Genetic manipulation of competition for nitrate between heterotrophic bacteria and diatoms

Image courtesy of Alessandra de Martino and Chris Bowler, Stazione Zoologica and Ecole Normale Supérieure.

Light Micrograph of Phaeodactylum tricornutum

TitleGenetic manipulation of competition for nitrate between heterotrophic bacteria and diatoms
Publication TypeJournal Article
Year of Publication2016
AuthorsDiner RE, Schwenck SM, McCrow JP, Zheng H, Allen AEllis
JournalFrontiers in Microbiology
Date Published2016/06
Type of ArticleOriginal Research
ISBN Number1664-302X
KeywordsBacteria,Diatoms,competition,RNA-Seq,Genetic Manipulation,nitrate

Diatoms are a dominant group of eukaryotic phytoplankton that contribute substantially to global primary production and the cycling of important elements such as carbon and nitrogen. Heterotrophic bacteria, including members of the gammaproteobacteria, are commonly associated with diatom populations and may rely on them for organic carbon while potentially competing with them for other essential nutrients. Considering that bacterioplankton drive oceanic release of CO2 (i.e., bacterial respiration) while diatoms drive ocean carbon sequestration vial the biological pump, the outcome of such competition could influence the direction and magnitude of carbon flux in the upper ocean. Nitrate availability is commonly a determining factor for the growth of diatom populations, particularly in coastal and upwelling regions. Diatoms as well as many bacterial species can utilize nitrate, however the ability of bacteria to compete for nitrate may be hindered by carbon limitation. Here we have developed a genetically tractable model system using the pennate diatom Phaeodactylum tricornutum and the widespread heterotrophic bacteria Alteromonas macleodii to examine carbon-nitrogen dynamics. While subsisting solely on P. tricornutum derived carbon, A. macleodii does not appear to be an effective competitor for nitrate, and may in fact benefit the diatom; particularly in stationary phase. However, allochthonous dissolved organic carbon addition in the form of pyruvate triggers A. macleodii proliferation and nitrate uptake, leading to reduced P. tricornutum growth. Nitrate reductase deficient mutants of A. macleodii (ΔnasA) do not exhibit such explosive growth and associated competitive ability in response to allochthonous carbon when nitrate is the sole nitrogen source, but could survive by utilizing solely P. tricornutum-derived nitrogen. Furthermore, allocthonous carbon addition enables wild-type A. macleodii to rescue nitrate reductase deficient P. tricornutum populations from nitrogen starvation, and RNA-seq transcriptomic evidence supports nitrogen-based interactions between diatoms and bacteria at the molecular level. This study provides key insights into the roles of carbon and nitrogen in phytoplankton-bacteria dynamics and lays the foundation for developing a mechanistic understanding of these interactions using co-culturing and genetic manipulation.

Short TitleDiatom bacteria competition for nitrate

Using the genetically tractable model system developed in this study, we have described mechanisms of interaction between diatoms and bacteria that may be of global biogeochemical significance. Our data strongly suggests bidirectional exchange of N substrates between diatoms and bacteria, and revealed putative diatom genes and pathways that may be impacted by the presence of bacteria and involved in N exchange. Furthermore, we have demonstrated that under certain environmental conditions (i.e., high DOC), marine bacteria are able to effectively compete with diatoms for NO−3, which may influence predictions of primary productivity and nutrient utilization by phytoplankton in the ocean and associated estimates of C export via the biological pump.

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