Antimicrobial activity of coral-associated beneficial bacteria against coral disease-causing microbial pathogens

Microbial infection of immune-compromised corals influences disease severity, resulting in coral mortality. However, coral-associated beneficial bacteria are known to produce antimicrobial compounds that prevent the growth of potential pathogens and invading microbes. Hence, beneficial bacteria associated with coral Porites lutea were isolated and antimicrobial protein and bioactive secondary metabolites were extracted and tested for their antimicrobial activity against putative prokaryotic and eukaryotic coral pathogens. Bioactive secondary metabolites exhibited remarkable antagonism against various coral pathogens such as Serratia marcescens, Vibrio species, and Aspergillus sydowii. Besides, the metabolites of Cobetia marina, Cobetia amphilecti, Pseudoalteromonas neustonica, and Virgibacillus halodenitrificans manifested notable inhibition against the protozoan ciliates (Uronema marinum, Holosticha diademata, Cohnilembus verminus, and Euplotes vannus) and zooplankton that are known to be involved in the secondary pathogenesis in coral diseased lesion progression. Thus, the present study may benefit in understanding coral-associated beneficial bacteria for their antagonistic interactions with microbial pathogens, as well as their potential involvement in reducing coral disease severity.

Possible beneficial interactions of ciliated protozoans with coral health and resilience

Microbial interactions contribute significantly to coral health in the marine environment. Most beneficial associations have been described with their bacterial communities, but knowledge of beneficial associations between protozoan ciliates and corals is still lacking. Ciliates are important bacterial predators and provide nutrition to higher trophic-level organisms. The mucus secreted by corals and the microenvironment of the coral surface layer attract ciliates based on their food preferences. The mixotrophic and heterotrophic ciliates play a major role in nutrient cycling by increasing nitrogen, phosphorus, and extractable sulfur, which can enhance the proliferation of coral beneficial microbe. Besides, bacterial predator ciliates reduce the pathogenic bacterial population that infects the coral and also act as bioindicators for assessing the toxicity of the reef ecosystem. Thus, these ciliates can be used as a beneficial partner in influencing coral health and resilience under various stress conditions. Herein, we explore the urgent need to understand the complex beneficial interactions of ciliates that may occur in the coral reef ecosystem.

Ciliated protozoan occurrence and association in the pathogenesis of coral disease

Various microbial infections have significantly contributed to disease progression leading to the mortality of corals. However, the holobiont and the external surfaces of coral, including the secreted mucus, provide a varied microenvironment that attracts ciliates based on their feeding preferences. Besides, some ciliates (e.g., Philasterine scuticociliate) may enter through the injuries or lesions on corals or through their indirect interactions with other types of microbes that influence coral health. Thus, ciliates occurrence and association are described with 12 different diseases worldwide. White syndrome disease lesions have diverse ciliate associations, and higher ciliate diversity was observed with diseased genera Acropora. Also, it was described, about sixteen ciliate species ingest coral Symbiodiniaceae and histophagous ciliates for coral tissue loss as secondary invaders. However, the ciliates nature of association with the coral disease remains unclear for primary or opportunistic secondary pathogenicity. Herein, we explore the urgent need to understand the complex interactions of ciliates in coral health.