Dataset Information

Unusual marine cyanobacteria/haptophyte symbiosis relies on N₂ fixation even in N-rich environments.

ABSTRACT: The microbial fixation of N₂ is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N₂-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N₂ fixation to DIN is unknown. We used stable isotope incubations, catalyzed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry (nanoSIMS), to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N₂ fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO₃^-) and meet little of their N demands via ammonium (NH₄⁺) uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N₂ fixation to supply large portions of the haptophyte's N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N₂ fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by NO₃^- additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. The results suggest N₂ fixation may be an evolutionarily viable strategy for diazotroph-eukaryote symbioses, even in N-rich coastal or high latitude waters.

SUBMITTER: Mills MM

PROVIDER: S-EPMC7490277 | biostudies-literature | 2020 Oct

REPOSITORIES: biostudies-literature

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Unusual marine cyanobacteria/haptophyte symbiosis relies on N2 fixation even in N-rich environments.

Mills Matthew M MM Turk-Kubo Kendra A KA van Dijken Gert L GL Henke Britt A BA Harding Katie K Wilson Samuel T ST Arrigo Kevin R KR Zehr Jonathan P JP

The ISME journal 20200610 10

The microbial fixation of N2 is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N2-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N2 fixation to DIN is unknown. We used stable isotop ...[more]

PMID: 32523086

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Project description:The stabilization of host-symbiont mutualism against the emergence of parasitic individuals is pivotal to the evolution of cooperation. One of the most famous symbioses occurs between legumes and their colonizing rhizobia, in which rhizobia extract nutrients (or benefits) from legume plants while supplying them with nitrogen resources produced by nitrogen fixation (or costs). Natural environments, however, are widely populated by ineffective rhizobia that extract benefits without paying costs and thus proliferate more efficiently than nitrogen-fixing cooperators. How and why this mutualism becomes stabilized and evolutionarily persists has been extensively discussed. To better understand the evolutionary dynamics of this symbiosis system, we construct a simple model based on the continuous snowdrift game with multiple interacting players. We investigate the model using adaptive dynamics and numerical simulations. We find that symbiotic evolution depends on the cost-benefit balance, and that cheaters widely emerge when the cost and benefit are similar in strength. In this scenario, the persistence of the symbiotic system is compatible with the presence of cheaters. This result suggests that the symbiotic relationship is robust to the emergence of cheaters, and may explain the prevalence of cheating rhizobia in nature. In addition, various stabilizing mechanisms, such as partner fidelity feedback, partner choice, and host sanction, can reinforce the symbiotic relationship by affecting the fitness of symbionts in various ways. This result suggests that the symbiotic relationship is cooperatively stabilized by various mechanisms. In addition, mixed nodule populations are thought to encourage cheater emergence, but our model predicts that, in certain situations, cheaters can disappear from such populations. These findings provide a theoretical basis of the evolutionary dynamics of legume-rhizobia symbioses, which is extendable to other single-host, multiple-colonizer systems.

Dataset Information

Unusual marine cyanobacteria/haptophyte symbiosis relies on N₂ fixation even in N-rich environments.

Publications

Unusual marine cyanobacteria/haptophyte symbiosis relies on N<sub>2</sub> fixation even in N-rich environments.

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