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