# Induced pathogenicity toward open-ocean diatoms by a filterable bacterium Ekhidna algicida sp. nov

## TL;DR

A new filterable bacterium, Ekhidna algicida, was found to kill open-ocean diatoms, potentially affecting marine ecosystems and nutrient cycles.

## Contribution

Discovery of a novel filterable marine bacterium that exhibits induced pathogenicity toward diatoms.

## Key findings

- Ekhidna algicida strain To15 kills various diatoms through exudates produced during co-culture.
- The genome of E. algicida encodes a type IX secretion system and candidate proteases, suggesting protein-mediated pathogenicity.
- Twenty additional algicidal Ekhidna strains were isolated, all passing through 0.2 μm filters.

## 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.