Project description:Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effect of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive phenotype of enlarged cells with multiple plastids and nuclei, similar to what was observed when the diatom was co-cultured with the live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.
Project description:We isolate the cultivable microbiome of a diatom and show that different bacteria have commensal, antagonistic, or synergistic effects on the diatom. One synergistic bacterium enhances growth of the diatom by production of auxin, a phytohormone. The diatom and its synergistic bacterium appear to use auxin and tryptophan as signaling molecules that drive nutrient exchange. Detection of auxin molecules and biosynthesis gene transcripts in the Pacific Ocean suggests that these interactions are widespread in marine ecosystems.
Project description:Phytoplankton-bacteria interactions are pivotal in marine ecosystems, influencing primary production and biogeochemical cycles. Diatoms engage in diverse relationships with bacteria, ranging from mutualism to pathogenicity. This study explores the interaction between a novel Alteromonas macleodii strain from the Equatorial Pacific and the model Thalassiosira pseudonana across the diatom different growth phases. We demonstrate that A. macleodii’s algicidal activity depends on the diatom’s growth phase, defensive capacity, and nutrient availability. The algicidal effect manifests during the diatom’s stationary phase or with external nutrient supplementation, implicating organic matter availability as a key driver. Transcriptomic analysis reveals that A. macleodii shifts from motility-associated to growth-associated gene expression based on the diatom’s physiology and coculture duration. Filtrate assays and fluorescence microscopy suggest a two-stage infection model: initial bacterial motility and exudate secretion induce diatom death, followed by bacterial aggregation around debris. Comparative transcriptomics with other algal hosts highlights host-specific bacterial responses, underscoring the context-dependent nature of these interactions. Our findings provide a deeper understanding of the molecular mechanisms driving diatom-bacteria interactions, shedding light on their role in marine microbial ecology and ecosystem functioning.
Project description:Diatom-derived polyunsaturated aldehydes (PUAs) significantly influence marine bacterial dy-namics, yet the underlying proteomic mechanisms remain elusive. We employed high-resolution comparative proteomics to decipher the functional reprogramming of two bacterial communi-ties—one naturally associated with a PUA-producing diatom (N-community) and another with a non-PUA producer (I-community)—under ecologically relevant PUA exposure. While growth rates and cell densities remained unaffected, indicating an absence of acute toxicity, proteomics revealed pronounced community-specific reorganization. N-communities displayed stable, regula-tion-oriented adjustments consistent with physiological accommodation, whereas I-communities exhibited dose-dependent stress responses, shifting toward protein repair and antioxidant defense. Our findings demonstrate that PUAs trigger profound proteomic reprogramming conditioned by the communities' prior ecological history. This functional divergence provides a molecular basis for understanding bacterial fitness and succession during diatom blooms, where PUA-mediated in-teractions could act as a selective filter shaping the phycosphere's microbial landscape. Polyunsaturated aldehydes (PUA) produced by diatoms have been proposed to exert a wide range of effects on marine bacteria, from inhibitory or stress-inducing responses to neutral or potentially beneficial effects. However, the bacterial proteomic responses remain elusive. Here, we employed a high-resolution comparative proteomic approach to decipher the functional reprogramming of two distinct bacterial communities under ecologically relevant PUA exposure. One community was composed by bacteria naturally associated with a PUA-producing diatom (N- communy and, a second community associated with a non-PUA-producing diatom (I-community). Bacterial growth rates and final cell densities were not significantly affected by any treatment, indicating the absence of toxic effects even at high PUA concentrations. Dissolved organic carbon consumption did not provide evidence that PUA was the relevant carbon source. Interestingly, comparative proteomic analyses revealed pronounced community-specific reorganization in response to PUA expo-sure.Our results show that PUAs trigger a profound proteomic reprogramming rather than a simple stress response. While I-community prioritized antioxidant defense and protein repair, N-community showed a metabolic shift towards energy conservation. These findings suggest that the metabolic history of bacterial assemblages determines their success in the phycosphere, providing a molecular basis for microbial succession during diatom blooms.