least a passage channel. Experimental data (Fig. 1.1) from Sunda and Huntsman
(1997) show that, at growth-limiting concentrations of ferric iron, phytoplankton cells
of all sizes have equal (the diffusion-limited maximum) rates of iron uptake per unit
area. Because the iron requirement is general, and only met by the Fe3+ ion, sufficient
areal density of transporters evolves in most (all?) species, such that uptake is, in fact,
diffusion-limited, not transporter-limited. Smaller cells with less mass relative to
surface area, however, receive enough iron to sustain growth when large cells are
iron-limited. In addition, small, oceanic phytoplankton have evolved to require less
iron per unit mass by substantial rearrangements of both photosynthetic and oxidative
metabolism.
Fig. 1.1 Cellular uptake rate (per m^2 of cell surface area) of ferric iron from a culture
medium (on a logarithmic scale) by phytoplankton cells of different mean diameters
as listed, two species of Thalassiosira (diatoms) and two of Prorocentrum
(dinoflagellates). Uptake was determined with radioactive iron as a tracer. Fe′ is a
total of dissolved Fe(3+) species. Above [Fe(3+)] ≈ 0.75 nM at 20°C, ferric hydroxide
precipitates, dashed curve, maintaining Fe′ at ≈0.75 nM. In the experiments, Fe′ was
set by means of an iron chelating agent
(After Sunda & Huntsman 1997).
Because phytoplankton are small, they are also individually ephemeral compared to
terrestrial plants or to algae attached along the shore. Grazing terrestrial animals
typically take a bite from a plant, which then heals; pelagic grazers typically ingest
the entire phytoplankton cell, so it is gone. Therefore, maintenance of a population of
cells, a phytoplankton stock, depends upon their rapid reproduction. And reproduction
can be rapid. Many (not all) phytoplankton can double in number one or more times
per day. Thus, if grazers are few and growth conditions (light, nutrients, temperature)
are good, then stocks can grow exponentially. Doubling once per day, they can
increase 1000-fold in 10 days. Rapidly growing diatoms can increase twice that fast.
This potentially rapid increase is the basis for phytoplankton population outbursts or