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1.
腐殖质是地表普遍存在的天然有机物,对海洋中重要的微量营养元素-铁（Fe）的分布及生物地球化学循环具有重要的影响作用。本文对腐殖质的来源、分布及对海水中溶解态铁的迁移转化的影响做了总结,特别论述了其在河口及沿岸水域的行为。大量研究表明河口、沿岸及开放海水中溶解态铁分布的变化可以用腐殖质的浓度及其铁结合能力的变化来解释。腐殖质的络合作用不仅能够阻止溶解态铁（DFe）在河口、沿岸等水域被去除,而且能够通过洋流将DFe迁移至外海及大洋区域,此外还能增加铁的溶解度及对海水中浮游植物的生物可利用性,并且促进铁的氧化还原循环。研究还发现两者之间的络合强度受到盐度、pH等理化因素的影响。盐度是影响HS与DFe配合能力的重要影响因素,盐度增加,导致HS中可以与Fe配合的位点数量降低,配合总量呈现指数降低,而pH的增加可以增加HS与DFe的配合量。另外HS还能影响海水中DFe的氧化还原,并以此影响浮游植物对DFe的吸收利用。因此腐殖质对溶解态铁的有机络合作用是影响其海洋生物地球化学循环的一个重要参数,进一步研究海水中腐殖质的浓度和分布具有重要的意义。  相似文献   

2.
Evidence for organic complexation of iron in seawater   总被引：1，自引：0，他引：1  
Iron occurs at very low concentrations in seawater of oceanic origin and its low abundance is thought to limit primary production in offshore waters (Martin and Fitzwater, 1988). A new electrochemical method, cathodic stripping voltammetry (CSV), is used here to determine the speciation of iron in seawater originating from the Western Mediterranean taking advantage of ligand competition of an added electroactive ligand with the natural organic complexing matter to evaluate whether iron is organically complexed. The measurements indicate that iron occurs 99% (or 99.9% depending on which value is selected for αFe′) complexed by organic complexing ligands throughout the water column of the Western Mediterranean and by analogy probably also in other oceanic waters. The composition of the organic complexing ligands is as yet unknown, but the data indicate a major source from microorganisms (bacteria or phytoplankton) in and immediately below the fluorescence maximum in the upper water column. The organic complexes are apparently reversible releasing iron when the competing ligand is added and binding more iron when its concentration is increased. The organic complexing ligands occur at concentrations well above those of iron ensuring full complexation of this biologically essential element, and buffer the free iron concentration at a very low level against fluctuations as a result of removal by primary producers or inputs from atmospheric sources. The new data indicate that a re-evaluation of the concept of the bioavailable fraction of iron is required.  相似文献   

3.
海水中铁的几种形态对海生小球藻生长的影响   总被引：2，自引：0，他引：2  
通过实验室一次性培养 ,研究不同形态的铁 :Fe3 + ,Fe2 + ,Fe3 + - EDTA,Fe2 + - EDTA以及胶体水合氧化铁在不同条件下对海生小球藻 (Chlorella vulgaris)生长的影响。实验结果表明 ,铁对小球藻生长影响的主要生物活性形式是有机络合态铁和胶体水合氧化铁 ,光诱导还原态 Fe2 + 也很重要。它们都积极促进了藻类的生长繁殖 ,其中 EDTA与铁离子结合的动力学过程并不明显影响藻类吸收铁。小球藻相对生长率的提高和叶绿素 A(Chl.a)的增加取决于铁的总量及其转变为可利用状态的速率 *和藻类的有效吸收。在不同的培养体系中 ,常量营养盐 N、P与 Fe的缺乏对藻类生长起着交互限制作用 ,其中缺 P比缺 N更能减少 EDTA络合铁对小球藻生长繁殖的促进作用。  相似文献   

