Iron is known to limit the growth of phytoplankton, especially diatoms, in large regions of the oceans. Because of their large size and silica ballast, diatoms are particularly important in the export of organic matter to the deep ocean. Previous laboratory data from our group and others have shown that diatoms take up the free iron (i.e., the iron that is not bound to strong chelating agents) from solution. But, according to field data, the free iron in surface seawater is too low to support the growth of these dominant primary producers. Both new and old laboratory data show that diatoms can obtain their iron by reducing Fe(III) in some chelates, including iron bound to siderophores. We have established that Fe(III) reduction at the diatom surface is an essential step in the uptake of iron in all cases and developed a model for uptake kinetics that reconciles all available data (Shaked et al. 2005). This model provides a chemical framework to quantify the bioavailability of Fe in seawater. In the course of this study, we have also shown that diatoms produce an abundant quantity of superoxide (O2-), by extracellular reduction of oxygen (Kustka et al. 2005). Superoxide is an extremely reactive radical able to reduce and oxidize many solutes, including Fe, in seawater.
To obtain a more mechanistic understanding of the iron uptake system of diatoms, we have studied the effect of Fe availability on the expression of genes coding for the proteins that are thought to be responsible for the reduction of Fe(III) and the transport of Fe in the two diatoms whose full genomes have been sequenced, Thalassiosira pseudonana and Phaedactylum tricornutum (Kustka et al. 2007). As expected, the transcription of putative ferric reductases is upregulated under iron stress. But the steps downstream from the initial Fe(III) reduction appear to be different in the two model organisms. Recently, we have demonstrated that diatoms and other marine phytoplankters are able to acquire Fe from iron storage proteins such as ferritins and Dps proteins (Castruita et al. in press). These proteins may thus play an important role in the biological cycling of iron in surface seawater .
