Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast Alaska

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM influences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation–emission fluorescence spectroscopy, specific UV absorbance (SUVA₂₅₄) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four different wetland and forest soils: bog, forested wetland, fen and upland forest. There were significant differences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA₂₅₄ and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9–32 mg C l⁻¹ and between 23–42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation–emission fluorescence spectroscopy showed that protein-like fluorescence was positively correlated (r ² = 0.82; P < 0.001) with BDOC for all soil types taken together. This finding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARAFAC modeling confirmed that the four soil types contain DOM with distinct chemical properties and have unique fluorescent fingerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes differently by influencing temporal patterns in stream heterotrophic productivity.     

Chemical composition of aquatic dissolved organic matter in five boreal forest catchments sampled in spring and fall seasons

The chemical composition and carbon isotope signature of aquatic dissolved organic matter (DOM) in five boreal forest catchments in Scandinavia were investigated. The DOM was isolated during spring and fall seasons using a reverse osmosis technique. The DOM samples were analyzed by elemental analysis, FT-IR, solid-state CP-MAS ¹³C-NMR, and C-1s NEXAFS spectroscopy. In addition, the relative abundance of carbon isotopes (¹²C, ¹³C, ¹⁴C) in the samples was measured. There were no significant differences in the chemical composition or carbon isotope signature of the DOM sampled in spring and fall seasons. Also, differences in DOM composition between the five catchments were minor. Compared to reference peat fulvic and humic acids, all DOM samples were richer in O-alkyl carbon and contained less aromatic and phenolic carbon, as shown by FT-IR, ¹³C-NMR, and C-1s NEXAFS spectroscopy. The DOM was clearly enriched in ¹⁴C relative to the NBS oxalic acid standard of 1950, indicating that the aquatic DOM contained considerable amounts of organic carbon younger than about 50 years. The weight-based C:N ratios of 31 ± 6 and the values of indicate that the isolated DOM is of terrestrial rather than aquatic origin. We conclude that young, hydrophilic carbon compounds of terrestrial origin are predominant in the samples investigated, and that the composition of the aquatic DOM in the studied boreal forest catchments is rather stable during low to intermediate flow conditions.     

Cu(II): a “signaling molecule” of the mangrove endophyte <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> ZZF51?

We previously reported the isolation of Cu–fusaric acid (Cu–FA) complex from the mangrove endophyte Fusarium oxysporum ZZF51. In this study, we explored the mechanism of Cu–FA production in the strain ZZF51 by comparing with that of another endophyte Fusarium sp. B2, which produced FA but not Cu–FA in the same culture condition. The results allowed us to hypothesize that Cu²⁺ may act as a “signaling molecule” to awaken the silent FA biosynthetic genes in ZZF51, inducing intracellular production of FA followed by chelation with Cu²⁺. This signaling network was triggered specifically by Cu²⁺ and may be interfered by other metal ions.     

Uptake of ammonium and soluble reactive phosphorus in forested streams: influence of dissolved organic matter composition

Many microbes responsible for inorganic nutrient uptake and transformation utilize dissolved organic matter (DOM) as a nutrient or energy source, but little is known about whether DOM composition is an important driver of nutrient uptake in streams. Our goal was to determine whether incorporating DOM composition metrics with other more commonly considered biological, physical, and chemical variables improved our ability to explain patterns of ammonium (\({\text{NH}}_{4}^{ + }\)–N) and soluble reactive phosphorus (SRP) uptake across 11 Lake Superior tributaries. Nutrient uptake velocities (V_f) ranged from undetectable to 14.6 mm min^?1 for \({\text{NH}}_{4}^{ + }\)–N and undetectable to 7.2 mm min^?1 for SRP. Logistic regressions suggested that DOM composition was a useful predictor of where SRP uptake occurred (4/11 sites) and \({\text{NH}}_{4}^{ + }\)–N concentration was a useful predictor of where \({\text{NH}}_{4}^{ + }\)–N uptake occurred (9/11 sites). Multiple regression analysis revealed that the best models included temperature, specific discharge, and canopy cover, and DOM composition as significant predictors of \({\text{NH}}_{4}^{ + }\)–N V_f. Partial least squares revealed fluorescence index (describing the source of aquatic fulvic acids), specific ultraviolet absorbance at 254 nm (an indicator of DOM aromaticity), temperature, and conductivity were highly influential predictors of \({\text{NH}}_{4}^{ + }\)–N V_f. Therefore, streams with higher temperatures, lower solute concentrations, more terrestrial DOM signal and greater aromaticity had greater \({\text{NH}}_{4}^{ + }\)–N V_f. Our results suggest that DOM composition may be an important, yet often overlooked, predictor of \({\text{NH}}_{4}^{ + }\)–N and SRP uptake in deciduous forest streams that should be considered along with commonly measured predictors.     

