Abstract
N2-fixing cyanobacteria represent a major source of new nitrogen and carbon for marine microbial communities, but little is known about their ecological interactions with associated microbiota. In this study we investigated the interactions between the unicellular N2-fixing cyanobacterium Cyanothece sp. Miami BG043511 and its associated free-living chemotrophic bacteria at different concentrations of nitrate and dissolved organic carbon and different temperatures. High temperature strongly stimulated the growth of Cyanothece, but had less effect on the growth and community composition of the chemotrophic bacteria. Conversely, nitrate and carbon addition did not significantly increase the abundance of Cyanothece, but strongly affected the abundance and species composition of the associated chemotrophic bacteria. In nitrate-free medium the associated bacterial community was co-dominated by the putative diazotroph Mesorhizobium and the putative aerobic anoxygenic phototroph Erythrobacter and after addition of organic carbon also by the Flavobacterium Muricauda. Addition of nitrate shifted the composition toward co-dominance by Erythrobacter and the Gammaproteobacterium Marinobacter. Our results indicate that Cyanothece modified the species composition of its associated bacteria through a combination of competition and facilitation. Furthermore, within the bacterial community, niche differentiation appeared to play an important role, contributing to the coexistence of a variety of different functional groups. An important implication of these findings is that changes in nitrogen and carbon availability due to, e.g., eutrophication and climate change are likely to have a major impact on the species composition of the bacterial community associated with N2-fixing cyanobacteria.
Highlights
Nitrogen is one of the key elements limiting the primary productivity of large parts of the oceans (Vitousek and Howarth, 1991; Moore et al, 2013)
POPULATION DYNAMICS Regular microscopic inspection revealed that the microbial consortium in our experiments consisted of healthy non-senescent Cyanothece cells and free-living chemotrophic bacteria; we did not observe bacteria attached to Cyanothece cells or other forms of cell aggregates
When neither dissolved organic carbon (DOC) nor nitrate was added to the medium (–C–N) chemotrophic bacteria density remained low throughout the experiment at all temperatures (Figures 1A,E,I)
Summary
Nitrogen is one of the key elements limiting the primary productivity of large parts of the oceans (Vitousek and Howarth, 1991; Moore et al, 2013). The input of new nitrogen by dinitrogen (N2)-fixing cyanobacteria will benefit the diazotrophs themselves but may affect other members of the oceanic plankton community. Field studies have shown transfer of fixed nitrogen from large diazotrophic cyanobacteria to picoplankton in the Baltic Sea (Ohlendieck et al, 2000) and in the Southwest Pacific (Garcia et al, 2007). Trichodesmium colonies are typically associated with chemotrophic bacteria (Paerl et al, 1989; Sheridan et al, 2002), both positive and negative interactions of N2-fixing cyanobacteria with chemotrophic bacteria have been described. Tseng et al (2005) reported a higher abundance and productivity of chemotrophic bacteria during a Trichodesmium bloom. Renaud et al (2005) observed a decreased bacterial abundance and activity during a Trichodesmium bloom and Nausch (1996) observed lower bacterial abundance and thymidine incorporation within a Trichodesmium bloom compared to the adjacent water
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