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

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

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

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

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

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

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

Respiration and growth rate E. coli Glycerol limited continuous culture Farmer and Jones (1976) Dilution rate (= growth rate) (h -1 )

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)

Azotobacter vinelandii Free living soil bacteria Occupies aerobic environments Fixes nitrogen asymbiotically Post et al, Arch. Microbiol (1982) 0.5 μ m

Specific substrate consumption and growth rate as function of external O 2 Azotobacter vinlandii Kuhler and Oelze (1988) Increasing ambient [O 2 ]

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)

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

Keisuke Inomura

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

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…