Distribution of dissolved organic carbon and nitrogen in a coral reef

Dissolved organic matter (DOM) concentrations in a fringing coral reef were measured for both carbon and nitrogen with the analytical technique of high-temperature catalytic oxidation. Because of high precision of the analytical system, not only the concentrations of dissolved organic carbon and nitrogen (DOC and DON, respectively) but the C:N ratio was also determined from the distribution of DOC and DON concentrations. The observed concentrations of DOC and DON ranged 57–76 and 3.8–5.6 μmol l⁻¹, respectively. The C:N ratios of the DOM that was produced on the reef flat were very similar between seagrass- and coral-dominated areas; the C:N ratio was 10 on average. The C:N ratio of DOM was significantly higher than that of particulate organic matter (POM) that was produced on the reef flat. Production rates of DOC were measured on the reef flat during stagnant periods and accounted for 3–7% of the net primary production, depending on the sampling site. The production rate of DON was estimated to be 10–30% of the net uptake of dissolved inorganic N in the reef community. Considering that the DOM and POM concentrations were not correlated with each other, a major source of the reef-derived DOM may be the benthic community and not POM such as phytoplankton. It was concluded that a widely distributed benthic community in the coral reef released C-rich DOM to the overlying seawater, conserving N in the community.     

Effect of organic complexation on the behaviour of dissolved Cd, Cu and Zn in the Scheldt estuary

Samples from the Scheldt estuary have been assayed for dissolved Cd, Cuand Zn using differential pulse anodic stripping voltammetry, either astotal (after UV irradiation) or labile concentrations. Under theseexperimental conditions, labile concentrations ranged between51–65% of total Cu, 16–66% of total Zn and53–91% of total Cd. The metal–organic interactions wereassessed by evaluating (a) the distribution coefficient K_d forthe distribution of the metals between the liquid phase (complexation) andtheir binding to particulate matter, and (b) the competitive effect exertedby inorganic complexing ligands using a multi–element interactionmodel. The proportion of organically bound metals (strong and labile) wasestimated, in this speciation scheme, to range from 86 to 99% for Cu,from 90 to 96% for Zn, and from 10 to 35% for Cd. From thedissolved organic carbon distribution in the Scheldt (≤ 10 mg Cl^-1 and taking into account competition from major cations Caand Mg, free ligand concentrations available for heavy metal complexationwere estimated to be ≤ 0.15 mg C l^-1. With these values,conditional stability constants for the chelation of Cu, Zn and Cd werecalculated assuming either a single-step or a two-step complexation in thedissolved phase. Given the assumptions made in these models, stabilityconstants in the range of 10^7.8–10^10.6 forCu, 10^7.0–10^9.1 for Zn and10^6.9–10^8.9 for Cd were obtained. Therelevance of these data to previous in vitro and in situ studies isdiscussed taking into consideration current concepts of metal bindingaffinity for organic ligands. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stream dissolved organic matter bioavailability and composition in watersheds underlain with discontinuous permafrost

