Editor's note: The South China Sea, vast and vast, with its vast islands and reefs, is rich in resources, beautiful and rich. In order to better understand the South China Sea and recognize the South China Sea, the Voice of the Chinese Academy of Sciences and the South China Sea Institute of Oceanology of the Chinese Academy of Sciences jointly set up a column on "Beautiful South China Sea" to interpret the relationship between the "monsoon" of the South China Sea and daily life, uncover the 10,000-year imprint formed by the "lithosphere" resources of the South China Sea, open the magical door of the interaction of "biodiversity" in the South China Sea, and travel happily in the ocean of knowledge.
How important are colorful corals, lobsters with teeth and claws, cute clownfish, sea creatures of all shapes, and beautiful and rich coral reefs important to the ocean?
Figure 1 Coral reefs on the seafloor of the South China Sea
Coral reefs are highly productive and biodiverse ecosystems in the oceans, providing a living environment for many flora and fauna and feeding a quarter of marine life. Among them, the long-term deposition of calcium in the ocean by reef-building stony corals is the basis for the formation of coral reefs. In recent years, corals and coral reefs have faced a series of challenges, including ocean acidification and rising sea temperatures, due to the impacts of climate change and human activities, and coral reefs have been significantly reduced globally.
There is a close and mutually beneficial symbiotic relationship between zooxanthellae and reef-building corals. Zooxanthellae use co2 produced by coral respiration for photosynthesis to produce oxygen and nutrients to supply corals, and studies have reported that zooxanthellae are closely related to the formation of coral calcareous bones. In return, corals provide zooxanthellae with protection, shelter, nutrition (mainly waste containing nitrogen and phosphorus) and a constant supply of CO2 needed for photosynthesis. In addition to zooxanthellae, there are complex microbial communities in corals. Currently, microbiologists refer to corals and aggregates of zooxanthellae, fungi, bacteria, archaea and viruses that coexist with corals as coral holobiont (Figure 2).
Figure 2 Schematic diagram of coral symbiosis in general (quoted from Peixoto RS, et al. Front. Microbiol. 2017）
Coral symbiotic bacteria are an important part of the coral symbiosis community. The study found that isolating and purifying bacterial strains from corals could be probiotics, non-pathogenic bacteria or even pathogenic bacteria of corals. In 2006, microbiologist Professor Eugene Rosenburg proposed the coral "probiotic hypothesis" based on the results of research and observations on coral albinism. He found that corals infected with the pathogenic bacterium Vibrio shiloi gradually developed albino resistance, which he believed was the result of the probiotics in the environment inhibiting the outbreak of pathogenic Vibrio. Therefore, the community structure of the coral symbiotic microbiota, including different bacterial species and abundance, is very important for the overall health of coral symbiosis.
Viruses (bacteriophages) in coral symbiotic populations play an important regulatory role in regulating the community structure of coral symbiotic flora and avoiding bacterial overplication. Bacteriophages are viruses with bacteria as the host, and can be divided into two types: strong bacteriophages and mild bacteriophages according to whether they lyse the host. Mild bacteriophages do not directly lyse the host after infestation with bacteria, but by integrating their own genetic material into the bacterial genome and replicating with bacterial genome replication, under certain conditions can be detached from the bacterial genome to cause host lysis, affecting bacterial community dynamics and maintaining stability for a long time (Figure 3).
Figure 3 Bacteriophage replication cycle (quoted from Ron Feiner, et al. Nat. Rev. Microbiol. 2015）
On June 30, Wang Xiaoxue's team at the Key Laboratory of Tropical Marine Biological Resources and Ecology, South China Sea Institute of Oceanography, Chinese Academy of Sciences, used the non-invasive infestation model of reef-building stony corals for the first time to analyze the mechanism of the coral pathogen Vibrio coralliilyticus that broke through the barrier of coral symbiotic flora and triggered stony coral tissue shed disease coralliilyticuskills non-pathogenic holobiont competitors by triggering prophage induction", published online in Nature Ecology & Evolution.
Stony coral exfoliation disease is found in a variety of reef-building corals around the world and kills infected corals in a matter of weeks, with a mortality rate of more than 90%. The researchers took the typical reef-building stony coral in the South China Sea - clump helmet coral as the research object, and found the pathogen vibrio coral, Vibrio coral lysis, in the digestive circulation fluid of the clump helmet coral. After infection with this pathogen, severe tissue ulceration under thermal stress is a typical form of coral tissue shedding disease (Figure 4).
Fig. 4 Clump of helmet-shaped corals with health and "tissue shedding disease"
The researchers compared the phenotypes of Vibrio coralis and other coral strains isolated from clump helmets, and found that the biofilm formation and swimming motility of Vibrio lysolyticus were significantly weaker than those isolated in other corals. According to the coral "probiotic hypothesis", the probiotics in corals inhibit the outbreak of pathogenic bacteria, so how does Vibrio coral lysolytic overcome this inhibition, which in turn leads to tissue shedding disease in corals?
To solve the mystery of the rapid outbreak of Vibrio corals, the researchers conducted a large number of laboratory and in-coral experimental studies, and finally reported a new interaction between Vibrio coral lysis and indigenous coral bacteria, and the results of this competitive action determine the health of corals (Figure 5). Vibrio coralles encodes L-lysine-ε-oxidase LodA, which can oxidize L-lysine and produce hydrogen peroxide. Vibrio corallytica uses LodA to activate the protophages of the endozoicomonas sp. and non-pathogenic Vibrio coral co-epiphytic bacteria, thereby eliminating these important coral symbiotic bacteria.
The study also found that LodA is also ubiquitous in important microbial taxa in ecosystems such as the human gut, soil and ocean, suggesting that using LodA to activate protophages may be a common strategy for competing among microbes that co-inhabit the same niche. This study reveals a new mechanism of pathogenicity of coral pathogenic bacteria, and obtains a new understanding of the overall health of coral symbiosis from the perspective of the function and interaction mode of symbiotic bacteria, which provides technical support for the use of probiotics to protect the health of reef-building corals.
Figure 5 Pattern diagram of coral pathogenic bacteria interacting with coral symbiotic bacteria
Wang Weiquan, a 2017 direct doctoral student of the South China Sea Institute of Oceanography, Tang Kaihao, an associate researcher, and Wang Pengxia, an associate researcher, are the co-first authors of the paper, and researcher Wang Xiaoxue is the corresponding author of the paper. Professor Yan Wang's team at Hainan University provided support in sampling coral samples and establishing an infestation system. This research work has been funded by the National Natural Science Foundation of China, the Key R&D Program of the Ministry of Science and Technology of the People's Republic of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the Guangdong Provincial Local Innovation Team, and the Guangdong Provincial Laboratory of Southern Marine Science and Engineering.
Article link: https://doi.org/10.1038/s41559-022-01795-y
Wang, W. et al. The coral pathogen Vibrio coralliilyticuskills non-pathogenic holobiont competitors by triggering prophage induction. Nat. Ecol. Evol. doi:10.1038/s41559-022-01795-y
Source: South China Sea Institute of Oceanography, Chinese Academy of Sciences