phytoplankton were N-limited. Low PO 4 3− concentrations in the Atlantic compared to
the Pacific Ocean, have prompted the suggestion of phosphorus limitation there;
however, Van Mooy et al. (2009) elucidated an intriguing adaptation of Atlantic
phytoplankton to low phosphorus. Phytoplankton in the Sargasso Sea reduce their
cellular phosphorus requirements by substituting non-phosphorus membrane lipids for
phospholipids. The Prochlorococcus genome is radically minimized (Dufresne et al.
2003), further saving on phosphate.
Use of Organic Forms of Nitrogen and Phosphorus
(^) Potential nutrients for phytoplankton growth (as well as heterotrophic organisms)
include organic forms of nitrogen and phosphorus. These can be a significant
component of the dissolved nutrient inventories in some regions, and net changes over
annual cycles suggest that use of organic forms of nutrients may extend the productive
season after inorganic forms are depleted (Banoub & Williams 1973). There are
multiple sources and sinks for dissolved organic material, so generalizations about
their significance are difficult, except to say that: (i) pool sizes are small; (ii) turnover
rates can be high; and (iii) it can be difficult to separate the roles of autotrophic and
heterotrophic microorganisms.
Trace Metals and Primary Production
(^) As mentioned above, phytoplankton require trace metals as cofactors for some
enzymes and for components of electron-transfer chains in mitochondria and
chloroplasts. These requirements are reflected in the vertical distributions of essential
metals (e.g. zinc and iron) in the ocean. Most essential metals are depleted at the
surface and increase with depth (Fig. 3.16).
Fig. 3.16 Vertical profiles of dissolved zinc at (32°41′N, 144°59′W) and iron at (50°N,
145°W) in the North Pacific Ocean.
(^) (After Bruland 1980, and Martin et al. 1989.)