Low Fe availability has been suggested as a potential constraint on primary production in the Great Lakes. Here we report on the use of a cyanobacterial bioreporter to assess available Fe in Lake Erie during summer and fall field seasons in 2 1 and 2 2. Bioreporter luminescence was derived from a luciferase reporter controlled by an iron-responsive promoter element isiAB. Filtered (< .2 mm) water sampled from the western basin during summer 2 1 and 2 2 yielded low bioreporter response indicating Fe-replete conditions [pFe (-log [Fe3+]) < 2 .8]. Likewise, water collected from the eastern basin following autumnal mixing in November 2 1 yielded a Fe-replete bioreporter response. In contrast, surface water collected at pelagic stations located in central and eastern basins during summer 2 2 indicated a seasonal depletion of bioavailable Fe. Whereas water sampled from these locations during July and August was characterized as Fe-replete (pFe < 2 .8), samples collected during September elicited a high luminescent response from the bioreporter (pFe > 21). Contrary to the characterization provided by the filtered samples, assay of bioreporter response in unfiltered water conducted during the September 2 2 cruise indicated these samples to be Fe-replete (pFe < 2 .6). Although this suggests that the dominant pool of bioavailable Fe is contained in the particulate fraction, we cannot discount the possibility that the bioreporter was rendered Fe replete by Fe regenerated predominantly from grazed bioreporter cells themselves. Thus, while it is clear that regenerative processes contribute to the pool of bioavailable Fe, it is equally clear that future efforts using the bioreporter with unfiltered water samples must account for the potential influence of biogenic Fe added with the reporter cells.
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