Biological Importance of Fe

Iron is an essential micronutrient for all organisms. It is a required element in cytochromes and the Fe-S centers of redox proteins involved in key metabolic processes such as photosynthesis, respiration, and the reduction of nitrate. Given the importance of Fe in these major metabolic enzymes, microorganisms under Fe-deficient conditions reduce their cellular Fe quotas (Fe C content) (Sunda et al., 1991) and have reduced photosynthetic (Raven, 1988 Green et al., 1994) and bacterial growth (Tortell et al., 1996) efficiencies. Thus ecosystem C metabolism is regulated in part by the availability of Fe. 

Bacteria have high Fe requirements and account for up to 50% of the biological iron pool in Fe-limited regions (Tortell et al., 1996). Bacterial production is stimulated with the addition of Fe (Pakulski et al., 1996). Under conditions of Fe deprivation, microorganisms release small molecules (MW 1000 Da) called siderophores that chelate Fe(III) with a strong affinity (Neilands, 1981, 1995). In controlled laboratory experiments, both 

BIOLOGICAL Fe DEMAND IN BROWN- VERSUS CLEARWATER SYSTEMS 

Second, in humic systems, there is an increase in the production of biologically harmful oxygen radicals such as 02 , H202, and OH , all of which are toxic by-products of the photo-oxidation of DOM (Zarifou et al., 1984). Even low concentrations of added H202 ( 100 nM) have been demonstrated to have negative effects on aquatic bacteria (Xenopoulos and Bird, 1997) 

Even in the absence of DOM and/or biological activity, Fe chemistry is complex. In oxygenated water at pH 1.0, iron exists primarily as dissolved Fe3+ (or Fe(H20)g3+). However, as the pH increases toward circumneutral, Fe(III) undergoes stepwise hydrolysis  

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