Susan A. Huntsman

Iron uptake and growth limitation in oceanic and coastal phytoplankton

Iron concentrations in open ocean are orders of magnitude lower than levels in coastal waters. Experiments with coastal and oceanic phytoplankton clones representing different algal groups and cell sizes indicate that cellular iron uptake rates are similar among the species when rates are normalized to cell surface area. This similarity in rates apparently is explained by evolutionary pressures that have pushed iron uptake in all species toward the maximum limits imposed by diffusion and ligand exchange kinetics. Because of these physical/chemical limits on uptake, oceanic species have been forced to decrease their cell size and/or to reduce their growth requirements for cellular iron by up to 8-fold. The biochemical mechanisms responsible for this reduction in metabolic requirements are unknown.

Interrelated influence of iron, light and cell size on marine phytoplankton growth

The sub-optimal growth of phytoplankton and the resulting persistence of unutilized plant nutrients (nitrate and phosphate) in the surface waters of certain ocean regions has been a long-standing puzzle,. Of these regions, the Southern Ocean seems to play the greatest role in the global carbon cycle,, but controversy exists as to the dominant controls on net algal production. Limitation by iron deficiency,, light availability,, and grazing by zooplankton have been proposed. Here we present the results from culture experiments showing that the amount of cellular iron needed to support growth is higher under lower light intensities, owing to a greater requirement for photosynthetic iron-based redox proteins by low-light acclimatized algae. Moreover, algal iron uptake varies with cell surface area, such that the growth of small cells is favoured under iron limitation, as predicted theoretically. Phytoplankton growth can therefore be simultaneously limited by the availability of both iron and light. Such a co-limitation may be experienced by phytoplankton in iron-poor regions in which the surface mixed layer extends below the euphotic zone—as often occurs in the Southern Ocean,—or near the bottom of the euphotic zone in more stratified waters. By favouring the growth of smaller cells, iron/light co-limitation should increase grazing by microzooplankton, and thus minimize the loss of fixed carbon and nitrogen from surface waters in settling particles,.

Processes regulating cellular metal accumulation and physiological effects: Phytoplankton as model systems

Trace metals exist in a variety of redox states and coordination species which markedly influences their geochemical behavior and biological availability. Most exist in natural waters as metal cations that are complexed to varying degrees by inorganic and organic ligands. Metal ions are generally taken up into cells by membrane transport proteins designed for acquisition of nutrient metals (Mg, Fe, Mn, Zn, Co, Cu, Mo). Organic ligands compete with these transport proteins for binding metal ions, and consequently, organic complexation substantially decreases metal uptake rates. Typically, the chelated metal is not directly available for cellular uptake, and metal uptake is controlled by either the concentration of free aquo ions or that of kinetically labile inorganic species. The coordination sites of transport proteins, however, are not entirely specific for intended nutrient metals, and consequently will bind with and transport non-nutritive or toxic metals. Competition for membrane transport sites and for intracellular metabolic binding sites can substantially influence the uptake of both nutrient and toxic metals and resultant effects on growth rate. Such growth rate effects provide feedback on cellular metal concentrations since the amount of metal accumulated within cells represents a balance between the rate of metal uptake and the cellular growth rate, the effective biodilution rate. Phytoplankton have provided useful model systems for investigating the processes and associated chemical and biological factors regulating cellular metal accumulation and resultant physiological effects.

