Nitrogen fixation iron availability

Berman-Frank, L, Cullen, J., Hareh, Y., SherreU, R., and Falkowski, P. G. (2001a). Iron availability, cellular iron quotas, and nitrogen fixation in Trichodesmium. Limnol. Oceanogr. 46, 1249—1277. 

It has been suggested that rates of nitrogen fixation in the modem ocean are limited by the availability of iron. The iron requirement of Trichodesmium, however, turns out to be much lower than previously estimated. Instead, it is the availability of phosphate that controls the upper limit of nitrogen fixation in the modem ocean (Sanudo-Wilhelmy et al, 2001). 

In some cases flavodoxins are expressed under iron limiting conditions to compensate for the lack of functional ferredoxin. In other cases, for example NifF, flavodoxins are expressed as a necessary component of a specific pathway. The low molecular weight ( 20kDa) proteins contain FMN which theoretically has three oxidation states available, oxidized, the 1 e reduced semiquinone, and the 2 e reduced hydroquinone. In practice, these proteins are thought to function in hydrogen metabolism or nitrogen fixation by cycling between the semiquinone and hydroquinone states. 

So, although N may be the proximate limiter of primary production, P is likely to be the ultimate limiter (Tyrrell 1999).The dependence of nitrogen fixation on iron availability introduces a complicating factor (Wu et al. 2000). 

The availability of soil phosphate is highly pH dependent and, as with nitrogen, Is only partially understood. The main mechanism for phosphate fixation (decreased availability) under acid conditions appears to be the precipitation of highly insoluble iron and aluminium phosphates. Phosphate availability also tends to decrease at high... 

The role of organic phosphorus in N -fixation and greenhouse gas emissions from the oceans was demonstrated by Sanudo-Wilhelmy et al. (2001). Many studies have reported that N -fixation is limited principally by the availability of iron (e.g. Falkowski, 1997), an essential coprotein for nitrogenase (Atlas and Bartha, 1993). However, Sanudo-Wilhelmy et al. (2001) recently reported that rates of N -fixation were highly correlated with the phosphorus content of a cyanobacterial colony rather than iron availability or intracellular iron content. The study of Sanudo-Wilhelmy et al. (2001) provides further evidence of the coupling of the phosphorus cycle with the carbon and nitrogen cycles. Indeed, each of... 

---