Flavodoxin and ferredoxin can be used as molecular markers for determination of biolimitation of iron in phytoplankton. TwoELISA tests have been used for determination of flavodoxin and ferredoxin respectively in samples from three endorheiclagoons.

Quantification of the iron nutritional status of phytoplankton is of great interest not only for the study of the oceans but also for fresh waters. Flavodoxin is a small flavoprotein proposed as a molecular marker for iron stress, since it is induced as a consequence of iron deprivation, replacing the iron-sulphur protein ferredoxin. Flavodoxin and ferredoxin from Scenedesmus vacuolatus have been immunoquantified in cells grown under different iron nutritional conditions. Flavodoxin and ferredoxin levels correlate with the iron availability, and the calculated flavodoxin index can be used as an iron-stress marker. Other physiological parameters such as copper deficiency, heterotrophic or mixotrophic growth, nitrogen source and salt stress were also tested as potential factors influencing flavodoxin expression. Salt stress and heterotrophic growth conditions alter flavodoxin and ferredoxin expression. Once flavodoxin expression is repressed by iron (and severe deficiency alleviated), S.vacuolatus still increases its ferredoxin from 0.5 to 1.6 mol of ferredoxin per mole of ferredoxin-NADP+ reductase, and this ratio can be used for the evaluation of mild deficiency.

Susceptibility to oxygen and properties relative to oxygen metabolism were compared in metronidazole-resistant and susceptible strains of Trichomonas vaginalis. The study involved clinical isolates displaying the aerobic type of resistance, as well as resistant strains developed in vitro, both with aerobic (MR-3) and anaerobic (MR-5, MR-100) resistance. Elevated sensitivity to oxygen of the resistant clinical isolates was observed. Progressive increase of susceptibility to oxygen also accompanied in vitro development of resistance. No correlation was found between the activity of NADH oxidase and aerobic resistance, while the in vitro derivative with fully developed anaerobic resistance (MR-100) showed about 50% decrease of NADH oxidase activity. The superoxide dismutase (SOD) activity was elevated in both resistant clinical isolates and in in vitro-developed resistant strains. The changes in levels of ferredoxin were insufficient to support ferredoxin deficiency as a cause of aerobic metronidazole resistance. Western blot analysis and electron paramagnetic resonance spectroscopy of purified hydrogenosomes showed that ferredoxin is expressed in aerobically resistant strains and has intact iron-sulfur clusters. Down-regulation of ferredoxin was demonstrated only in the late phase of development of the anaerobic resistance (MR-100). The results support a link between aerobic resistance and defective oxygen scavenging. The increased levels of intracellular oxygen, beneficial to resistant parasites when they interact with the drug, may have adverse effects on their fitness as shown by their increased sensitivity to oxidative stress.

We report the cloning and characterization of a 1881 bp Thalassiosira weissflogii (Bacillariophyceae, Centrales) plastid genomic fragment containing the ferredoxin gene (petF) and several other genes. The organization of this fragment is exactly the same as its equivalent region from the plastid genome of another centric diatom, Odontella sinensis, and the sequences share high homology in both coding and non-coding regions. Traces of a probable vestigial protein-coding gene and of a partial trn gene are found just after the trnR gene in both diatoms as well as in the plastid genome of the red alga Porphyra purpurea, illustrating the close relationship between red algal and heterokont algal plastid genomes and arguing in favour of the origin of the latter by secondary endosymbiosis. In accordance with phylogenetic analyses of most plastid genes, phylogenetic trees inferred from 69 ferredoxin sequences show that both the green and the red algal plastid genes arose from cyanobacterial genes; our results furthermore suggest that each of these plastid lineages is related to different extant cyanobacterial genes, implying either that different endosymbiotic events were responsible for the origin of green and red plastids or that each lineage kept only one (and a different one) of several paralogous cyanobacterial genes present in the ancestral cyanobacterial genome.

Thioredoxins are ubiquitous small-molecular-weight proteins (typically 100–120 amino-acid residues) containing an extremely reactive disulphide bridge with a highly conserved sequence -Cys-Gly(Ala/Pro)-Pro-Cys-. In bacteria and animal cells, thioredoxins participate in multiple reactions which require reduction of disulphide bonds on selected target proteins/enzymes. There is now ample biochemical evidence that thioredoxins exert very specific functions in plants, the best documented being the redox regulation of chloroplast enzymes. Another area in which thioredoxins are believed to play a prominent role is in reserve protein mobilization during the process of germination. It has been discovered that thioredoxins constitute a large multigene family in plants with different subcellular localizations, a unique feature in living cells so far. Evolutionary studies based on these molecules will be discussed, as well as the available biochemical and genetic evidence related to their functions in plant cells. Eukaryotic photosynthetic plant cells are also unique in that they possess two different reducing systems, one extrachloroplastic dependent on NADPH as an electron donor, and the other one chloroplastic, dependent on photoreduced ferredoxin. This review will examine in detail the latest progresses in the area of thioredoxin structural biology in plants, this protein being an excellent model for this purpose. The structural features of the reducing enzymes ferredoxin thioredoxin reductase and NADPH thioredoxin reductase will also be described. The properties of the target enzymes known so far in plants will be detailed with special emphasis on the structural features which make them redox regulatory. Based on sequence analysis, evidence will be presented that redox regulation of enzymes of the biosynthetic pathways first appeared in cyanobacteria possibly as a way to cope with the oxidants produced by oxygenic photosynthesis. It became more elaborate in the chloroplasts of higher plants where a co-ordinated functioning of the chloroplastic and extra chloroplastic metabolisms is required.