We examined the impact of permafrost on dissolved organic matter (DOM) composition in Caribou-Poker Creeks Research Watershed (CPCRW), a watershed underlain with discontinuous permafrost, in interior Alaska. We analyzed long term data from watersheds underlain with varying degrees of permafrost, sampled springs and thermokarsts, used fluorescence spectroscopy, and measured the bioavailabity of dissolved organic carbon (DOC). Permafrost driven patterns in hydrology and vegetation influenced DOM patterns in streams, with the stream draining the high permafrost watershed having higher DOC and dissolved organic nitrogen (DON) concentrations, higher DOC:DON and greater specific ultraviolet absorbance (SUVA) than the streams draining the low and medium permafrost watersheds. Streams, springs and thermokarsts exhibited a wide range of DOC and DON concentrations (1.5–37.5 mgC/L and 0.14–1.26 mgN/L, respectively), DOC:DON (7.1–42.8) and SUVA (1.5–4.7 L mgC⁻¹ m⁻¹). All sites had a high proportion of humic components, a low proportion of protein components, and a low fluorescence index value (1.3–1.4), generally consistent with terrestrially derived DOM. Principal component analysis revealed distinct groups in our fluorescence data determined by diagenetic processing and DOM source. The proportion of bioavailable DOC ranged from 2 to 35%, with the proportion of tyrosine- and tryptophan-like fluorophores in the DOM being a major predictor of DOC loss (p < 0.05, R ² = 0.99). Our results indicate that the degradation of permafrost in CPCRW will result in a decrease in DOC and DON concentrations, a decline in DOC:DON, and a reduction in SUVA, possibly accompanied by a change in the proportion of bioavailable DOC.     

Copper complexation by natural organic matter in contaminated and uncontaminated ground water

Abstract

Ground-water samples were collected from an uncontaminated and a contaminated site. Copper complexation was characterized by ion- selective electrode (ISE), fluorescence quenching (FQ), and cathodic stripping voltammetric (CSV) titrations. All of the samples were titrated at their natural pH values and some of the samples were also titrated at other pH values. For a total Cu concentration of 10^?6 M, the free Cu²⁺ concentrations in the samples from the uncontaminated site were all less than 10^?7 M, while free Cu²⁺ in the samples from the contaminated site were all less than 10^?8 M. For a particular sample and total Cu concentration, the free Cu²⁺ concentration decreased as the pH increased. Relative to ISE, FQ underestimated and CSV overestimated the degree of Cu²⁺ binding. The Cu²⁺ -complexing properties of the ground waters are similar to many published results for the same pH and for ligand concentrations normalized to T.O.C. Chemical equilibrium computations indicate that organic complexes would dominate Cu speciation in the uncontaminated ground waters for 10^?7 to 10^?5 M total Cu. In the contaminated ground waters, sulfide complexes would be the predominant Cu species for total Cu less than the total S(?11) concentration. Organic complexes would dominate Cu speciation for total Cu greater than total S(?11).     

The effect of metal diffusion and supply limitations on conditional stability constants determined for durum wheat roots

Conditional stability constants (log K), and binding site densities (Γ_max) for dissolved metals and biota are important input parameters for the Biotic Ligand Model. However, determination of these binding parameters is likely to be influenced by solution kinetics because roots have a large metal-binding capacity and can accumulate metals rapidly. The aim of this study was to determine if the rate of free metal ion diffusion to the root surface, and amount of metal in the bulk solution, is sufficient to accommodate the maximum root–metal accumulation capacity. The extent to which these kinetic limitations affect the magnitude of log K and Γ_max values was also assessed. Seven day old hydroponically grown durum wheat (Triticum turgidum L. var durum, cv ‘Arcola’) were exposed to solutions with p{Cu²⁺}s ranging from 10.54 to 2.26 (~20 °C, pH = 6.0, ionic strength = 0.03 M). Exposure solutions were prepared with and without the metal buffer nitrilotriacetic acid (NTA) so that the total amount of metal in the exposure solution, and net flux of metal to the root, could be varied. The results demonstrate that NTA enhances Cu accumulation at exposure p{Cu²⁺}s between 10 and 6. Comparison of the diffusive flux to the root with the metal flux into the root, for (−NTA) and (+NTA) Cu exposures, showed that the flux of the un-buffered free metal ion to the root was not large enough to accommodate the maximum Cu binding capacity between 10 and 6 p{Cu²⁺} in solution. The total amount of Cu in solution may have limited uptake for exposure p{Cu²⁺}s of 10.01 and 9.01, but the background concentrations of Cu in the control plants prevented definitive conclusions from being made within this exposure range. Similar results were found for Mn and Ni. For Cd, which had lower background concentrations in the roots, the amount of metal in solution did not limit uptake until a p{Cd²⁺} of 10.01. Limiting the supply of Cu²⁺ to the root (i.e. low {Cu²⁺}s with no NTA) caused only a moderate bias in Γ_max values, but suppressed the log K value by 3.44 log units. The log K values for Cd, Mn and Ni, in the absence of NTA, were more similar than expected, which suggests that the kinetics of free ion re-supply to the root surface limited metal uptake, as it did for Cu. Section Editor: T. B. Kinraide     

