ABSTRACT: Under conditions of iron stress, many organisms replace the common iron-sulfur redox protein ferredoxin with flavodoxin, a functionally equivalent, non-iron-containing protein. These 2 proteins have been proposed to be indicators of iron nutritional status in marine phytoplankton, but little is known of their expression and regulation. This study characterized their expression by: (1) testing 17 marine phytoplankton isolates from 4 different algal classes for their ability to induce flavodoxin under iron limitation, (2) determining the effect of ecologically relevant limiting factors (other than iron) on flavodoxin expression using the marine centric diatom Thalassiosira weissflogii as a model organism, and (3) examining, in detail, the relationship between iron availability and relative ferredoxin/flavodoxin abundance again using T. weissflogii as a model. In the organisms examined, the most common response (12 of 17 isolates) to iron limitation was induction of flavodoxin and suppression of ferredoxin expression. The remaining 5 isolates, largely of coastal origin, were never observed to produce flavodoxin. These 5 non-inducing organisms have been shown to have high intrinsic Fe requirements for growth and should therefore not present a problem for field measurements of ferredoxin and flavodoxin in iron-poor areas. Expression of flavodoxin in T. weissflogii was found to be specific to iron limitation, and was not induced by nitrate, phosphate, silicate, zinc or light deficiency. The prevalence of the flavodoxin response and its insensitivity to other limiting factors support its use as an indicator of the presence of iron limitation. Iron regulation of relative ferredoxin and flavodoxin abundance (the Fd index) and iron availability was examined in greater detail by measuring the Fd index in T. weissflogii grown over a range of limiting iron concentrations. The relationship between Fd index and growth rate (a proxy for iron availability) is composed of 2 distinct regions. In the first region, at low growth rates, ferredoxin is undetectable and the Fd index is uniformly zero. In the second region, at moderate-to-fast growth rates, ferredoxin and flavodoxin co-occur in the cells. This implies that flavodoxin substitution is not a simple 'on-or-off' response. Flavodoxin expression is also very sensitive to iron limitation, occurring even at fast growth rates (80 to 90% µ_max). When the 2 proteins co-occurred in cells, their relative abundance (the Fd index) tended to increase along with increasing iron availability. Thus, variation in the Fd index has the potential to indicate spatial and temporal changes in the severity of iron stress in the phytoplankton community.

KEY WORDS: Ferredoxin · Flavodoxin · Iron · Phytoplankton

Published in MEPS Vol. 184 (1999) on July 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999