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Published byGeorge Gilbert Modified over 9 years ago
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Nitrogen fixing (diazotrophic) phytoplankton: e.g. Image: Annette Hynes 1 mm 1 μ m Trichodesmium Croccosphaera watsonii Image: WHOI
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Where are diazotrophs? Trichodesmium LaRoche and Breitbarth (2005) See also Sohm et al (2011), Luo et a (2012)
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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
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Slow max growth rate Breitbarth et al (2005) Temperature dependent growth rate of Trichodesmium Moore et al (1995) Temperature dependent growth rate of picocyanobacteria
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Trade-offs define biogeography Observations of Trichodesmium: Breitbarth and LaRoche (2005) Model, all diazotrophs: Monteiro et al (2010)
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Interpret in terms of resource competition Monteiro et al (2011) - following Tilman (1977), … Ambient Fe concentration = minimum subsistence concentration
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Nitrogen fixing photo-autotroph Why slow growth rate? Energetic cost of breaking triple bond Intracellular oxygen management Intracellular iron management
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Respiration and growth rate E. coli Glycerol limited continuous culture Farmer and Jones (1976) Dilution rate (= growth rate) (h -1 )
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Respiration and growth rate E. coli Glycerol limited continuous culture Farmer and Jones (1976) Dilution rate (= growth rate) (h -1 ) Intercept: maintenance respiration Slope related to efficiency (1/yield)
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Azotobacter vinelandii Free living soil bacteria Occupies aerobic environments Fixes nitrogen asymbiotically Post et al, Arch. Microbiol (1982) 0.5 μ m
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Specific substrate consumption and growth rate as function of external O 2 Azotobacter vinlandii Kuhler and Oelze (1988) Increasing ambient [O 2 ]
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Specific substrate consumption and growth rate as function of external O 2 For same specific substrate supply, higher growth rate in lower oxygen environment Strong impact on maintenance uptake/respiration Oxygen management to preserve nitrogenase Azotobacter vinlandii Kuhler and Oelze (1988)
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Model Conserve internal fluxes of mass, electrons and energy McCarty (1965), Vallino et al (1996) … Biophysical model of substrate and O 2 uptake Pasciak and Gavis (1974), Staal et al (2003), … Keisuke Inomura pyruvate “biomass” sucrose NH 4 + O2O2 CO 2 O2O2 N2N2 C5H7O2NC5H7O2N Molecular diffusion
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Keisuke Inomura
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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
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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 Azotobacter vinelandii growth Model for e.g. Croccosphaera ? Experimental data for marine organism More general application…
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