4.
Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.  相似文献   

5.
The distribution of dissolved iron and its chemical speciation (organic complexation and redox speciation) were studied in the northeastern Atlantic Ocean along 23°W between 37 and 42°N at depths between 0 and 2000 m, and in the upper-water column (upper 200 m) at two stations further east at 45°N10°W and 40°N17°W in the early spring of 1998. The iron speciation data are here combined with phytoplankton data to suggest cyanobacteria as a possible source for the iron binding ligands. The organic Fe-binding ligand concentrations were greater than that of dissolved iron by a factor of 1.5–5, thus maintaining iron in solution at levels well above it solubility. The water column distribution of the organic ligand indicates in-situ production of organic ligands by the plankton (consisting mainly of the cyanobacteria Synechococcus sp.) in the euphotic layer and a remineralisation from sinking biogenic particles in deeper waters. Fe(II) concentrations varied from below the detection limit (<0.1 nM) up to 0.55 nM but represented only a minor fraction of 0% to occasionally 35% of the dissolved iron throughout the water column. The water column distribution of the Fe(II) suggests biologically mediated production in the deep waters and photochemical production in the euphotic layer. Although there was no evidence of iron limitation in these waters, the aeolian iron input probably contributed to a shift in the phytoplankton assemblage towards increased Synechococcus growth.  相似文献   

6.
The complexation of dissolved Ni has been evaluated in a rapidly-flushed, rural estuary (Tweed, UK) by ligand exchange-adsorptive cathodic stripping voltammetry. Results suggest the presence of strongly binding ligands, L, throughout, with average stability constants of about 1019 and which are saturated by ambient Ni concentrations. Equilibrium speciation calculations incorporating these constants in WHAM, version 6, predict an increase in Ni complexation (as NiL) from about 50% of total dissolved Ni in fresh water to over 90% in sea water. Equivalent calculations using the default-mode fulvic and humic substances (FS and HS, respectively) encoded in the WHAM database predict a reduction in complexation (as NiFS + NiHS) from about 20% in fresh water to less than 1% in sea water. Discrepancies arising from the two approaches are largely attributed to the different analytical detection windows employed. Thus, a better representation of Ni complexation is derived from including both types of complexant in the speciation calculations, resulting in estimates of net complexation in excess of 60% of total dissolved Ni throughout the estuary. The uncertainties and assumptions inherent in all computations illustrate the difficulty in measuring or predicting metal complexation in estuaries.  相似文献   

7.
The ubiquitous algal species, Emiliania huxleyi, was incubated in sea water supplemented only with nitrate and phosphate (N and P) without chelating agents to control metal speciation. Growth was slow in a “low-iron” culture containing 1.3 nM iron and was found to be iron-limited, growth-accelerating when a 1-nM iron addition was made. The growth rate in a “high-iron” culture (5.4 nM iron) was greater, reaching 0.4 div day−1 but this culture too was found to have become iron-limited when a 9-nM iron addition was made on day 17 of the incubation. Both cultures were found to release iron-complexing ligands in excess of the iron concentration, 6 nM in the low-iron culture, and 10 nM in the high-iron culture. More ligands were produced after the iron addition taking the ligand concentration to 11 nM in the low-iron culture. The data show that the ligands are released in response to the iron addition, when at least some of the iron had already been taken up. This type of release is contrary to the concept of a siderophore, which is supposed to be released in periods of lack of iron; however the increase in the ligand concentration is similar to that released by the natural community in response to the iron addition in the IRON-EX II experiment [Rue, E.L., Bruland, K.W., 1997. The role of organic complexation on ambient iron chemistry in the equatorial Pacific Ocean and the response of a mesoscale iron addition experiment. Limnol. Oceanogr. 42, 901–910]. The enhanced growth in the cultures when more iron was added indicated that the organically complexed iron present in the cultures was not immediately available to the organisms (or at least not at sufficiently high rate), and that the organisms responded to freshly added, inorganic, iron.  相似文献   

