People: If this image does not appear press reload or turn on images

Robert Strzepek


Figure 1

Figure 2

Curriculum Vitae

Robert Strzepek

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.

top home