Nitrogen fixing (diazotrophic) phytoplankton: e.g.

Image: Annette Hynes

1 mm

1 μm

Trichodesmium

Croccosphaerawatsonii

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 demandNitrogenase

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)

Specific respiration rate (mol O

2 (g cell)

-1

s-1)

Dilution rate (= growth rate) (h-1)

Respiration and growth rate

E. coli

Glycerol limited continuous culture

Farmer and Jones (1976)

Specific respiration (mol O

2 (g cell)

-1

s-1)

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)

0.5μm

Specific substrate consumption and growth rate as function of external

O2

• Azotobacter vinlandii

• Kuhler 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 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 O2 uptake• Pasciak and Gavis

(1974), Staal et al (2003), …

Keisuke Inomura

pyruvate

“biomass”

sucroseNH4

+

O2

CO2

O2

CO2

N2

C5H7O2N

Moleculardiffusion

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

SummaryProvided 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…