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Isaac, A.

Publications and source records attributed to Isaac, A..

2 recordsLinked to original sources
bioRxiv2020

Quorum Sensing Regulates 'swim-or-stick' Lifestyle in the Phycosphere

Originality-significance statementMotility and biofilm formation are processes regulated by quorum sensing (QS) in bacteria. Both functions are believed to play an important role in interactions between bacteria and phytoplankton. Here, we show that two bacterial symbionts from the microbial community associated with a ubiquitous diatom switch their motile lifestyle to attached cells while an opportunist bacterium from the same community is incapable of attachment, despite possessing the genetic machinery to do so. Further work indicated that the opportunist lacks QS signal synthases while the symbionts produce three QS signals, one of which is mainly responsible for regulating symbiont colonization of the diatom microenvironment. These findings suggest that QS regulates colonization of diatom surfaces and further work on these model systems will inform our understanding of particle aggregation and bacterial attachment to marine snow and how these processes influence the global carbon cycle. SummaryInteractions between phytoplankton and bacteria play major roles in global biogeochemical cycles and oceanic nutrient fluxes. These interactions occur in the microenvironment surrounding phytoplankton cells, known as the phycosphere. Bacteria in the phycosphere use either chemotaxis or attachment to benefit from algal excretions. Both processes are regulated by quorum sensing (QS), a cell-cell signaling mechanism that uses small infochemicals to coordinate bacterial gene expression. However, the role of QS in regulating bacterial attachment in the phycosphere is not clear. Here, we isolated a Sulfitobacter pseudonitzschiae F5 and a Phaeobacter sp. F10 belonging to the marine Roseobacter group and an Alteromonas macleodii F12 belonging to Alteromonadaceae, from the microbial community of the ubiquitous diatom Asterionellopsis glacialis. We show that only the Roseobacter group isolates (diatom symbionts) can attach to diatom transparent exopolymeric particles. Despite all three bacteria possessing genes involved in motility, chemotaxis, and attachment, only S. pseudonitzschiae F5 and Phaeobacter sp. F10 possessed complete QS systems and could synthesize QS signals. Using UHPLC-MS/MS, we identified three QS molecules produced by both bacteria of which only 3-oxo-C16:1-HSL strongly inhibited bacterial motility and stimulated attachment in the phycosphere. These findings suggest that QS signals enable colonization of the phycosphere by algal symbionts.

ecology↗
bioRxiv2020

Diatom Modulation of Microbial Consortia Through Use of Two Unique Secondary Metabolites

Unicellular eukaryotic phytoplankton, such as diatoms, rely on microbial communities for survival despite lacking specialized compartments to house microbiomes (e.g., animal gut). Microbial communities have been widely shown to benefit from diatom excretions that accumulate within the microenvironment surrounding phytoplankton cells, known as the phycosphere. However, mechanisms that enable diatoms and other unicellular eukaryotes to nurture specific microbiomes by fostering beneficial bacteria and repelling harmful ones are mostly unknown. We hypothesized that diatom exudates may attune microbial communities and employed an integrated multi-omics approach using the ubiquitous diatom Asterionellopsis glacialis to reveal how it modulates its naturally associated bacteria. We show that A. glacialis reprograms its transcriptional and metabolic profiles in response to bacteria to secrete a suite of central metabolites and two unusual secondary metabolites, rosmarinic acid and azelaic acid. While central metabolites are utilized by potential bacterial symbionts and opportunists alike, rosmarinic acid promotes attachment of beneficial bacteria to the diatom and simultaneously suppresses the attachment of opportunists. Similarly, azelaic acid enhances growth of beneficial bacteria, while simultaneously inhibiting growth of opportunistic ones. We further show that the bacterial response to azelaic acid is widespread in the worlds oceans and taxonomically restricted to a handful of bacterial genera. Our results demonstrate the innate ability of an important unicellular eukaryotic group to modulate their microbial consortia, similar to higher eukaryotes, using unique secondary metabolites that regulate bacterial growth and behavior inversely in different bacterial populations.

microbiology↗