Quantitative and Qualitative Aspects of Dissolved Organic Carbon Leached from Senescent Plants in an Oligotrophic Wetland

We conducted a series of experiments whereby dissolved organic matter (DOM) was leached from various wetland and estuarine plants, namely sawgrass (Cladium jamaicense), spikerush (Eleocharis cellulosa), red mangrove (Rhizophora mangle), cattail (Typha domingensis), periphyton (dry and wet mat), and a seagrass (turtle grass; Thalassia testudinum). All are abundant in the Florida Coastal Everglades (FCE) except for cattail, but this species has a potential to proliferate in this environment. Senescent plant samples were immersed into ultrapure water with and without addition of 0.1% NaN₃ (w/ and w/o NaN₃, respectively) for 36 days. We replaced the water every 3 days. The amount of dissolved organic carbon (DOC), sugars, and phenols in the leachates were analyzed. The contribution of plant leachates to the ultrafiltered high molecular weight fraction of DOM (>1 kDa; UDOM) in natural waters in the FCE was also investigated. UDOM in plant leachates was obtained by tangential flow ultrafiltration and its carbon and phenolic compound compositions were analyzed using solid state ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (¹³C CPMAS NMR) spectroscopy and thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH thermochemolysis), respectively. The maximum yield of DOC leached from plants over the 36-day incubations ranged from 13.0 to 55.2 g C kg⁻¹ dry weight. This amount was lower in w/o NaN₃ treatments (more DOC was consumed by microbes than produced) except for periphyton. During the first 2 weeks of the 5 week incubation period, 60–85% of the total amount of DOC was leached, and exponential decay models fit the leaching rates except for periphyton w/o NaN₃. Leached DOC (w/ NaN₃) contained different concentrations of sugars and phenols depending on the plant types (1.09–7.22 and 0.38–12.4 g C kg⁻¹ dry weight, respectively), and those biomolecules comprised 8–34% and 4–28% of the total DOC, respectively. This result shows that polyphenols that readily leach from senescent plants can be an important source of chromophoric DOM (CDOM) in wetland environments. The O-alkyl C was found to be the major C form (55±9%) of UDOM in plant leachates as determined by ¹³C CPMAS NMR. The relative abundance of alkyl C and carbonyl C was consistently lower in plant-leached UDOM than that in natural water UDOM in the FCE, which suggests that these constituents increase in relative abundance during diagenetic processing. TMAH thermochemolysis analysis revealed that the phenolic composition was different among the UDOM leached from different plants, and was expected to serve as a source indicator of UDOM in natural water. Polyphenols are, however, very reactive and photosensitive in aquatic environments, and thus may loose their plant-specific molecular characteristics shortly. Our study suggests that variations in vegetative cover across a wetland landscape will affect the quantity and quality of DOM leached into the water, and such differences in DOM characteristics may affect other biogeochemical processes.     

径流雨水中溶解性有机质特征演化及其对典型污染物迁移和生物有效性的影响

地表径流污染已经逐渐成为城市面源污染的重要组成部分。其中溶解性有机质DOM (Dissolved organic matter)是有机污染物的主要组成部分。DOM中因为含有大量不饱和结构、官能团,其中包括羟基、羧基、羰基、胺基等,这些结构容易与径流中重金属结合发生络合反应,改变重金属的赋存形态,从而对其迁移转化及其生物有效性产生很大的影响。本文以北京市地表径流为研究对象,研究城市地表径流在冬、夏季不同功能区不同下垫面中溶解性有机质特征及其与典型重金属的作用机制,通过荧光淬灭滴定实验,研究夏季径流雨水中DOM的不同组分与重金属Cu~(2+)、Pb~(2+)、Zn~(2+)之间的结合机制。研究结果显示PARAFAC将获得的样品都分解成2类6个不同的组分,1种腐殖酸,1种类蛋白;类蛋白物质的荧光强度与Cu~(2+)、Pb~(2+)的淬灭率要强于腐殖酸,而Zn~(2+)则呈现相反趋势;通过使用二维相关同步光谱发现DOM对重金属Cu~(2+)、Pb~(2+)、Zn~(2+)的敏感性呈现递减趋势,二维相关异步光谱发现Cu~(2+)、Pb~(2+)会先与位于270—300nm附近类蛋白光谱带反应,Zn~(2+)则会先与330nm附近的腐殖酸光谱带反应。     

