Determination of Bioavailable Fe in Lake Erie Using a Luminescent Cyanobacterial Bioreporter

Low Fe bioavailability 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 2001-02. Bioreporter luminescence was derived from a luciferase reporter controlled by iron-responsive promoter element isiAB. Filtered (< 0.2 μm) water sampled from the western basin during summer 2001-02 yielded low bioreporter response indicating Fe sufficient conditions [-log [free Fe3+] (pFe) < 20.8]. Likewise, water collected from the eastern basin following autumnal mixing in November 2001 yielded a Fe sufficient bioreporter response. In contrast, surface water collected at pelagic stations located in central and eastern basins during summer 2002 indicated a seasonal depletion of bioavailable Fe. Whereas water sampled from these locations during July and August was characterized as Fe sufficient (pFe < 20.8), 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 2002 cruise indicated these samples to be Fe sufficient (pFe < 20.6). 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 sufficient by Fe regenerated predominantly from 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 Fe introduced by the added reporter cells.