Effect of sunlight on redox cycles of manganese in the southwestern Sargasso Sea

The cycling of manganese between soluble Mn(II) and particulate manganese oxides was investigated in the upper 750 m at a station in the southwestern Sargasso Sea. Dissolved manganese was present at a maximum concentration (4.3 ± 0.6 nM) in the surface mixed layer (0–40 m) and decreased to 0.67 ± 0.19 nM at depths of 400–750 m. Particulate manganese, on the other hand, occurred at a minimum concentration (0.034 ± 0.012 nM) in the mixed layer and increased to a maximum of 0.41–0.48 nM at depths of 120–250 m. All of the increase in particulate manganese with depth occurred within the fraction that could be reductively dissolved by 0.3 mM ascorbic acid at ambient seawater pH, indicating that the rise resulted from an increase in the concentration of manganese oxides. Oxides were undetectable in the mixed layer, but a mean of 94% of the particulate manganese appeared to be associated with oxides at depths of 80–250 m. Radiotracer (54MnCl2) measurements of particulate formation rates and concomitant steady-state calculations of particulate turnover rates indicate that the low near-surface concentrations of particulate manganese resulted from both low formation rates and high turnover rates of particulate manganese. The formation of particulate manganese was sharply inhibited by sunlight, consistent with photoinhibition of manganese oxidizing microorganisms. Likewise, sunlight caused a 12-fold increase in the dissolution rate of 54Mn-labeled particulate manganese, which we attribute to photoreduction of manganese oxides. Both photo-effects appear to be important in sharply reducing the concentration of particulate manganese in near-surface seawater, and thereby minimizing the removal of manganese via particulate sinking. The resulting reduction in removal rates should be a major factor contributing to the surface maximum in manganese concentrations, which is a prominent feature of manganese profiles in much of the worlds ocean.

Photoreduction of manganese oxides in seawater

Experiments were conducted on the photoreductive dissolution of 54Mn-labeled synthetic oxides, prepared from MnO42− oxidation of 54Mn(II), and natural labeled oxides formed in seawater from microbial oxidation of 54Mn(II). Sunlight increased the dissolution rate of synthetic oxides in seawater, an effect that increased with the duration of light exposure. The photodissolution of these oxides was found to result primarily from Mn reduction by H2O2, produced in seawater from the photoreduction of O2 by dissolved organic matter. This conclusion was based on the previously observed marked stimulation of photodissolution by added humic compounds, the observed reductive dissolution of the oxides by added H2O2 and on the almost complete reversal of photodissolution by enzymatic (catalase) removal of H2O2. Sunlight had an even larger stimulatory effect on the reductive dissolution of 54Mn-labeled natural oxides. It increased specific dissolution rates to values of 6–13% h−1, 6–70 times higher than rates in the dark. In contrast to synthetic oxides, rates for natural oxides did not increase measurably with the duration of light exposure, were not appreciably altered by humic acid addition or by photolytic removal of natural organic matter, and were not substantially reduced by catalase addition. Furthermore, rates for reductive dissolution of natural oxides by H2O2 were only about 1/6th of those for synthetic oxides. These results indicate that the photoreductive dissolution of natural oxides in seawater is not primarily related to the photoproduction of H2O2, although such production appears to account for a small portion (ca. 10–20%) of the overall effect. Instead, both the chromophore and the reductant(s) involved in the reaction appear to reside with the bacterial/Mn oxide aggregates themselves. Although several possibilities can be postulated, the exact mechanism of the photochemical reaction remains obscure.

The use of chemiluminescence and ligand competition with EDTA to measure copper concentration and speciation in seawater

A batch chemiluminescence method for measurement of copper in seawater was developed from an existing flow injection technique. The method involved the reaction of copper in the sample with 1,10-phenanthroline, and subsequent release of photons from the oxidation of copper(II)-phenan-throline chelates by H2O2 at alkaline pH. The method was extremely sensitive with a detection limit in the range of 0.05–for 200 μl samples. Because of its sensitivity, it could be used to measure copper directly in seawater without a preconcentration step. To measure total copper, samples had to be acidified to pH 2 and stored for weeks to months (and in some cases also photo-oxidized by UV light) to release copper from kinetically inert species. Total copper concentrations measured by the chemiluminescence method in natural seawater agreed well with those obtained by more traditional methods. The chemiluminescence method was sensitive to copper speciation due to the slow dissociation of many bound copper species. This property was utilized to develop a ligand competition technique for measurement of free cupric ion concentrations. The method involved additions of EDTA and copper to seawater and was based on the lack of detectability of CuEDTA chelates by the chemiluminescence method. Free cupric ion concentrations measured by this method ranged from 10−12.9 M for a relatively clean estuarine sample containing 5 nM total dissolved copper to 10−10.9 M for a sample from a polluted harbor which contained 38 nM copper. Cupric ion concentrations measured by the new method were in good agreement with those measured in the same samples by a previously published method: EDTA competition combined with copper adsorption on to C18 Sep—Pak cartridges. Copper was found to be heavily (>99%) complexed by natural organic ligands in both shelf and estuarine water samples and the concentrations of these ligands generally increased with dissolved copper concentration and decreasing sample salinity. These results agree with those of several previous studies.