A megaplasmid transforms a bacterium from opportunistic pathogen to mutualist providing thermal resilience to corals

Singapore’s marine environment is 1° north of the equator, driven by alternating monsoon seasons from the South China Sea and the Malacca Strait. The Malacca Strait carries humic rich waters from peat forests, which coupled to high sediment load, make this ecosystem unique and extreme. The Singapore turbid coral reefs, within the Southeast Asia Coral Triangle, are high diversity and resilient, experiencing fewer bleaching events of lower intensity despite being the world’s busiest shipping port in a highly urbanized city state. We hypothesized that bacteria within the coral holobiont confer traits that enhance resilience. Bacteria were isolated from the scleractinian coral Pachyseris speciosa, the majority identified as Pseudovibrio spp., with some of these carrying a megaplasmid (~490 kbp). Manipulative experiments demonstrated that the novel Pseudovibrio sp. is an opportunistic pathogen capable of causing bleaching disease at elevated temperature. However, acquisition of the megaplasmid alters its strategy to mutualism with P. speciosa, enhancing Pseudovibrio sp. attachment to the coral and extending its thermal range. Pseudovibrio isolates show both contact-independent (antibiotic) and contact-dependent (T6SS) inhibitory activity against Vibrio spp.