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Nitrogen fixing ( diazotrophic ) phytoplankton: e.g.

Nitrogen fixing ( diazotrophic ) phytoplankton: e.g. Trichodesmium. 1 μm. Croccosphaera watsonii. 1 mm. Image: Annette Hynes. Image: WHOI. Where are diazotrophs? Trichodesmium. LaRoche and Breitbarth (2005) See also Sohm et al (2011), Luo et a (2012).

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Nitrogen fixing ( diazotrophic ) phytoplankton: e.g.

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  1. Nitrogen fixing (diazotrophic) phytoplankton: e.g. Trichodesmium 1 μm Croccosphaera watsonii 1 mm Image: Annette Hynes Image: WHOI

  2. Where are diazotrophs?Trichodesmium LaRoche and Breitbarth (2005) See also Sohm et al (2011), Luo et a (2012)

  3. Trade-offs defining diazotrophs Relative to others, nitrogen fixers have • freedom from limitation by fixed nitrogen • high cellular iron demand • Nitrogenase • low maximum growth rate

  4. Slow max growth rate Moore et al (1995) Temperature dependent growth rate of picocyanobacteria Breitbarth et al (2005) Temperature dependent growth rate of Trichodesmium

  5. Trade-offs define biogeography Observations of Trichodesmium: Breitbarth and LaRoche (2005) Model, all diazotrophs: Monteiro et al (2010)

  6. Interpret in terms of resource competition Ambient Fe concentration = minimum subsistence concentration Monteiro et al (2011) - following Tilman (1977), …

  7. Why slow growth rate? • Energetic cost of breaking triple bond • Intracellular oxygen management • Intracellular iron management Nitrogen fixing photo-autotroph

  8. Respiration and growth rate • E. coli • Glycerol limited continuous culture • Farmer and Jones (1976) Specific respiration rate (mol O2 (g cell)-1 s-1) Dilution rate (= growth rate) (h-1)

  9. Respiration and growth rate Slope related to efficiency (1/yield) • E. coli • Glycerol limited continuous culture • Farmer and Jones (1976) Specific respiration (mol O2 (g cell)-1 s-1) Intercept: maintenance respiration Dilution rate (= growth rate) (h-1)

  10. Azotobactervinelandii • Free living soil bacteria • Occupies aerobic environments • Fixes nitrogen asymbiotically 0.5μm Post et al, Arch. Microbiol (1982)

  11. Specific substrate consumption and growth rate as function of external O2 Increasing ambient [O2] • Azotobactervinlandii • Kuhler and Oelze (1988)

  12. Specific substrate consumption and growth rate as function of external O2 • For same specific substrate supply, higher growth rate in lower oxygen environment • Strong impact on maintenance uptake/respiration • Oxygen management to preserve nitrogenase • Azotobactervinlandii • Kuhler and Oelze (1988)

  13. Model • Conserve internal fluxes of mass, electrons and energy • McCarty (1965), Vallino et al (1996) … • Biophysical model of substrate and O2 uptake • Pasciak and Gavis (1974), Staal et al (2003), … O2 CO2 Molecular diffusion O2 CO2 “biomass” C5H7O2N pyruvate sucrose NH4+ N2 Keisuke Inomura

  14. Keisuke Inomura

  15. Oxygen management over-rides energetic demand • Maintenance (intercept) very sensitive to cell size • Modeled yields too high • “biomass” stoichiometry? • exudation of fixed N? Keisuke Inomura

  16. Summary • Provided appropriate physiological trade-offs and environment are imposed, diverse system will plausibly self-organize • For diazotrophs, slow population growth rate is a key trait • Cartoon “flux balance”/biophysical model captures key aspects of Azotobactervinelandiigrowth • Model for e.g. Croccosphaera? • Experimental data for marine organism • More general application…

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