Seasonal changes in the chemical quality and biodegradability of dissolved organic matter exported from soils to streams in coastal temperate rainforest watersheds

The composition and biodegradability of streamwater dissolved organic matter (DOM) varies with source material and degree of transformation. We combined PARAFAC modeling of fluorescence excitation–emission spectroscopy and biodegradable dissolved organic carbon (BDOC) incubations to investigate seasonal changes in the lability of DOM along a soil-stream continuum in three soil types: bog, forested wetland and upland forest. The percent BDOC ranged from 7 to 38% across all sites, and was significantly greater in soil compared to streamwater in the bog and forested wetland, but not in the upland forest. The percent BDOC also varied significantly over the entire sampling period in soil and streamwater for the bog and forested wetland, as BDOC peaked during the spring runoff and was lowest during the summer months. Moreover, the chemical quality of DOM in wetland soil and streamwater was similar during the spring runoff and fall wet season, as demonstrated by the similar contribution of protein-like fluorescence (sum of tyrosine and tryptophan fluorescence) in soil water and in streams. These findings suggest that the tight coupling between terrestrial and aquatic ecosystems is responsible for the delivery of labile DOM from wetland soils to streams. The contribution of protein-like fluorescence was significantly correlated with BDOC (p < 0.001) over the entire sampling period indicating DOM is an important source of C and N for heterotrophic microbes. Taken together, our findings suggest that the production of protein-rich, labile DOM and subsequent loss in stream runoff might be an important loss of labile C and N from coastal temperate watersheds.     

Chemical Characteristics and Origin of Dissolved Organic Matter in the Yukon River

Monthly (or bi-weekly) water samples were collected from the Yukon River, one of the largest rivers in North America, at a station near the US Geological Survey Stevens Village hydrological station, Alaska from May to September 2002, to examine the quantity and quality of dissolved organic matter (DOM) and its seasonal variations. DOM was further size fractionated into high molecular weight (HMW or colloidal, 1 kDa–0.45 μm) and low molecular weight (LMW, <1 kDa) fractions. Dissolved organic carbon (DOC), colored dissolved organic matter (C-DOM) and total dissolved carbohydrate (TCHO) species were measured in the size fractionated DOM samples. Concentrations of DOC were as high as 2830 μmol-C l⁻¹ during the spring breakup in May and decreased significantly to 508–558 μmol-C l⁻¹ during open-water season (June–September). Within the DOC pool, up to 85% was in the colloidal fraction (1 kDa–0.45 μm) in early May. As DOC concentration decreased, this colloidal portion remained high (70–85% of the bulk DOC) throughout the sampling season. Concentrations of TCHO, including monosaccharides (MCHO) and polysaccharides (PCHO), varied from 722 μmol-C l⁻¹ in May to 129 μmol-C l⁻¹ in September, which comprised a fairly constant portion of bulk DOC (24±2%). Within the TCHO pool, the MCHO/TCHO ratio consistently increased from May to September. The C-DOM/DOM ratio and the size fractionated DOM increased from May to September, indicating that DOM draining into the Yukon River contained increased amounts of humified materials, likely related to a greater soil leaching efficiency in summer. The average composition of DOM was 76% pedogenic humic matter and 24% aquagenic CHO. Characteristics of soil-derived humic substances and low chlorophyll-a concentrations support a dominance of terrestrial DOM in Yukon River waters.     

