Characterization of ferredoxin and flavodoxin as markers of iron limitation in marine phytoplankton

Abstract

Under conditions of iron stress, many organisms replace the comlnon iron-sulfur redox protein ferredoxin with flavodoxin, a functionally equivalent, non-iron-containing protein These 2 proteins have been proposed to be ind~cators of iron nutritional status in marine phytoplankton, but llttle is known of their expression and regulation. This study charactenzed their expression by. (1) testing 17 marine phytoplankton lsolates from 4 different algal classes for their ability to induce flavodoxin under iron limtatlon, ( 2 ) determining the effect of ecologically relevant limiting factors (other than iron) on flavodoxin expression using the marine centric diatom Thalassiosira weissflogu as a model organism, and (3) exdmining, in detail, the relationship between iron availability and relatlve ferredoxin/flavodoxin abundance again using T welssflogii as a model. In the organisms exammed, 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 we~ssflogii was found to b e 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 a n inhcator of the presence of iron limitation. Iron regulation of relative ferredoxin and flavodoxln abundance (the Fd index) and iron availabhty was examined in greater detail by measuring the Fd index in T. welssflogii grown over a range of h t i n g 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, ferredoxln is undetectable and the Fd index is uniformly zero In the second region, at moderate-to-fast growth rates, ferredoxin and flavodoxln co-occur in the cells. This implies that flavodoxin substitution is not a sunple 'on-or-off' response Flavodoxin expression is also very sensitive to iron limitation, occurring even at fast growth rates (80 to 90% pmau). 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 ~ndlcate spatial and temporal changes in the severity of iron stress in the phytoplankton community