Cryptic bacterial pathogens of diatoms peak during senescence of a winter diatom bloom

Abstract

Summary Diatoms are globally abundant microalgae that form extensive blooms in aquatic ecosystems. Certain bacteria behave antagonistically towards diatoms, killing or inhibiting their growth. Despite their crucial implications to diatom bloom and population health, knowledge of diatom antagonists in the environment is fundamentally lacking. We report systematic characterisation of the diversity and seasonal dynamics of bacterial antagonists of diatoms via plaque assay sampling in the Western English Channel, where diatoms frequently bloom. Unexpectedly, peaks in detection did not occur during characteristic spring diatom blooms, but coincided with a winter bloom of Coscinodiscus, suggesting that these bacteria likely influence distinct diatom host populations. We isolated multiple bacterial antagonists, spanning 4 classes and 10 bacterial orders. Notably, a diatom attaching Roseobacter Ponticoccus alexandrii was isolated multiple times, indicative of a persistent environmental presence. Moreover, many isolates had no prior reports of antagonistic activity towards diatoms. We verified diatom growth inhibitory effects of eight isolates. In all cases tested, these effects were activated by pre‐exposure to diatom organic matter. Discovery of widespread ‘cryptic’ antagonistic activity indicates that bacterial pathogenicity towards diatoms is more prevalent than previously recognised. Finally, examination of the global biogeography of WEC antagonists revealed co‐occurrence patterns with diatom host populations in marine waters globally.

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Exchange or Eliminate: The Secrets of Algal-Bacterial Relationships

“…Diatoms, prominent microalgae in the oceans, are a key population of phytoplankton. Bacteria have the capacity to either promote or inhibit their growth and proliferation [ 135 , 136 ], and diatoms can also change their metabolic profile in response to bacteria [ 137 ]. Among these microalgae, the pennate Phaeodactylum tricornutum , with its diverse morphotypes, alongside the centric Thalassiosira pseudonana , has emerged as a model for diatom research [ 10 , 138 , 139 ].…”