Models of marine microbes

Nitrogen fixing (diazotrophic) phytoplankton: e.g.

Image: Annette Hynes

1 mm1 mTrichodesmiumCroccosphaerawatsoniiImage: WHOIWhere are diazotrophs?Trichodesmium

LaRoche and Breitbarth (2005) See also Sohm et al (2011), Luo et a (2012)Trade-offs defining diazotrophsRelative to others, nitrogen fixers havefreedom from limitation by fixed nitrogenhigh cellular iron demandNitrogenaselow maximum growth rateSlow max growth rate

Breitbarth et al (2005) Temperature dependent growth rate of Trichodesmium

Moore et al (1995)Temperature dependent growth rate of picocyanobacteriaTrade-offs define biogeography

Observations of Trichodesmium:Breitbarth and LaRoche (2005)Model, all diazotrophs:Monteiro et al (2010)

RESOURCE RATIO THEORY is a powerful tool5Interpret in terms of resource competition

Monteiro et al (2011) - following Tilman (1977),

Ambient Fe concentration = minimum subsistence concentrationDiazotrophs present where ambient iron concentration greater than/equal to the lowest subsistence concentration for any diazotrophBUT WHAT CONTROLS IRON CONCENTRATION - competitors6

Nitrogen fixing photo-autotrophWhy slow growth rate?Energetic cost of breaking triple bondIntracellular oxygen managementIntracellular iron managementRespiration and growth rateE. coliGlycerol limited continuous cultureFarmer and Jones (1976)

Specific respiration rate(mol O2 (g cell)-1 s-1)Dilution rate (= growth rate) (h-1)Respiration and growth rateE. coliGlycerol limited continuous cultureFarmer and Jones (1976)

Specific respiration(mol O2 (g cell)-1 s-1)Dilution rate (= growth rate) (h-1)Intercept: maintenance respirationSlope related to efficiency (1/yield)Azotobacter vinelandiiFree living soil bacteriaOccupies aerobic environmentsFixes nitrogen asymbiotically

Post et al, Arch. Microbiol (1982)0.5mSpecific substrate consumption and growth rate as function of external O2

Azotobacter vinlandiiKuhler and Oelze (1988)Increasing ambient [O2]Specific substrate consumption and growth rate as function of external O2

For same specific substrate supply, higher growth rate in lower oxygen environmentStrong impact on maintenance uptake/respirationOxygen management to preserve nitrogenaseAzotobacter vinlandiiKuhler and Oelze (1988)ModelConserve internal fluxes of mass, electrons and energyMcCarty (1965), Vallino et al (1996) Biophysical model of substrate and O2 uptakePasciak and Gavis (1974), Staal et al (2003), Keisuke InomurapyruvatebiomasssucroseNH4+O2CO2O2CO2N2C5H7O2NMoleculardiffusion

Keisuke Inomura

Oxygen management over-rides energetic demandMaintenance (intercept) very sensitive to cell sizeModeled yields too highbiomass stoichiometry?exudation of fixed N?

Keisuke Inomura

SummaryProvided appropriate physiological trade-offs and environment are imposed, diverse system will plausibly self-organizeFor diazotrophs, slow population growth rate is a key traitCartoon flux balance/biophysical model captures key aspects of Azotobacter vinelandii growth Model for e.g. Croccosphaera?Experimental data for marine organismMore general application