Thalassiosira pseudonana growth phase determines gene expression and algicidal behavior of a new Alteromonas macleodii strain

Phytoplankton-bacteria interactions are pivotal in marine ecosystems, influencing primary production and biogeochemical cycles. Diatoms engage in diverse relationships with bacteria, ranging from mutualism to pathogenicity. This study explores the interaction between a novel Alteromonas macleodii strain from the Equatorial Pacific and the model Thalassiosira pseudonana across the diatom different growth phases. We demonstrate that A. macleodii's algicidal activity depends on the diatom's growth phase, defensive capacity, and nutrient availability. The algicidal effect manifests during the diatom's stationary phase or with external nutrient supplementation, implicating organic matter availability as a key driver. Transcriptomic analysis reveals that A. macleodii shifts from motility-associated to growth-associated gene expression based on the diatom's physiology and coculture duration. Filtrate assays and fluorescence microscopy suggest a two-stage infection model: initial bacterial motility and exudate secretion induce diatom death, followed by bacterial aggregation around debris. Comparative transcriptomics with other algal hosts highlights host-specific bacterial responses, underscoring the context-dependent nature of these interactions. Our findings provide a deeper understanding of the molecular mechanisms driving diatom-bacteria interactions, shedding light on their role in marine microbial ecology and ecosystem functioning. Overall design: T. pseudonana cultures, in either Exponential or Stationary phases, were cocultured with a novel A. macleodii strain isolated from the North Pacific. RNA was extracted at three time points: T1 (30 minutes), T2 (36 hours), and T3 (8 days) for each coculture condition. All experiments were conducted in triplicates (labeled A, B, and C). For Ribodepletion calibration, RNA was extracted from exponentially growing T. pseudonana once and subsequently divided for Ribodepletion, poly(A) selection, and Total RNA sequencing.