Jiasui Li1, Marwan E. Majzoub1,2, Ezequiel M. Marzinelli3,4,5, Zhicong Dai1,6, Torsten Thomas1 and Suhelen Egan1
1Centre for Marine Science and Innovation, School of Biological, Earth and Environmental Sciences, Faculty of Science, The University of New South Wales, Kensington, NSW, 2052, Australia
2School of Medical Sciences, Faculty of Medicine, The University of New South Wales, Kensington, NSW, 2052, Australia
3The University of Sydney, School of Life and Environmental Sciences, Sydney, NSW, 2006, Australia
4Singapore Centre for Environmental Life Sciences Engineering, Nanyang Technological University, Singapore 637551, Singapore
5Sydney Institute of Marine Science, Mosman, NSW, 2028, Australia
6Institute of Environment and Ecology, School of the Environment and Safety Engineering, Jiangsu University, Jiangsu, 212013, P. R. China
Delisea pulchra is a red seaweed residing in the subtidal regions of the south-eastern coast of Australia. It suffers from a bleaching disease caused by high-temperature stress and the activities of opportunistic pathogens. Our studies identified over 30 isolates that were antagonistic against bleaching pathogens, including a TDA-producing Phaeobacter sp. BS52 that can increase the disease resistance of D. pulchra by mitigating the pathogen-induced shifts in host-associated microbiota (i.e., preventing dysbiosis). Further, a phylogenetically closely related strain Phaeobacter sp. BS23, has neither protective nor pathogenic effects towards D. pulchra and did not prevent the pathogen-induced dysbiosis. These findings indicate that Phaeobacter spp. have species- or strain-specific ecological roles in the bleaching disease of D. pulchra. To identify functions underpinning this phenotypic variation, we sequenced, and de novo assembled the whole genomes of D. pulchra protective BS52 and two representative commensals Phaeobacter spp. BS23 and BS34. Phylogenomic analysis designated the strains BS52 and BS23 to species Phaeobacter piscinae and BS34 to Phaeobacter inhibens. Using comparative genomic analysis, the study identified putative functions such as phage infection and antibiotic production that may contribute to the protective ability of BS52. This study highlights that subspecies level variations can lead to different disease protective phenotypes in Phaeobacter spp. It is hoped that these findings will benefit future probiotics discovery and disease management in seaweeds with applications for conservation and aquaculture.