Publications

4509

Abstract Phytoplankton are the base of marine food webs. They form intricate interactions with heterotrophic bacteria ranging from mutualistic to pathogenic that together impact oceanic carbon and nutrient cycling. Our understanding of these interactions in marine environments remains primarily limited to laboratory-based studies of model organisms. Here, we report the discovery and characterization of Ekhidna algicida sp. nov. strain To15, isolated from the oligotrophic Pacific Ocean (16°N, 140°W) based on its algicidal effect on the pelagic diatom Thalassiosira oceanica. Subsequent co-culture experiments demonstrate that E. algicida is lethal within days to a diverse array of diatoms, including diatoms isolated from similar locations, with the effect mediated by bacterial exudates produced during co-culture with susceptible diatoms. Exudates of E. algicida monoculture are not algicidal, suggesting a pathogenic shift upon interaction with susceptible diatoms. The genome of E. algicida To15 encodes for a type IX secretion system (T9SS), together with candidate secreted proteases, suggesting a potential protein-mediated pathogenicity. Twenty additional algicidal Ekhidna strains were subsequently isolated from the Pacific Ocean. All these algicidal bacteria pass through 0.2 μm pore-size filters, highlighting the importance of the often-overlooked group of “filterable” marine bacteria. Our findings reveal E. algicida as a Pacific Ocean diatom pathogen, with potential impacts on microbial community composition dynamics in pelagic ecosystems.

‘ Candidatus Liberibacter asiaticus’ (Las) is a gram-negative bacterial pathogen associated with citrus huanglongbing (HLB) or greening disease. Las is transmitted by the Asian citrus psyllid (ACP) where it colonizes the phloem tissue, resulting in substantial economic losses to the citrus industry worldwide. Despite extensive efforts, effective management strategies against HLB remain elusive, necessitating a deeper understanding of the pathogen's biology. Las undergoes cell-to-cell movement through phloem flow and colonizes different tissues in which Las may have varying interactions with the host. Here, we investigate the transcriptomic landscape of Las in citrus seed coat vasculatures, enabling a complete gene expression profiling of Las genome and revealing unique transcriptomic patterns compared with previous studies using midrib tissues. Comparative transcriptomics between seed coat, midrib, and ACP identified specific responses and metabolic states of Las in different host tissue. Two Las virulence factors that exhibit higher expression in seed coat can suppress callose deposition. Therefore, they may contribute to unclogging sieve plate pores during Las colonization in seed coat vasculature. Furthermore, analysis of regulatory elements uncovers a potential role of LuxR-type transcription factors in regulating Liberibacter effector gene expression during plant colonization. Together, this work provides novel insights into the pathogenesis of the devastating citrus HLB. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .