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Development of a field assay of iron limitation in nutrient rich lakes

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  • Severe blooms of cyanobacteria (blue-green algae) affect many nutrient-rich Oregon lakes. However, the availability of iron may limit bloom development in certain lakes. Iron uptake and assimilation by phytoplankton have been the subjects of a number of studies, particularly with regard to possible iron limitation of primary productivity. An understanding of iron assimilation by phytoplankton has been hindered by a lack of data on the biological availability of aqueous iron and the corresponding iron nutritional status of cells. The physiological response of phytoplankton to iron stress appears to be the best' way of obtaining this information but analytical problems·had previously limited this approach. In this report we demonstrate that cellular concentrations of the redox proteins ferredoxin and flavodoxin are practical indicators of iron stress, both in laboratory cultures and in natural populations of cyanobacteria. Experiments with the N2-fixing cyanobacterium Anabaena sp. 7120 reveal that ferredoxin levels decrease and flavodoxin levels increase in proportion to the overall iron stress of the cells. Iron stress is a function of both the iron provided in the medium (iron availability) as well as the iron requirements of the cells. Thus for a given level of available iron, N2-fixing and nitrate­ reducing cells, which have high iron requirements, exhibited greater iron stress than cells growing on ammonium. This indicates an important linkage between iron availability and nitrogen availability: N2-fixation induces a greater need for iron, and low iron may limit the rates of N2-fixation. Predictions of iron limitation in lakes must take into account the nitrogen resources of the lake. Conversely, studies of nitrogen assimilation in lakes should consider the effects of iron availability.
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  • The activities on which this report is based were financed in part by the Department of the Interior, U.S. Geological Survey, through the Oregon Water Resources Research Institute.
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