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Coral reefs are among the most energetic marine ecosystems, taking a place in ecological balance. Microbes participate in the energy exchange in coral ecosystems, which may affect coral resistance and resilience. However, there is still a lack of understanding regarding the microbial structure between corals and seawater. In this study, microbial structure and interactions in coral and seawater were studied using quantitative PCR and high-throughput sequencing. We found that the abundance and diversity of microbes in corals were higher than those in seawater. Corals, as nonfluidic ecosystems, limited the dispersal of microbes, causing broader stochasticity in the ecological shaping process, whereas ecological drift and homogeneous selection were the most important assembly mechanisms in seawater. The microbial niche width was larger in the coral group than in the seawater group, indicating strong adaptability. A group of microbes (e.g., Caldilineaceae, Burkholderiaceae, and Nitrosopumilaceae) may become microbial indicators which differentiate seawater samples (e.g., SAR11 and SAR86 clades, Cryomorphaceae), which may be due to separated habitats and nutrition. Coral microbes may participate in nitrogen-metabolism to maintain a nitrogen limited microenvironment. Nodes linked in the coral co-occurrence network showed coexisting interactions and higher complexity based on the comparison of topological values. Collectively, significant differences in microbial fractions were demonstrated observed in terms of diversity, composition, co-occurrence patterns, assembly processes and predicted functions between coral and seawater samples, showing potential microbial interactions and providing in-depth insights into the metacommunity diversity in coral reef ecosystems.

Coral reefs are among the most biodiverse ecosystems, providing habitats for various organisms. Studies on coral bleaching have been increasing recently, but little is known about the distribution and community assembly of coral pathogenic bacteria (e.g., several Vibrio species). We elucidated the distribution pattern and interaction relationships of total bacteria and Vibrio spp. in sediments from the Xisha Islands, which are characterized by their high coverage and diversity of coral resources. Vibrio spp. showed significantly higher relative abundance values in the Xisha Islands (1.00 x 108 copies/g) than in other areas (approximately 1 x 104 to 9.04 x 105 copies/g), indicating that the coral bleaching event of 2020 may have promoted the bloom of vibrios. A spatial shift in community composition was observed between the northern (Photobacterium rosenbergii and Vibrio ponticus) and southern (Vibrio ishigakensis and Vibrio natriegens) sites, accompanied by a clear distance-decay pattern. The spatial distance and coral species (e.g., Acroporidae and Fungiidae) had much greater correlations with the Vibrio community than did environmental factors. However, complex mechanisms may exist in the community assembly of Vibrio spp. due to the large proportion of unexplained variation. Stochastic processes may play an important role, as shown by the neutral model. Vibrio harveyi had the highest relative abundance (77.56%) and niche breadth, compared to other species, and it was negatively correlated with Acroporidae, likely reflecting its strong competitive ability and adverse effects on specific corals. Our study provides insights into the bloom and underlying assembly mechanisms of sedimentary vibrios in the Xisha Islands, thereby contributing to identify the potential indicator of coral bleaching and provide inspiration for the environmental management of coral reef areas. [ABSTRACT FROM AUTHOR]

Alginate is an important polysaccharide in the ocean that supports the growth of marine microorganisms. Many widespread Vibrio species possess alginate lyases and can utilize alginate as a carbon source, but the detailed alginate degradation mechanism in Vibrio remains to be further explored. In this study, we obtained a highly efficient alginate-degrading strain, Vibrio pelagius WXL662, with 11 alginate lyases (VpAly-I to -XI) and further elucidated its molecular mechanism of alginate degradation. Three alginate utilization loci (AUL) were identified in different parts of WXL662's genome, comprising six alginate lyases (VpAly-I, -II, -VIII, -IX, -X, and -XI) and other genes related to alginate degradation. Most of the alginate-degrading genes are strongly induced when alginate is provided as the sole carbon source. Ten alginate lyases (VpAly-I to -X) had been purified and characterized, including six from polysaccharide lyase family 7 (PL7), three from PL17, and one from PL6. These recombinant alginate lyases existing in different cellular locations were active at a wide temperature (10 to 50°C) and pH (4.0 to 9.0) range, with different substrate preferences and diverse degradation products, enabling WXL662 to efficiently utilize alginate in a changing marine environment. Importantly, outer membrane vesicles (OMVs) can act as vectors for alginate lyases (VpAly-II, -V, and -VI) in WXL662. Further investigations of public Vibrio genomes revealed that most alginate-degrading vibrios possess one AUL instead of previously reported "scattered" system. These results emphasize the specific alginate degradation strategy in Vibrio pelagius WXL662, which can be used as a model strain to study the ecological importance of effective alginate-degrading vibrios in the ocean. [ABSTRACT FROM AUTHOR]