8.
As part of a larger program focused on understanding the biogeochemistry of large river plumes, we participated in two expeditions during 2000 to sample the Mississippi River plume. Surface water samples were collected using a trace metal clean towed fish and analyzed for total dissolved Fe, organic Fe complexing ligands and their associated conditional stability constants. The ligands in the river plume have conditional stability constants (log K′FeL between 10.5 and 12.3 with an average of 11.2 and standard deviation of 0.6) very similar to ligands found in the open ocean. Comparison of high flow and low flow regimes indicates that variability in flow may be the main cause of the variability in Fe concentrations in the plume. The organic Fe complexing ligands are in greatest excess during a time of higher flow. These ligands are responsible for maintaining very high (5 nM) Fe concentrations throughout the plume. Due to complexation with these organic ligands, the concentration of Fe remains above the Fe-hydroxide solubility level until a salinity above 35 is reached where there appears to be a sink for Fe in the less productive waters. Therefore, Fe is transported a great distance from the river source and is available for biological utilization in the coastal zone.  相似文献   

9.
Recent electrochemical measurements have shown that iron (Fe) speciation in seawater is dominated by complexation with strong organic ligands throughout the water column and have provided important thermodynamic information about these compounds. Independent work has shown that iron exists in both soluble and colloidal fractions in the Atlantic Ocean. Here we have combined these approaches in samples collected from a variety of regimes within the Atlantic Ocean. We measured the partitioning of Fe between soluble (< 0.02 μm) and colloidal (0.02 to 0.4 μm) size classes and characterized the concentrations and conditional stability constants of Fe ligands within these size classes. Results suggest that equilibrium partitioning of Fe between soluble and colloidal ligands is partially responsible for the distribution of Fe between soluble and colloidal size classes. However, a significant fraction of the colloidal Fe was inert to ligand exchange as soluble Fe concentrations were generally lower than values predicted by a simple equilibrium partitioning model.In surface waters, strong ligands with conditional stability constants of 1013 relative to total inorganic Fe appeared to dominate speciation in both the soluble and colloidal fractions. In deep waters these ligands were absent, and instead we found ligands with stability constants 12–15 fold smaller that were predominantly in the soluble pool. Nevertheless, significant levels of colloidal Fe were found in these samples, which we inferred must be inert to coordination exchange.  相似文献   

10.
The redox speciation of dissolved iron in seawater was evaluated at 121 locations in the Pacific Ocean at depths of 15-1000 m, using the method of luminol chemiluminescence. The results indicate that reduced iron, Fe(II), is ubiquitous in surface seawater with a relatively consistent pattern of occurrence. Surface maxima were present in most profiles, with median concentrations of 25-30 pM representing 12-14% of the total dissolved iron. Concentrations decreased monotonically with depth to<12 pM within the upper euphotic zone. This pattern was observed during both day and nighttime sampling events, which suggests that non-photochemical production mechanisms can produce photochemical-like signatures. Further, if theoretical rates of Fe(II) oxidation are applicable to the open ocean, then the employed sampling methods precluded assessment of photochemically-produced Fe(II), regardless of ambient light conditions. For this and other reasons, the concentrations reported here for the upper water column likely represent lower limits of labile iron concentration, and suggest that dissolved iron may be more available for uptake than previously believed. Deeper in the water column, Fe(II) was also frequently detected, though it constituted a small fraction of the total dissolved iron. Possible source mechanisms at these depths include thermal (dark) reduction of Fe(III) organic complexes or remineralization of sinking biogenic particles containing Fe(II). In the northern Philippine Sea between the Japanese coast and the Izu-Bonin volcanic arc system, Fe(II) concentrations were found to be atypically high, possibly because of high atmospheric dust deposition near the surface and transport of sediment-derived iron at depth.  相似文献   

