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ABSTRACT
Algae and bacteria establish complex relationships reciprocally influencing their growth. Associated bacterial communities alter microalgae metabolism and physiology, affecting biomass yield and quality. In this study, 203 bacterial isolates (33 different genera), recovered from Tetraselmis suecica F&M-M33 cultures, were identified and phenotypically characterized in relation to the microalgal growth-promoting (MGP) features of indol-3-acetic acid (IAA) and siderophore production. IAA production was observed in 30 isolates belonging to 10 genera, whilst siderophore production was observed in 12 isolates belonging to nine genera, indicating that bacteria which are phylogenetically different within the phycosphere community may contribute in similar ways to boosting algal growth. Moreover, strains belonging to a single genus (e.g. Bacillus, Muricauda, Labrenzia and Amorphus) showed different MGP features, but none of these isolates were capable of producing both growth-promoting factors. Twenty-two selected strains were further tested in co-culture assays to evaluate their effect on T. suecica F&M-M33 growth; four induced an increase in microalgal productivity. The exo-metabolome of T. suecica F&M-M33 cultures, either axenically or when co-cultured with either Vitellibacter strain AAD2 or Sphingopyxis flavimaris strain AG5, was determined using a non-targeted metabolomics approach using an Ultra High Performance Liquid Chromatography (UHPLC) system. This allowed for the detection of 133 entities (small chemical molecules, e.g. metabolites), of which 84 were identified. Most of the entities were common in all three cultures, showing that the T. suecica F&M-M33 phycosphere was rich in small peptides and organic acids, and a high number of terpenes were also observed. Seventeen entities were culture-specific, suggesting that they were directly related to microalgae–bacteria interactions.
Supplementary information
The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at http://doi.org/10.1080/09670262.2020.1765024
Supplementary table 1. Identification of bacterial isolates by 16S rDNA sequencing. Strains were isolated from different cultures of Tetraselmis suecica F&M-M33.
Supplementary table 2. Bacterial strains, previously isolated from Tetraselmis suecica F&M-M33 cultures, used in this work.
Supplementary table 3. Bacterial strains used for co-culture assays with Tetraselmis suecica F&M-M33.
Supplementary table 4. Entities identified in the supernatant of Tetraselmis suecica F&M-M33 cultures. The supernatants of cultures of T. suecica F&M-M33 axenic (AX), T. suecica F&M-M33 co-cultured with Vitellibacter strain AAD2 (AX+AAD2) and T. suecica F&M-M33 co-cultured with Sphingopyxis flavimaris strain AG5 (AX+AG5) were analysed through a non-targeted metabolomics approach. Data refer to the analysis conducted in positive ionization mode. Entities were identified using METLIN metabolite and lipid databases.
Supplementary table 5. Entities identified in the supernatant of Tetraselmis suecica F&M-M33 cultures. The supernatants of cultures of T. suecica F&M-M33 axenic (AX), T. suecica F&M-M33 co-cultured with Vitellibacter strain AAD2 (AX+AAD2), and T. suecica F&M-M33 co-cultured with Sphingopyxis flavimaris strain AG5 (AX+AG5) were analysed through a non-targeted metabolomics approach. Data refer to the analysis conducted in negative ionization mode. Entities were identified using MELTIN metabolite and lipid databases.
Author contributions
E. Piampiano: drafted the manuscript, analysed the samples and compiled the data, commented on and contributed to the final manuscript. F. Pini: drafted the manuscript, analysed the samples and compiled the data, commented on and contributed to the final manuscript. N. Biondi: collected the samples, analysed the samples and compiled the data, commented on and contributed to the final manuscript. C.J. Garcia: analysed the samples and compiled the data, commented on and contributed to the final manuscript. F. Decorosi: analysed the samples and compiled the data, commented on and contributed to the final manuscript. F.A. Tomàs-Barberàn: provided material support and funding for this project, commented on and contributed to the final manuscript. L. Giovannetti: conceived the project, drafted the manuscript, provided material support and funding for this project, commented on and contributed to the final manuscript. C. Viti: conceived the project, drafted the manuscript, provided material support and funding for this project, commented on and contributed to the final manuscript.