Coral enhances its environmental adaptability by adjusting trophic status

Coral bleaching and the ecological degradation of coral reefs have become increasingly severe due to global warming and human activities. As mixotrophic organisms, scleractinian coral can not only obtain energy through photosynthesis of symbiotic zooxanthellae (autotrophy) but also ingest nutrients in seawater through the coral host (heterotrophy).

However, the influence of coral's trophic flexibility on environmental adaptability remains unclear. Coral reefs are widely distributed in the South China Sea (SCS), spanning about 20 latitudes from north to south. The environmental conditions of coral reef areas at different latitudes are significantly different. The Sanya coral reef in the northern SCS (relatively higher latitude) is seriously affected by human activities; coral reefs in the central and southern SCS (relatively lower latitude) are less affected by human activities due to their distance from the mainland. Exploring coral trophic patterns among different coral reefs on a large spatial scale in the SCS is conducive to revealing the adjustment mechanism of coral trophic status and their effects on coral's environmental adaptability.

A recent study has revealed the effects of coral's unique trophic flexibility on their environmental adaptability. In this study, a total of 70 samples of Favia palauensis, a type of scleractinian coral, were collected from coral reefs at different latitudes (Sanya, Xisha Islands, and Nansha Islands) in the SCS. Firstly, researchers have analyzed the spatial differences of zooxanthellae density (ZD). The results show that ZD decrease with the decreasing latitude. Further analysis found that changes in nutrient level, sea surface temperature (SST), and light intensity are the main reasons for the spatial difference of ZD in the SCS.

In addition, through the separation of coral polyps and their symbiotic zooxanthellae, as well as the stable carbon isotope composition(δ13C) analysis of the coral polyps and zooxanthellae, revealed the significant spatial differences in coral trophic patterns (i.e. relative autotrophic photosynthesis and heterotrophic predation intensity).

The results show that corals in Sanya have higher photosynthetic rates and autotrophic abilities, while corals in the Xisha and Nansha Islands have relatively higher heterotrophic abilities. This means that the conspecific coral can adjust the intensity of autotrophic/heterotrophic under different environmental conditions. The flexible adjustment of trophic patterns, especially the improvement of heterotrophic predation ability, is conducive to enhance its adaptability to different environmental conditions.

This study illustrates the importance of trophic flexibility to coral's environmental adaptability from the perspective of energy supply. The research conclusions provide new indicators and information for assessing the adaptation potential of coral in the SCS under global warming and human activities.

Journal Information: Shendong Xu et al, Spatial variations in the trophic status of Favia palauensis corals in the South China Sea: Insights into their different adaptabilities under contrasting environmental conditions, Science China Earth Sciences (2021). DOI: 10.1007/s11430-020-9774-0  

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