The mechanism of bacterial (Roseobacter) colonization of phytoplankton phycospheres

Involved team members: Ashley Isaac

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Among the most consistent and seemingly important microbial associations in marine ecosystems is between diatoms and members of the bacterial family Rhodobacteraceae, commonly referred to as the roseobacters. Roseobacters are one of the major bacterial phylotypes present in marine ecosystems and are metabolically versatile. Although several members have traditionally been described as generalists that scavenge organic matter from numerous sources, many observations suggest other members are specialized in colonizing the phycosphere, the microenviroment surrounding diatom and phytoplankton cells. However, the molecular mechanisms that differentiate Roseobacter generalists and phycosphere colonizers are poorly understood. To elucidate these niche differences, we employ co-culture experiments and field sampling during phytoplankton blooms, coupled with bioinformatics and -omics techniques.

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We investigated roseobacters in the North Sea during the 2010–2012 diatom spring blooms using a time series of 38 deeply sequenced metagenomes and 10 metaproteomes collected throughout these events. Roseobacter metagenome assembled genomes (MAGs) were among the most recurrent and abundant throughout the datasets. We found that the relative abundance of specific roseobacters strongly correlated to dominating diatom species during these blooms. Comparative genomics showed that these roseobacters MAGs (e.g., Roseovarius and Sulfitobacter species), which we categorized as putative phycosphere colonizers, possessed the tight adherence (tad) gene cluster, while other MAGs (e.g., Planktomarina and Planktotalea species) did not. In addition, putative phycosphere colonizers possessed higher prevalence of biosynthetic gene clusters for secondary metabolites, particularly, homoserine lactones. Collectively, the gene products may facilitate and help regulate bacterial attachment to the phycosphere, allowing for closer interactions and exchange of metabolites. Current work involves the generation of tad knockout mutants to confirm its importance in roseobacter attachment and phycosphere colonization. Additionally, florescent in-situ hybridization (FISH) is being employed to validate and quantify the lifestyles of representative roseobacters from these niches.