Iron (Fe) Deficiency in Cyanobacteria

Iron (Fe) Deficiency i*n *Cyanobacteria Fe in cyanobacteria serves as an essential redox component important to diverse metabolic pathways. Fe-rich systems in cyanobacteria such as the photosynthetic apparatus and the respiratory electron transport system are dependent on Fe supply (Raven et al., 1999). Other important cellular processes such as nitrogen assimilation (Raven, 1988), ribonucleotide synthesis, chlorophyll synthesis and oxygen radical detoxification are also Fe dependent. Prokaryotes, including cyanobacteria, have developed several strategies to enhance survival during periods of Fe deficiency. Some of these include the release of internal Fe stores contained within proteins such as bacterioferritin (Keren et al., 2004), the production of ferric ion scavenging molecules known as siderophores (Wilhelm et al., 1996), and the substitution of Fe dependent catalysts with isofunctional proteins that do not require Fe. In gram negative bacteria and bacteria with chromosomes low in GC content, many of the genes activated in response to Fe deficiency are globally regulated by the transcriptional repressor Fur (ferric uptake regulator). Under Fe replete conditions, Fe2+ serves as a coregulator of Fur, promoting binding of Fur to a consensus sequence (FUR box) on the operator region of Fe-regulated genes. Fur binding effectively prevents the binding of RNA polymerase and subsequent transcription of these genes (Ochsner and Vasil, 1996). The cyanobacterial Fur homolog controls a regulon, which includes genes involved in siderophore production (Ghassemian and Straus, 1996), the induction of flavodoxin for substitution of ferredoxin in the photosynthetic electron transport chain, and the production of a novel Fe stress- specific PS I binding protein IsiA, which forms an 18 mer around PSI to protect it from photooxidation (Bibby, 2001). The Fe Hypothesis Martin speculated that adding 0.3 million tons of the trace metal Fe into the Southern Ocean would be