# Comparative genomics reveals the molecular basis for divergent algicidal strategies in two Alteromonas macleodii strains

## TL;DR

This study compares two related marine bacteria strains to understand how their genomes enable different strategies for killing algae.

## Contribution

The study reveals the genomic basis for divergent algicidal strategies in two Alteromonas macleodii strains.

## Key findings

- Strain FDHY-03 uses a broad-spectrum strategy with polysaccharide lyases.
- Strain FDHY-CJ targets diatoms with GH16 isoforms, chemotaxis, and quorum sensing.
- Genomic plasticity enables niche partitioning within a single species.

## Abstract

Marine bacteria such as Alteromonas are key players in regulating algal blooms, yet the genomic basis for their strain-specific algicidal activities remains poorly understood. Here, we use comparative genomics to dissect the mechanisms of functional divergence between two closely related Alteromonas macleodii strains: strain FDHY-03, which employs a broad-spectrum strategy, and FDHY-CJ, which has adapted a narrow-spectrum strategy specifically targeting diatoms. We reveal that these distinct predatory strategies are underpinned by divergent genomic architectures. The broad-spectrum strain FDHY-03 leverages a versatile, synergistic enzymatic arsenal rich in polysaccharide lyases to enable its broad-spectrum attack. In contrast, the specialist FDHY-CJ has evolved an integrated, high-precision system comprising: (i) a specialized CAZyme toolkit, uniquely enriched with GH16 isoforms, tailored to breach diatom-specific defenses; (ii) an enhanced chemotaxis system (Tsr) to home in on its algal targets; and (iii) a complex quorum sensing network (AHL/solo-LuxR) to coordinate its behavior in diatom-rich niches. Our findings provide a high-resolution model for microbial microevolution, demonstrating how genomic plasticity enables rapid niche partitioning within a single species. This work illuminates the molecular details of marine microbial warfare and provides a blueprint for the genome-informed selection of targeted biocontrol agents for harmful algal blooms.

Frequent harmful algal blooms pose a severe threat to global biogeochemical cycles. Algicidal bacteria, acting as natural antagonists, serve as effective biological tools for controlling harmful algal blooms. While extensive research has been conducted on the isolation and identification of algicidal bacteria, the genomic basis for their strain-specific algicidal activity remains unclear. This study employs comparative genomics to analyze the genomic architecture of two closely related Alteromonas macleodii strains, revealing distinctly different algicidal strategies. Our findings offer valuable insights into the molecular basis of microbial warfare in marine environments, ultimately contributing to the advancement of microbial-based approaches for mitigating harmful algal blooms.