Harboring a range of life forms at various scales, including zooxanthellae, coral cells, associated bacteria, viruses, and archaea, coral is a complex holobiont.
A novel interplay occurring between a coral pathogen and other coral commensal bacteria has been identified by a research group headed by Professor Xiaoxue Wang from the South China Sea Institute of Oceanology (SCSIO) of the Chinese Academy of Sciences. This discovery will also aid in the direct determination of the health of a stony coral.
The study was reported in the journal Nature Ecology & Evolution on June 30th, 2022.
At an unparalleled rate, changes have been noted in the tropical ocean environment. Ocean warming is causing a threat to the coral reefs’ health. Additionally, coral bleaching, and stony coral tissue loss disease (SCTLD), a disease caused due to a coral pathogen, are becoming more of a serious issue in numerous tropical oceans.
The so-called Galaxea fascicularis (also referred to as galaxy coral) is a huge reef-building coral and is generally found next to the Xi Sha Islands and Hainan Island of the South China Sea. The huge polyps of galaxy coral tend to resemble a starburst and are tipped in white, giving it a sparkling and crystal-like appearance.
At the time of their survey, the scientists discovered that the galaxy coral suffers SCTLD in a few regions. Gastric fluids present in healthy and unhealthy corals were sampled by researchers from the South China Sea.
Amongst the fluids, scientists determined the temperature-dependent pathogen Vibrio coralliilyticus. On being infected with this pathogen, galaxy colonies exhibited serious tissue loss under acute heat stress, thereby ascertaining that Vibrio coralliilyticus is a causative agent of SCTLD in galaxy coral.
The main enzyme, called LodA (L-lysine-epsilon-oxidase), was found by the scientists in this coral pathogen. LodA helps oxidize L-lysine and produces hydrogen peroxide. This aids in removing a nonpathogenic Vibrio species that cohabit in the coral by inducing a prophage that is hidden in its genome.
This coral pathogen also makes use of LodA to outcompete other bacteria that are symbiotic or linked to coral, for example, Endozoicomonas spp., via LodA-dependent prophage induction in wild coral.
Hence, the scientists illustrated that the ecological significance of LodA, which was earlier linked to antimicrobial activities, is activating prophage induction.
Bacterial hosts, for example, native bacteria present in coral, could increase their fitness via the acquisition of prophages when lacking nutrients; But, “the risk is that this hidden prophage can be manipulated by competitors and can be converted into a bomb which kills the host bacterial cells,” stated Xiaoxue Wang, the lead researcher of the study.
The coral pathogen, the invader, helps trigger this hidden bomb in the native microbiota and removes competitors, thus colonizing corals and ultimately killing the galaxy corals.
This study illustrates that the coral pathogen V. coralliilyticus invades the coral G. fascicularis by killing resident strains through the generation of hydrogen peroxide to induce SCTLD.
Clearly, pathogens have evolved many strategies to evade host defenses to colonize the gastric cavity of wild corals, and the results can now be taken into account for coral reef conservation.
Xiaoxue Wang, Study Lead Researcher, South China Sea Institute of Oceanology, Chinese Academy of Sciences
Wang, W., et al. (2022) The coral pathogen Vibrio coralliilyticus kills non-pathogenic holobiont competitors by triggering prophage induction. Nature Ecology & Evolution. doi.org/10.1038/s41559-022-01795-y.