11.
Measurements of zinc and zinc complexation by natural organic ligands in the northeastern part of the Atlantic Ocean were made using cathodic stripping voltammetry with ligand competition. Total zinc concentrations ranged from 0.3 nM in surface waters to 2 nM at 2000 m for open-ocean waters, whilst nearer the English coast, zinc concentrations reached 1.5 nM in the upper water column. In open-ocean waters zinc speciation was dominated by complexation to a natural organic ligand with conditional stability constant (log KZnL′) ranging between 10.0 and 10.5 and with ligand concentrations ranging between 0.4 and 2.5 nM. The ligand was found to be uniformly distributed throughout the water column even though zinc concentrations increased with depth. Organic ligand concentrations measured in this study are similar to those published for the North Pacific. However the log KZnL′ values for the North Atlantic are almost and order of magnitude lower than those reported by Bruland [Bruland, K.W., 1989. Complexation of zinc by natural organic-ligands in the central North Pacific. Limnol. Oceanogr., 34, 269–285.] using anodic stripping voltammetry for the North Pacific. Free zinc ion concentrations were low in open-ocean waters (6–20 pM) but are not low enough to limit growth of a typical oceanic species of phytoplankton.  相似文献   

12.
《Marine Chemistry》2001,76(3):175-187
Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms, including phytoplankton, which are responsible for the primary productivity in the world's oceans. However, due to the low solubility of Fe in seawater, phytoplankton are often limited by their inability to incorporate enough Fe to allow for optimal growth rates in regions with dissolved Fe concentrations below 1 nM. It has been postulated that certain phytoplankton may produce compounds to facilitate the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (>99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, origin, and chemical characteristics of these organic chelates are largely unknown. Although it has been implied that some components of natural Fe-binding ligands are siderophores, no direct analyses of such compounds from natural seawater have been conducted. Here, we present a simple solid-phase extraction technique employing Biobeads SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from large volumes (>200 l) of seawater. Additionally, we report on the first successful determination of molecular weight size classes and preliminary iron-binding functional group characterization within those size classes for isolates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affinities (with respect to inorganic iron—Fe′) of KFeL,Fe′cond=1011.5–1011.9 M−1, similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In addition, 63% of the extracted compounds from surface-collected samples fall within the defined size range of siderophores (300–1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophores extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-binding functional groups consistent with biologically produced siderophores. These results provide further insight into characteristics of the Fe-binding ligands that are thought to be important in controlling the biological availability of Fe in the oceans.  相似文献   

13.
Natural colloids are abundant in seawater and are an intermediary in the fate, transport and bioavailability of many trace elements. Knowledge of the pathways and mechanisms of the biological uptake of colloidal Fe and other Fe species is of paramount importance in understanding Fe limitation on marine phytoplankton and thus carbon sequestration in the ocean. Whether the natural colloids serve as a source for the biological Fe requirements of marine phytoplankton, or just as a sink for particle-reactive metals in the oceans remains largely unknown. This study examined the bioavailability of Fe bound with colloids from different regions to a coastal diatom (Thalassiosira pseudonana). Natural colloids were isolated by cross-flow ultrafiltration and radiolabeled with 59Fe before being exposed to phytoplankton. Control experiments were conducted to ensure that 59Fe radiolabeled onto the colloids remained mostly in the colloidal phase. Both the natural oceanic and coastal colloidal organic matter complexed Fe (1 nm–0.2 μm) can be biologically available to the marine diatom even though its uptake was lower than the low molecular weight counterparts. By comparing the measured Fe internalization fluxes and the calculated maximum diffusive uptake fluxes, it is evident that ligand exchange kinetics on the cell surface may control the internalization of macromolecular Fe. The calculated concentration factors under dark and light conditions were generally comparable. Colloidal Fe, as an important intermediary phase, can be actively involved in the planktonic food web transfer through biological uptake and regeneration processes. The bioavailable fraction of Fe may be substantially underestimated by only considering the truly dissolved Fe or overestimated when using the external fluxes, such as aerosol Fe, as the bioavailable fraction.  相似文献   

