The ice-ice disease black box: a disease complex triggered by bacterial associations in an ex situ milieu of biobanked Kappaphycus alvarezii

Abstract Ice-ice disease (IID) is widely associated with environmental stress and microbial presence in seaweed farms, but its expression under ex situ conditions remains poorly understood. This study examined the associated culturable bacterial community in independently maintained K. alvarezii stock cultures classified as healthy or diseased based on characteristic symptoms. Bacteria were isolated from healthy tissue, diseased tissue, and their corresponding bulk waters, identified using 16S rRNA gene sequencing, and analyzed through phylogenetic reconstruction, species richness estimation, and community similarity metrics. A total of 31 bacterial isolates representing six unique taxa were recovered. Diseased tissue exhibited higher bacterial richness and a distinct tissue-associated bacterial composition compared with healthy tissue. Healthy tissue exclusively harbored Rossellomorea aquimaris, whereas diseased tissue contained Vibrio and Photobacterium species, with representative isolates showing agarolytic and carrageenanolytic activity. In contrast, bulk waters from healthy and diseased cultures contained Pseudoalteromonas species. Although the study focuses on microbial associations, the emergence of IID under controlled gene bank conditions highlights the potential interaction of host condition, culture environment, routine handling, and opportunistic bacteria. Together, these factors illustrate the “black box” dynamics of IID in ex situ conditions and provide insights for disease monitoring and management in seaweed gene banks.