The Ks values for PO 4 3− uptake by Trichodesmium are low (implying high affinity), as
low as 100 nM (Moutin et al. 2005) at some ocean sites and for some cultures, but
still well above the SRP concentrations widespread in gyre surface layers. A capacity
for stripping PO 4 3− from organic forms has been repeatedly demonstrated in culture,
and alkaline phosphatase serving this function is present in Trichodesmium in the
field. Trichodesmium, particularly in colonies of tangled filaments, has variable
density regulated by the interaction of intracellular gas bubbles and carbohydrate
loading (e.g. White et al. 2006) and can both sink and rise. Possibly, it sinks to the
nutricline, takes up phosphate, and returns to better-lighted layers to fix nitrogen and
grow. Several studies (e.g. Villareal & Carpenter 2003) have found in some instances
that rising colonies have greater phosphorus content than sinking ones. At least
Trichodesmium can take up ammonium and nitrate, so it might be expected that
available fixed nitrogen would suppress fixation. It does, but only to about 30% of
maximum rates.
Fig. 11.30 Depth profiles of (a) whole-water and (b) size-fractionated (<10 μm) N 2
fixation rates based on ^15 N 2 uptake during November 2004 (•), February 2005 ( ),
and March 2005 ( ). Error bars are standard errors for means of three replicates.
These are likely to be underestimates (Mohr et al. 2010), but relative rates are likely
to be correct.
(After Grabowski et al. 2008.)
Experiments with field collections (e.g. Reuter 1988) and cultures show that
fixation rates in Trichodesmium depend on iron concentration, with limitation effects
within the range of concentrations observed in the field. Effects on diazotrophy in the
field of experimentally increasing iron availability are not well studied, but
geochemical arguments suggest that iron availability is key to the global rates and
distribution of fixation. The possibility of iron limitation of N 2 fixation leads directly
to a partial explanation for the distinction between the Sargasso Sea and NPSG in