The Effects of Iron and Light Limitation on the
Physiology of an Oceanic and a Coastal Diatom
Iron (Fe) limits marine phytoplankton primary production in
several oceanic regions. The mechanisms by which Fe reduces
photosynthetic rates and, hence, primary productivity are
not well understood. Similarly, the effect of environmental
variables on phytoplankton iron requirements has not been
adequately assessed. Light, for example, has been
hypothesized to dictate phytoplankton Fe requirements. In
the process of acclimation to light intensities (i.e.,
photoacclimation), phytoplankton modify their photosynthetic
apparatus (Photosystems I and II (PSI & PSII), and the
cytochrome b6f complex (Cyt b6f)). Given the abundance of Fe
proteins contained within these complexes (Fig.
1) acclimation to low light has been hypothesized to
increase cellular Fe requirements. This prediction is based
on the greater abundance of photosystem reaction centres and
electron transport components for cells grown in low
light.
In a previous set of experiments, the effects of
irradiance, temperature and nitrogen source on the cellular
Fe concentrations (i.e., Fe quotas) of a marine diatom were
examined. The predicted increase in cellular Fe quotas at
low irradiance was tested by measuring the intracellular
iron of the marine diatom Thalassiosira weissflogii
(Grunow) growing over a range of continuous photon flux
densities (6-150 µmol quanta m-2 s-1). The results of
two independent trials provide evidence for the predicted
increase in Fe quotas at diminished irradiance (Fig.
2).
Current Research
Presently, I am continuing to explore the relationship
between irradiance and phytoplankton Fe requirements. This
involves quantitation of the major photosystem electron
transfer components (PSII, PSI, Cyt b6f), in addition to Fe
quotas, for two diatom cultures grown over a range of photon
flux densities.
In a preliminary experiment to determine the changes in
reaction centres concurrent with different growth
irradiances, T. weissflogii (T-VIC) was grown
at 2 continuous light intensities: 266 µmol quanta m-2
s-1 (high-light, HL) and 33 µmol quanta m-2 s-1
(low-light, LL). PSII-O2 flash yields were measured and used
to calculate the number of functional PSII reaction centres
per cell. Cytochrome f and P700 were determined
spectrophotometrically and used to calculate the number of
functional Cyt b6f and PSI complexes per cell,
respectively.
As predicted, the concentrations of PSII and PSI increase in
low-light cells, doubling between 266 and 33 µmol
quanta m-2 s-1. However, the photosystem ratio remained
approximately constant. Cytochrome f concentration
(the proxy for the Cyt b6f complex) increased 4-fold in
low-light cultures. Based on the theoretical number of Fe
atoms per complex, the Cyt b6f complex represents the
largest Fe pool within the membrane bound photosynthetic
electron transport pathway. Summing the theoretical iron
requirements derived from reaction centre data, growth at LL
results in a 3.5-fold increase in Fe quotas over HL
conditions. Based on these findings, the photosynthetic
electron transport chain accounts for ca. 40% and 100% of
total cellular Fe for HL and LL cells, respectively.