Removing heavy metals from synthetic effluents using “kamikaze” Saccharomyces cerevisiae cells

One key step of the bioremediation processes designed to clean up heavy metal contaminated environments is growing resistant cells that accumulate the heavy metals to ensure better removal through a combination of biosorption and continuous metabolic uptake after physical adsorption. Saccharomyces cerevisiae cells can easily act as cation biosorbents, but isolation of mutants that are both hyperaccumulating and tolerant to heavy metals proved extremely difficult. Instead, mutants that are hypersensitive to heavy metals due to increased and continuous uptake from the environment were considered, aiming to use such mutants to reduce the heavy metal content of contaminated waters. In this study, the heavy metal hypersensitive yeast strain pmr1∆ was investigated for the ability to remove Mn²⁺, Cu²⁺, Co²⁺, or Cd²⁺ from synthetic effluents. Due to increased metal accumulation, the mutant strain was more efficient than the wild-type in removing Mn²⁺, Cu²⁺, or Co²⁺ from synthetic effluents containing 1–2 mM cations, with a selectivity $ {\text{Mn}}^{{{\text{2}} + }} > {\text{Co}}^{{{\text{2}} + }} ~ > {\text{Cu}}^{{{\text{2}} + }} $ {\text{Mn}}^{{{\text{2}} + }} > {\text{Co}}^{{{\text{2}} + }} ~ > {\text{Cu}}^{{{\text{2}} + }} and also in removing Mn²⁺ and Cd²⁺ from synthetic effluents containing 20–50 μM cations, with a selectivity Mn²⁺ > Cd²⁺.     

A large molecular size fraction of riverine high molecular weight dissolved organic matter (HMW DOM) stimulates growth of the harmful dinoflagellate Alexandrium minutum

An increase in the concentration of riverine dissolved organic matter (DOM) has been observed during the last decades, and this material can stimulate marine plankton in coastal waters with significant freshwater input. We studied the effect of two size fractions of riverine high molecular weight dissolved organic matter (HMW DOM), isolated with tangential ultrafiltration, on the harmful dinoflagellate Alexandrium minutum and a natural isolate of marine bacteria under laboratory conditions. Both A. minutum and bacteria grew significantly better with the low MW DOM compared to both the high MW DOM fraction and controls (no DOM additions). This experiment demonstrates that the harmful algae A. minutum and bacteria benefit from larger molecules of river HMW DOM, and highlights the potential of A. minutum to utilize organic nitrogen from large DOM molecules. This ability may enhance their likelihood of success in estuaries/costal waters with a humic rich freshwater input, especially when the relative amount of large molecules within DOM is more pronounced.     

Re-evaluating the effects of organic ligands on copper toxicity to barley root elongation in culture solution

Abstract

In the presence of weak ligands, both free ion activity and organic complexes of Cu should b considered when predicting Cu toxicity in aquatic and soil-plant systems. However, there is littl information about the quantitative contribution of Cu that is organically complexed to Cu toxicity. In thi study, a bioassay using barley root elongation in culture solution was used to investigate the effects o organic ligands with different conditional stability constants on Cu toxicity and the quantitativ contribution of the organically complexed Cu to the Cu toxicity. The results indicated that a significan decrease (p<0.05) in Cu toxicity, assessed by barley root elongation, was observed in response to th addition of organic ligands. The decrease differed, to some extent, with different organic ligands o disodium ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), oxalate and malate at low and constant free Cu²⁺ activity. Addition of EDTA or NTA resulted in strong reduction of Cu toxicity while modest reduction of Cu toxicity was observed for the addition of malate as the relatively wea ligand. Furthermore, the results of the present study revealed that the CuNTA^? and CuEDTA^2? complexes were not toxic, while the Cu–malate complexes were mildly toxic to barley root elongation More importantly, it was found that the toxicity of Cu–malate complexes were nearly 0.5-fold less than that of free Cu²⁺ ions.     

