifera can maintain a high degree of stable microbial communities despite of significant physiological changes in response to extremely high pCO2.
Highlights
Response to ocean acidification (OA), or whether theses changes alter host physiology
The rarefaction analyses revealed that the sequencing effort for each sample was sufficient to reflect the microbial diversity, and the rank-abundance curve showed that most operational taxonomic units (OTUs) had an abundance lower than 0.1%, which demonstrated that the microbial communities were occupied by rare species
There were no significant differences in beta diversity among the pCO2 treatments, which is in contrast to the findings of a previous report showing an increase in coral microbial diversity with decreasing pH, possibly caused by an intermediate disturbance[16]
Summary
Response to OA, or whether theses changes alter host physiology. Preliminary laboratory-based investigations have revealed a remarkable impact of increased pCO2 or reduced pH on coral microbial communities[12,16,18]. No significant changes were observed in the microbial communities of transplanted corals in natural CO2 vents[19] and associated with two Pacific corals after 8 weeks of exposure to increased pCO220. These contradictory findings underscore the need for further research. The potential of natural microbial communities in corals to acclimatize/adapt to OA cannot be overlooked. A. gemmifera colonies collected from this reef flat were exposed to three pCO2 levels to test our hypothesis that both the coral physiology and the microbial communities associated with this coral species are stable and resistant to OA exposure
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