Corals are complex holobionts, harboring a variety of life forms at different scales, including coral cells, zooxanthellae, and a diversity of bacteria, archaea, and viruses. A new study published in the journal Nature ecology and evolution investigated a new interaction between a coral pathogen (Vibrio coralliilyticus) and other coral commensal bacteria, which have a major impact on the health of hard corals.
Warming oceans are currently threatening the health of coral reefs around the world. In addition to coral bleaching, stony coral tissue loss disease (SCTLD) occurs in several tropical oceans. Scientists have examined a species of coral called Galaxea fascicularis (commonly known as galaxy coral), which is a huge reef-building coral widespread near Hainan Island and the Xi Sha Islands in the South China Sea.
The analyzes revealed that, in certain regions, the corals of the galaxies suffer from SCLTD. By sampling gastric fluids from healthy and unhealthy corals in the South China Sea, researchers identified infections with the temperature-dependent pathogen Vibrio coralliilyticuswhich causes severe tissue loss in acute heat stress and is therefore likely a causative agent of SCLTD.
Scientists have found a key enzyme, L-lysine-epsilon-oxidase (LodA), in this coral pathogen which oxidizes L-lysine and generates hydrogen peroxide. This substance helps to eliminate a non-pathogenic substance vibrio species that cohabit in corals by inducing a prophage hidden in their genome. In addition, V. corallilyticus also uses LodA to outperform symbiotic bacteria such as Endozoicomonas spp., by triggering the induction of prophages in wild coral.
While native coral bacteria can increase their fitness through the acquisition of prophages when nutrients are scarce, “the risk is that this hidden prophage can be manipulated by competitors and converted into a bomb that kills host bacterial cells” , as the study’s lead author Xiaoxue Wang, a researcher at the University of the Chinese Academy of Sciences, put it.
Thus, this coral pathogen is a dangerous invader that competitively kills native coral microbiota and eliminates competitors, colonizing and ultimately decimating galaxy corals.
“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,” concluded Dr Wang.
By Andrei Ionescu, Terre.com Personal editor