14.
南沙群岛海域表层沉积物中有机物、铁和锰的分布特征   总被引：1，自引：0，他引：1  
通过1997年11月（冬季）和1999年7月（夏季）两个航次对南沙群岛海域的现场调查，实测了南沙深海盆表层沉积物中的有机物，Fe和Mn的含量，讨论了沉积物中Fe、Mn的平面和深度分布。在沉积物的上层几厘米处Fe和Mn都出现了峰值，这是上层Mn^2 （Fe^2 ）氧化，再沉淀引起的，沉积物中Fe和Mn的深度分布是氧化锰（铁）和氢氧化锰（铁）的还原，扩散和再沉淀的结果，细菌在海洋环境的Fe、Mn循环中起着重要的作用，在大洋底的厌氧环境中细菌将Fe、Mn还原为低价离子或可溶性化合物向间隙水和上覆水移动，在沉积物表层的氧化条件下细菌又使环境中的Fe、Mn沉淀，使其再次富集。  相似文献   

15.
使用阴极溶出伏安法,利用2,3-二羟基苯丙氨酸(DHN)可以与Fe(Ⅲ)结合生成配合物,而BrO3-的加入可以催化该电化学反应的性质,系统研究了海水中溶解态Fe(Ⅲ)的最佳分析条件。结果表明,体系中添加20.0μmol/L DHN即可达到分析要求;添加BrO3-可以使溶出电流线性增大,选择的最终浓度为20.0mmol/L。当沉积电位为-0.20V,扫描速率为50.0mV/s,沉积时间为90s时,即可达到低铁海水的分析要求,在此条件下检测限为0.011nmol/L。紫外消解可以使测量灵敏度比未消解时提高13倍。用此方法测量得到的太平洋某处(158°15′E,22°23′N)海水表层水浓度为0.45nmol/L,75m处浓度为0.14nmol/L,1 500m处浓度为0.86nmol/L。  相似文献   

16.
Concentrations of organic matter, iron and manganese in the deep sea surface sediments in the Nansha Islands sea area, South China Sea are measured, Horizontal and vertical distributions of iron and manganese are discussed. The vertical distribution of iron and manganese in the sediments results from reduction, diffusion, and redeposition of manganese (or iron) oxide and hydroxide in the sediment. There are the maxima of iron and manganese in solid phase in the top of the sediment, which is caused by the penetration of O2 and the upward flux of Mn^2 ( or Fe^2 ). Manganese bacteria play a very important role in the cycle of solid-phase iron and manganese in the ocean environment. Manganese bacteria oxidize Mn^2 ( or Fe^2 ) in dissolved state to Mn^4 ( or Fe^3 ) in oxidized state under the aerobic condition, whereas they reduce iron and manganese in anaerobic conditions.  相似文献   

17.
Calibration of a chalcogenide glass membrane, Fe(III)ISE [Fe2.5(Ge28Sb12Se60)97.5], in buffered saline media has been undertaken in order to assess the suitability of this ISE for seawater analyses. The electrode slopes in saline citrate and salicylate buffers were 26.3 and 28.2 mV/decade, respectively, for Fe3+ concentrations ranging from 10−10 M to less than 10−25 M Fe3+. The calibration lines in the citrate and salicylate buffers were essentially collinear with the response in unbuffered chloride-free standards containing >10−5 M Fe3+, demonstrating that the response of the FeISE is unaffected by chloride ions. A mechanism involving a combination of charge transfer and ion-exchange of Fe(III), at the electrode diffusion layer, can be used to explain the ≈30 mV/decade slope of the FeISE. The response of the FeISE in UV photooxidised seawater containing 8 nM total Fe was measured as the pH was changed from 8.27 to 3.51. The slope of the response was 24.2 mV/decade [Fe3+] calculated as a function of pH using Fe(III) hydrolysis constants for seawater. Moreover, the response was essentially collinear with that in citrate buffers and in unbuffered solutions containing >10−5 M Fe3+ and the slope for the combined data was 26.2 mV/decade. This study was restricted to organic-free seawater because the certainty in Fe(III)–ligand stability constants is insufficient to warrant the selection of an ideal calibration buffer system, and there is evidence that powerful chelating ligands (e.g., EDTA along with humic and fulvic acids) may alter the response of the Fe(III)ISE. The Fe dissolution rate of the FeISE in UV photooxidised seawater was found to be 1.6×10−2 nmol Fe/min, as measured by cathodic stripping voltammetry (CSV). This would contaminate a 100-ml sample by 0.8–1.6 nM Fe over a typical measurement period of 5–10 min obtained using a stability criterion of 0.5 mV/min. Various methods are proposed for reducing the level of contamination in open ocean samples that contain sub-nanomolar concentrations of iron. The FeISE has the potential to detect free Fe3+ at concentrations typically found in natural seawater.  相似文献   

