Persistence of iron(II) in surface waters of the western subarctic Pacific (original) (raw)
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Controlling iron availability to phytoplankton in iron-replete coastal waters
Marine Chemistry, 2004
Recent work demonstrates that the micronutrient iron may strongly influence the magnitude and character of algal production in nearshore waters due in part to the higher but variable iron requirements of neritic phytoplankton. However, ascertaining the direct effects of iron nutrition in coastal waters has been forestalled by our inability to experimentally regulate ambient iron availability independent of other factors. We present here results from size-fractionated iron uptake experiments showing that increasing concentrations of the siderophore Desferriferrioxime B (DFB) progressively decreases the biological availability of iron tracer added to natural seawater. These findings extend those of previous studies showing that high concentrations of DFB induce iron limitation of phytoplankton in coastal waters. Similar tests with two siderophores (P1P and PCC7002 No. 1) isolated from marine prokaryotes showed little or no impact on short-term iron uptake in these natural population cultures. DFB additions did not influence the short-term uptake of carbon indicating that its inhibitory effect was not due to general toxicity to the cells. Uptake rates of iron tracer in the large (>5.0 Am) phytoplankton fraction decreased linearly with increasing DFB concentrations, becoming undetectable at z 3 nM DFB, or f 5 Â over ambient dissolved iron concentrations. The decrease in iron availability with DFB addition was equally dramatic for the ultraplankton (0.2-5.0 Am), but in this case low-level tracer uptake (f 10%) persisted even at high DFB concentrations (3-500 nM). Our experimental findings are combined with a preliminary kinetic model to suggest that iron equilibration among the natural ligand classes (L 1 , L 2) and DFB may require an adjunctive (or associative) ligand substitution mechanism to explain the very rapid effect that DFB exerts on iron uptake when added to seawater. Even so, several hours to days likely are needed for the equilibration of added iron among DFB and natural ligands when low-level (e.g., 0.5 nM) DFB concentrations are employed. Model results provide indirect support to earlier suggestions that large eukaryotic phytoplankton extract iron from the weaker class of natural ligands (Fe(III)L 2). The combination of iron enrichment and DFB amendments provides a practical means for studying how iron influences algal production, carbon cycling and phytoplankton species composition in nearshore waters.
Aquatic Microbial Ecology, 2004
Studies in high nutrient, low chlorophyll (HNLC) regions have demonstrated that increased Fe availability results in an increase in phytoplankton biomass and changes in community composition. Here we present experiments in which the availability of iron (Fe) was increased or reduced to monitor the response of individual groups of phytoplankton (large eukaryotes, picoeukaryotes and cyanobacteria) by flow cytometry. Additions (0.5 to 5.0 nM Fe) and reductions in available Fe (through addition of 1 to 10 nM of the fungal siderophore desferrioxamine B) were made to enclosed communities from the South American eastern boundary current off Peru, where ambient dissolved Fe concentrations were <100 pM. As predicted, chlorophyll concentrations increased in the added Fe treatments relative to the control, indicative of Fe limitation. Flow cytometry demonstrated that this was due to increases in the abundance of large eukaryotes that are Fe-starved under ambient conditions. Cyanobacterial abundance increased and decreased linearly with Fe availability, suggesting that cyanobacteria were Fe-limited but not Fe-starved. In contrast, picoeukaryote cell abundance increased with decreasing Fe availability, although chlorophyll cell -1 in this group responded in an inverse manner. The results demonstrate that members of the marine phytoplankton community respond differently to Fe availability, which may influence the outcome of biological competition among organisms in Fe-limited environments.
Global Biogeochemical Cycles, 2005
1] 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 highnitrate, 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 (F v /F m < 0.25, Ferredoxin Index < 0.2). During FeCycle, picophytoplankton (0.2-2 mm) and nanophytoplankton (2-20 mm) each contributed >40% of total chlorophyll. Whereas the picophytoplankton accounted for 5050% of total primary production, they were responsible for the majority of community iron uptake in the mixed layer. Thus ratios of 55 Fe: 14 C uptake were highest for picophytoplankton (median: 17 mmol:mol) and declined to 505 mmol: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 , 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.
Iron bioavailability to phytoplankton: an empirical approach
The ISME Journal, 2014
Phytoplankton are often limited by iron in aquatic environments. Here we examine Fe bioavailability to phytoplankton by analyzing iron uptake from various Fe substrates by several species of phytoplankton grown under conditions of Fe limitation and comparing the measured uptake rate constants (Fe uptake rate/ substrate concentration). When unchelated iron, Fe 0 , buffered by an excess of the chelating agent EDTA is used as the Fe substrate, the uptake rate constants of all the eukaryotic phytoplankton species are tightly correlated and proportional to their respective surface areas (S.A.). The same is true when FeDFB is the substrate, but the corresponding uptake constants are one thousand times smaller than for Fe 0 . The uptake rate constants for the other substrates we examined fall mostly between the values for Fe 0 and FeDFB for the same S.A. These two model substrates thus empirically define a bioavailability envelope with Fe 0 at the upper and FeDFB at the lower limit of iron bioavailability. This envelope provides a convenient framework to compare the relative bioavailabilities of various Fe substrates to eukaryotic phytoplankton and the Fe uptake abilities of different phytoplankton species. Compared with eukaryotic species, cyanobacteria have similar uptake constants for Fe 0 but lower ones for FeDFB. The unique relationship between the uptake rate constants and the S.A. of phytoplankton species suggests that the uptake rate constant of Fe-limited phytoplankton has reached a universal upper limit and provides insight into the underlying uptake mechanism.
Photosynthesis Research, 1994
Iron supply has been suggested to influence phytoplankton biomass, growth rate and species composition, as well as primary productivity in both high and low NO 3 surface waters. Recent investigations in the equatorial Pacific suggest that no single factor regulates primary productivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composition and growth rates. One goal of biological oceanography is to isolate the effects of single factors from this multiplicity of interactions, and to identify the factors with a disproportionate impact. Unfortunately, our tools, with several notable exceptions, have been largely inadequate to the task. In particular, the standard technique of nutrient addition bioassays cannot be undertaken without introducing artifacts. These so-called 'bottle effects' include reducing turbulence, isolating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and thus removing it from vertical movement through a light gradient, and exposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient additions from uncontrolled changes in other environmental and ecological factors. To overcome these limitations, oceanographers have sought physiological or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficiency of these catalysts. Reductions in photosynthetic efficiency often accompany nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolecular assemblages. Many catalysts involved in electron transfer and reductive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredoxin or cytochrome f content, nitrate assimilation rates, and dinitrogen fixation rates are amongst the diagnostics that have been used to infer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases in response to iron-limitation. These include cell surface iron-transport proteins, and the electron transfer protein flavodoxin which replaces the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobacteria.