During austral summer 2003, we tracked a patch of surface water infused with the tracer sulfur hexafluoride, but without addition of Fe, through subantarctic waters over 10 days in order to characterize and quantify algal Fe pools and fluxes to construct a detailed biogeochemical budget. Nutrient profiles characterized this patch as a high‐nitrate, low‐silicic acid, low‐chlorophyll (HNLSiLC) water mass deficient in dissolved Fe. The low Fe condition was confirmed by several approaches: shipboard iron enrichment experiments and physiological indices of Fe deficiency (Fv/Fm < 0.25, Ferredoxin Index < 0.2). During FeCycle, picophytoplankton (0.2–2 μm) and nanophytoplankton (2–20 μm) each contributed >40% of total chlorophyll. Whereas the picophytoplankton accounted for ∼50% of total primary production, they were responsible for the majority of community iron uptake in the mixed layer. Thus ratios of 55Fe:14C uptake were highest for picophytoplankton (median: 17 μmol:mol) and declined to ∼5 μmol:mol for the larger algal size fractions. A pelagic Fe budget revealed that picophytoplankton were the largest pool of algal Fe (>90%), which was consistent with the high (∼80%) phytoplankton Fe demand attributed to them. However, Fe regenerated by herbivory satisfied only ∼20% of total algal Fe demand. This iron regeneration term increased to 40% of algal Fe demand when we include Fe recycled by bacterivory. As recycled, rather than new, iron dominated the pelagic iron budget (Boyd et al., 2005), it is highly unlikely that the supply of new Fe would redress the imbalance between algal Fe demand and supply. Reasons for this imbalance may include the overestimation of algal iron uptake from radiotracer techniques, or a lack of consideration of other iron regeneration processes. In conclusion, it seems that algal Fe uptake cannot be supported solely by the recycling of algal iron, and may require an Fe “subsidy” from that regenerated by heterotrophic pathways.

Three unicellular cyanobacterial strains (PCC 7425, PCC 8303, PCC 9308) assigned to the genus Cyanothece Komárek 1976, which showed an unusually high content of light refractile inclusions when viewed by phase-contrast microscopy, were characterized by confocal laser scanning microscopy and transmission electron microscopy. All strains had concentric cortical thylakoids and a compact central nucleoid. Frequently, the two innermost thylakoid membranes protruded to form circular enclosures containing cytoplasm or electron-transparent granules, or both. The largest granules were partially immersed in the nucleoid region, but they remained attached to the inner cortical thylakoids by a single narrow connection. The pattern of binary cell division in strain PCC 7425 was different than that in strains PCC 8303 and PCC 9308. In the former, all cell wall layers invaginated simultaneously, whereas in the latter the invagination of the outer membrane was delayed compared to that of the cytoplasmic membrane and the peptidoglycan layer. Thus, prior to completion of cell division, the new daughter cells of strains PCC 8303 and PCC 9308 were transiently connected by a thick septum, which was not observed in strain PCC 7425. Nucleoid partitioning coincided with initiation of cell division in all three strains and was unlike that reported in other bacteria and in archaea, in which separation of the nucleoids precedes cell division. Based on the common morphological and ultrastructural features, the three strains of Cyanothece examined constitute a distinct cluster, which might deserve independent generic status.

The complex chemical speciation of Fe in aquatic systems and the uncertainties associated with biological assimilation of Fe species make it difficult to assess the bioavailability of Fe to phytoplankton in relation to total dissolved Fe concentrations in natural waters. We developed a cyanobacterial Fe‐responsive bioreporter constructed in Synechococcus sp. strain PCC 7942 by fusing the Fe‐responsive isiAB promoter to Vibrio harveyi luxAB reporter genes. A comprehensive physiological characterization of the bioreporter has been made in defined Fraquil medium at free ferric ion concentrations ranging from pFe 21.6 to pFe 19.5. Whereas growth and physiological parameters are largely constrained over this range of Fe bioavailability, the bioreporter elicits a luminescent signal that varies in response to Fe deficiency. A dose‐response characterization of bioreporter luminescence made over this range of Fe3+ bioavailability demonstrates a sigmoidal response with a dynamic linear range extending between pFe 21.1 and pFe 20.6. The applicability of using this Fe bioreporter to assess Fe availability in the natural environment has been tested using water samples from Lake Huron (Laurentian Great Lakes). Parallel assessment of dissolved Fe and bioreporter response from these samples reinforces the idea that measures of dissolved Fe should not be considered alone when assessing Fe availability to phytoplankton communities.

No abstract

There is increasing recognition that iron distribution and availability is significant in terms of global oceanic production. Low availability of iron and other nutritive trace metals may also constrain productivity in the North American Great Lakes. Despite its importance, the biogeochemistry of iron in the water column of lacustrine systems remains poorly characterized. In addressing the current state of iron biogeochemistry, a workshop organized a decade ago at the Bermuda Biological Station for Research brought together a cross-disciplinary team of chemists and biologists who sought to synthesize current knowledge and identify research priorities in this field. Key among goals identified during the workshop, and one that remains today for the most part unfulfilled, was to ‘develop techniques to quantify those fractions of Fe that are accessible to phytoplankton.’ Here we review recent progress toward meeting this objective, drawing on specific examples from Lake Superior where these approaches have been applied.