Abundance, Diversity and Functional Potentials of Planktonic Bacteria and Microeukaryotes in the Coral-Reef System of Xisha Islands, China

Corals influence microorganisms within the surrounding seawater, yet the diversities and functions of seawater bacteria and microeukaryotes in coral-reef systems have not been well addressed. We collected 40 seawater samples in outer coral reef flats and semi-closed inner lagoons from the surface, middle and bottom layers in the pristine coral-reef system of Xisha Islands, South China Sea. We detected the abundance, composition and distribution of bacteria and microeukaryotes using flow cytometry, qPCR and high throughput sequencing techniques, and profiled the potential ecological roles based on the information of 16S and 18S rDNA sequencing. In terms of flow cytometry, Prochlorococcus dominated the autotrophs with cell abundance ranging from 5.8 × 10² to 5.44 × 10³ cells mL⁻¹ seawater. Based on qPCR, the 16S rDNA copies were much higher in coral reef flats than in lagoons (P = 0.003). The bacterial communities held significantly lower diversity in bottom waters compared with surface and middle waters (P < 0.05), which were dominated by SAR11, Flavobacteriales, and Synechococcus. Alveolata represented most of the microeukaryotic communities with Dinophyceae and Syndiniales well represented in all samples. Neither bacterial nor microeukaryotic community exhibited distinct layer or niche pattern, however, Haptophyta and Picozoa decreased with depth and SAR 86, MAST-3 and Picozoa were enriched in lagoons (P < 0.05). To adapt the nutrient-poor and organic matter-rich environment, bacterial nitrogen fixation and assimilatory/dissimilatory nitrate reduction were active in the system, and mixotrophy was the most important trophic strategy among microeukaryotes. The study highlighted the ecological adaptability of seawater microbes to the unique coral-reef environments.