Interactions between co-existing microorganisms deeply affect the physiology of the involved organisms and, ultimately, the function of the ecosystem as a whole. Copiotrophic Alteromonas are marine gammaproteobacteria that thrive during the late stages of phytoplankton blooms in the marine environment and in laboratory co-cultures with cyanobacteria such as Trichodesmium. The response of this heterotroph to the sometimes rapid and transient changes in nutrient supply when the phototroph crashes is not well understood. Here, we isolated and sequenced the strain Alteromonas macleodii str. Te101 from a laboratory culture of Trichodesmium erythraeum IMS101, yielding a chromosome of 4.63 Mb and a single plasmid of 237 kb. Increasing salinities to ≥43 ppt inhibited the growth of Trichodesmium but stimulated growth of the associated Alteromonas. We characterized the transcriptomic responses of both microorganisms and identified the complement of active transcriptional start sites in Alteromonas at single-nucleotide resolution. In replicate cultures, a similar set of genes became activated in Alteromonas when growth rates of Trichodesmium declined and mortality was high. The parallel activation of fliA, rpoS and of flagellar assembly and growth-related genes indicated that Alteromonas might have increased cell motility, growth, and multiple biosynthetic activities. Genes with the highest expression in the data set were three small RNAs (Aln1a-c) that were identified as analogs of the small RNAs CsrB-C in E. coli or RsmX-Z in pathogenic bacteria. Together with the carbon storage protein A (CsrA) homolog Te101-05290, these RNAs likely control the expression of numerous genes in responding to changes in the environment.
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Acknowledgements This work was supported by the EU project MaCuMBA (grant agreement no. 311975) to WRH and FRV and by the German Israeli Foundation grant no. 1133 to IB-F and WRH. Support of ML-P by a postdoctoral fellowship from the Valencian Consellería de Educació, Investigació, Cultura i Esport (APOSTD/2016/051) and of SH by the China Scholarship Council is gratefully acknowledged. We thank Viktoria Reimann for expert technical assistance and Dr. Rahul Kulkarni for sharing the source code of CSRA_TARGET. The authors are grateful for the constructive comments and suggestions from three anonymous reviewers.
© 2017 International Society for Microbial Ecology.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics