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

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…It is not clear, then, why a genetic trait that confers such obvious advantages was not selected during the evolution of terrestrial plants. Fld has been found in the plastids of eukaryotic algae belonging to all major families (19), suggesting that the gene was lost in the green algal precursor or in the primitive plants. Fld loss might be related to ecological adaptations, but a systematic appraisal of this possibility will require an extensive survey of Fld-proficient and -deficient clones between algal groups and habitats.…”

“…An example of such an adaptation is provided by some cyanobacteria and marine algae that, when exposed to Fe-deficient conditions, induce the expression of flavodoxins (Fld), small redox proteins harboring one molecule of flavin mononucleotide as prosthetic group (6,(19)(20)(21). These soluble electron carriers are able to replace Fe-containing ferredoxin (Fd) in many different processes, including photosynthesis (22).…”

“…Fld expression relieves the burdens imposed by Fe deficit and restores normal rates of growth and reproduction. The importance of this flavoprotein in the dynamics of marine ecology is highlighted by its use as the most sensitive marker of Fe stress in the oceans (6,19). Up-regulation of Fld levels is also considered as one of the factors determining colonization of Fe-poor waters by phytoplankton (23), and photosynthetic microorganisms lacking the Fld gene are usually restricted to Fe-rich coastal environments (24).…”

Physiological and behavioral diagnostics of nitrogen limitation for the toxic dinoflagellate Alexandrium fundyense

“…At present there are only a few methods and indicators that have been identified for nutrient limitation of phytoplankton species in the natural environment. For example, diagnostics such as glutamine:glutamate (GLN/GLU; 14 Flynn et al 1989; and ferridoxin:flavodoxin (Fd index; Erdner et al 1999), have been related to specific limitations or stresses (nitrogen and iron) in laboratory cultures and in some cases have been used to monitor changes in nutritional stress in the natural environment (LaRoche et al 1996;Erdner and Anderson 1999). However, in most cases standard bulk analyses of the phytoplankton community, such as chlorophyll, primary production, or nutrient uptake, are not useful since they provide data for the entire planktonic community rather than an individual species, such as A. fundyense.…”

Comparative metatranscriptomics identifies molecular bases for the physiological responses of phytoplankton to varying iron availability

“…When iron limited, cells may temporarily replace iron-requiring proteins with less efficient iron-free functional equivalents to minimize their iron demands, including the alternation between the iron-containing photosynthetic electron transfer protein ferredoxin and the iron-free version flavodoxin (14,15). Most cultured isolates of diatoms preferentially use ferredoxin when grown under iron-replete conditions (16)(17)(18) although inconsistencies between laboratory and field studies have been reported (5,19). In some instances, reliance on iron-free versions of proteins may be permanent.…”