PHYSIOLOGICAL AND BIOCHEMICAL RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF 2 MARINE DIATOMS TO FE STRESS

Flavodoxin is a small electron-transfer protein capable of replacing f erredoxin during periods of Fe deficiency. When evaluating the suitabi lity of flavodoxin as a diagnostic indicator for Fe limitation of phyt oplankton growth, we examined its expression in two marine diatoms we cultured using trace-metal-buffered medium. Thalassiosira weissflogii and Phaeodactylum tricornutum were cultured in ethylenediaminetetraace tic acid-buffered Sargasso Sea water containing from 10 to 1000 nM add ed Fe. Trace-metal-buffered cultures of each diatom maintained high gr owth rates across the entire range of Fe additions. Similarly, decline s in chlorophyll/cell and in the ratio of photosystem II variable-to-m aximum fluorescence were negligible (P. tricornutum) to moderate (T. w eissflogii; 54% decline in chlorophyll/cell and 22% decrease in variab le-to-maximum fluorescence). Moreover, only minor variations in photos ynthetic parameters were observed across the range of additions. In co ntrast, flavodoxin was expressed to high levels in low-Fe cultures. De spite the inverse relationship between flavodoxin expression and Fe co ntent of the medium, its expression was seemingly independent of any o f the indicators of cell physiology that were assayed. It appears that flavodoxin is expressed as an early-stage response to Fe stress and t hat its accumulation need not be intimately connected to limitations i mposed by Fe on the growth rate of these diatoms.