(^) Finkel et al. (2006) conducted a more comprehensive study to compare the genetic
(phylogenetic) versus environmental (phenotypic) variations in iron demands of
different phytoplankton. They examined Fe, Mn, Zn, Cu, Co, and Mo levels in five
phytoplankton species grown at five different light intensities. Metal to phosphorus
ratios varied by one to three orders of magnitude. The Fe : P for all species examined
ranged from 2 to 1000 Fe : P (mmol : mol). Diatoms and cyanobacteria showed the
widest ranges of Fe : P 2–251 and 7–1053, respectively, while the dinoflagellate and
prasinophyte had much narrower ranges of 18–359 and 9–52 respectively. The results
show that variations in light levels had as great or greater effect on metal : P ratios,
than group or species differences.
(^) Iron and light interactively affect the cell composition, the rates of primary
production, and the growth of marine phytoplankton. The efficiency of the iron-
uptake mechanism; variations in iron requirements for PSI and PSII; and wide ranges
in iron cell quotas, all suggest the importance of successfully competing for limited
iron supplies. Experiments with laboratory cultures suggest some of the strategies
used by different phytoplankton. Mackey et al. (2008) examined variations in
photosynthetic electron flow in natural phytoplankton communities dominated by
either Synechococcus or Prochlorococcus in high-light, low-nutrient environments.
Both groups have oceanic clones with low PSI : PSII ratios, presumably an adaptation
to low iron availability. Low levels of PSI restrict the electron flow from PSII to PSI
and expose PSII to photo-damage. In these picoplankters, the cell prevents
photodamage with plastoquinol terminal oxidase (PTOX), an enzyme downstream of
PSII that uses electrons to reduce oxygen and regenerate water. This pathway
decouples oxygen cycling from CO 2 fixation in photosynthesis, and appears to be
widespread in oceanic surface layers.
Effects of Temperature Variation on Primary
Productivity
(^) Increased temperature affects phytoplankton growth as it does other metabolic
processes: it makes the reactions proceed faster. Since many reactions with a variety
of kinetics are involved in photosynthesis, there is no simple relation between either
photosynthetic rate or growth rate and temperature. Most importantly, different
species respond differently to temperature variation. Furthermore, temperature
interacts with other factors such as nutrient availability to determine rates, so that the
relationships can be complex. However, at an ecological level the effect of
temperature is to set an overall upper limit on growth rates. A data compilation by
Smayda (1976, 1980) of maximum cell division rates for many species (Fig. 3.17)
shows increasing exponents over ranges of 10°C or more, and then, for most species,