18.
To elucidate iron regeneration and organic iron(III)-binding ligand formation during microzooplankton and copepod grazing on phytoplankton, incubation experiments were conducted in the western subarctic Pacific. During 8 days of dark incubation of ambient water and that amended with plankton concentrate, dissolved iron and organic iron(III)-binding ligands accumulated, approximately proportionally to the decrease in chlorophyll a. The observed increases in dissolved iron concentration were much greater than those expected from the consumption of phytoplankton biomass and previously reported Fe:C value of cultured algal cells, suggesting resolution from colloidal or particulate iron adsorbed onto the algal cell surface. When copepods were added to the ambient water, organic iron(III)-binding ligands accumulated more rapidly than in the control receiving no copepod addition, although consumed phytoplankton biomass was comparable between the two treatments. Bioassay experiment using filtrates collected from the incubation experiment showed that organic ligands formed during microzooplankton grazing reduced the iron bioavailability to phytoplankton and suppressed their growth. Moreover, picoplankton Synechococcus sp. and Micromonas pusilla were more suppressed by the organic ligands than the diatom Thalassiosira weissflogii. In conclusion, through microzooplankton and copepod grazing on phytoplankton, organic iron(III)-binding ligands as well as regenerated iron are released into the ambient seawater. Because the ligands lower iron bioavailability to phytoplankton through complexation and the degree of availability reduction varies among phytoplankton species, grazing by zooplankton can shift phytoplankton community structure in iron-limited waters.  相似文献   

19.
Single species populations and natural populations of phytoplankton were grown in mixtures of surface sea water and deep ocean water. The yields of Phaeodactylum tricornutum and the natural species assemblages were positively correlated with the percentage of deep water in the mixture. Single species populations of Thalassiosira weissflogii and Amphidinum sp. showed yields which were positively correlated with per cent deep water at low deep water concentrations and negatively correlated at high deep water concentrations. Transition from the positive to the negative slope occurred at about 75% deep water for T. weissflogii and at about 25% deep water for Amphidinium. Populations were apparently limited by an inorganic nutrient, probably nitrogen, in the region of positive slope and by some other factor, probably organic, in the region of negative slope. The addition of EDTA increased the yields of T. weissflogii and Amphidinium in the presence of deep water and reduced lag times in the growth of the natural populations.  相似文献   

20.
The chemical speciation of iron was determined in the Southern Ocean along a transect from 48 to 70°S at 20°E. Dissolved iron concentrations were low at 0.1–0.6 nM, with average concentrations of 0.25±0.13 nM. Organic iron complexing ligands were found to occur in excess of the dissolved iron concentration at 0.72±0.23 nM (equivalent to an excess of 0.5 nM), with a complex stability of log KFeL′=22.1±0.5 (on the basis of Fe3+ and L′). Ligand concentrations were higher in the upper water column (top 200 m) suggesting in situ production by microorganisms, and less at the surface consistent with photochemical breakdown. Our data are consistent with the presence of stable organic iron-complexing ligands in deep global ocean waters at a background level of ∼0.7 nM. It has been suggested that this might help stabilise iron at levels of ∼0.7 nM in deep ocean waters. However, much lower iron concentrations in the waters of the Southern Ocean suggest that these ligands do not prevent the removal of iron (by scavenging or biological uptake) to well below the concentration of these ligands. Scavenging reactions are probably inhibited by such ligand competition, so it is likely that biological uptake is the chief cause for the further removal of iron to these low levels in waters that suffer from very low iron inputs.  相似文献   

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