experiments in the Sargasso, significant effects were only found for ammonium
enrichment (Fig. 11.28). The system is nitrogen limited. The absence of stimulation
by nitrate may be explained for Prochlorococcus by its lack of nitrate reductase
(Rocap et al. 2003; but see Martiny et al. 2009). While picoeukaryotes generally have
genes for use of nitrate, those may well not be active when [N+N] is at extremely low
levels. Clearly, the immediately limiting nutrient is fixed nitrogen. The addition of
both ammonium and phosphate might not have had a synergistic effect, as was seen in
the Sargasso, given the low N : P ratio in the NPSG. Van Mooy and Devol point out
that Prochlorococcus is metabolically poised to operate in the face of extremely low
phosphorus and iron availability. The phospholipids usual in cell membranes have
been replaced by sulfolipids – sulfate is copiously abundant in seawater relative to
biological needs. The genome is radically minimized (Dufresne et al. 2003), saving
on phosphate, and the absence of nitrate reductase saves on iron. Regardless of N : P,
the radically low euphotic zone levels of phosphate in subtropical gyres favor this
autotroph that needs an absolute minimum of it. Possibly the large South Pacific gyre
has completely different nutrient dynamics: while also very oligotrophic, it
consistently exhibits 0.2–3 μM inorganic phosphate.
Fig. 11.28 Rates of RNA synthesis by groups of particulate plankton as determined by
(^33) PO 4 3− labeling. Asterisks indicate statistically significant difference from the
control; error bars are standard deviations among three replicates.
(After Van Mooy & Devol 2008.)
(^) The NPSG ecosystem is likely also iron-limited in a sense, but the long-term rates
of eolian and advective iron supply are great enough to overcome the slow rate of
fixed nitrogen supply, so that nitrogen becomes fully depleted from the surface.
HNLC regions all are much more “baroclinic” and divergent, such that iron supply
cannot keep pace with nitrogen supply, so iron becomes, as John Martin liked to put