Bacterial Growth on Photochemically Transformed Leachates from Aquatic and Terrestrial Primary Producers

Abstract We measured bacterial growth on phototransformed dissolved organic matter (DOM) leached from eight different primary producers. Leachates (10 mg C liter⁻¹) were exposed to artificial UVA + UVB radiation, or kept in darkness, for 20 h. DOM solutions were subsequently inoculated with lake water bacteria. Photoproduction of dissolved inorganic carbon (DIC), ranging from 3 to 16 μg C liter⁻¹ h⁻¹, and changes in the absorptive characteristics of the DOM were observed for all leachates upon UV irradiation. The effects of irradiation exposure on DOM bioavailability varied greatly, depending on leachate and type of bacterial growth criterion. Bacterial carbon utilization (biomass production plus respiration) over the entire incubation period (120 h) was enhanced by UV radiation of leachate from the terrestrial leaves, relative to carbon utilization in non-irradiated leachates. Conversely, carbon utilization was reduced by radiation of the leachates from aquatic macrophytes. In a separate experiment, the stable C and N isotope composition of bacteria grown on irradiated and non-irradiated DOM was estimated. Bacterial growth on UV-irradiated DOM was enriched in ¹³C relative to the bacteria in the non-irradiated treatments; this result may be explained by selective assimilation of photochemically produced, isotopically enriched labile compounds. Received: 17 February 2000; Accepted: 1 May 2000; Online Publication: 28 August 2000     

Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m² d⁻¹) and accounted for 20%–53% of CFTB and 11%–33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5×) and dissolved P (10×) relative to lake water, peaked in the winter–spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%–20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Received 11 June 2001; Accepted 14 December 2001.     

Study of the Action of Se and Cu on the Growth Metabolism of <Emphasis Type="Italic">Escherichia coli</Emphasis> by Microcalorimetry

The biological effect of Se and Cu²⁺ on Escherichia coli (E. coli) growth was studied by using a 3114/3236 TAM Air Isothermal Calorimeter, ampoule method, at 37°C. From the thermogenesis curves, the thermokinetic equations were established under different conditions. The kinetics showed that a low concentration of Se (1–10 μg/mL) promoted the growth of E. coli, and a high concentration of Se (>10 μg/mL) inhibited the growth, but the Cu²⁺ was always inhibiting the growth of E. coli. Moreover, there was an antagonistic or positive synergistic effect of Se and Cu²⁺ on E. coli in the different culture medium when Se was 1–10 μg/ml and Cu²⁺ was 1–20 μg/ml. There was a negative synergistic effect of Se and Cu²⁺ on E. coli when Se was higher than 10 μg/ml and Cu²⁺ was higher than 20 μg/ml. The antagonistic or synergistic effect between Se and Cu²⁺ on E. coli was related to the formation of Cu–Se complexes under the different experimental conditions chosen.     

Association of Chernobyl-derived 239+240Pu, 241Am, 90Sr and 137Cs with different molecular size fractions of organic matter in the soil solution of two grassland soils

Radiocesium is normally bound only rather weakly and unspecifically by humic substances, in contrast to the actinides Pu and Am. Recently, however, it was observed that fallout ¹³⁷Cs in the soil solution from an Of-horizon of a podzol forest soil (slightly decomposed plant material) was associated essentially only with one single size fraction of the humic substances. In deeper soil layers with well humified material (AOh-horizon), radiocesium was associated with all size fractions of the dissolved organic matter (DOM). To examine whether this unexpected behaviour is also observable for DOM isolated from other soils, we determined the association of fallout ¹³⁷Cs,⁹⁰Sr,²³⁸Pu, ^239+240Pu and ²⁴¹Am with various size fractions of DOM from in situ soil solutions isolated from two layers (0–2 cm and 2–5 cm) of two grassland soils (a soddy podzolic soil and a peat soil) within the 10 km zone of the nuclear reactor at Chernobyl (Ukraine). The four size fractions of DOM as obtained by gel filtration of the soil solution were (mean nominal molecular weight in daltons): fraction I: ≥2000, fraction II: 1300; fraction III: 560, fraction IV: inorganic compounds. The results for the well humified DOM (humus accumulation horizon of podzol, deeper layer of peat soil) showed that Pu and Am are essentially associated with the high molecular weight fractions, while Sr is present only in the `inorganic' fraction. Radiocesium is found in all the size fractions separated. A quite similar pattern was also found for Pu, Am, and Sr in the soil solution from only slightly decomposed plant material (0–2 cm of peat soil), but not for radiocesium. This radionuclide was again essentially only observable in one single low molecular weight fraction of DOM. The above results thus support our recent observations in the different horizons of a forest podzol mentioned above, even though no reason for the different binding of radiocesium by well humified soil organic matter and by only slightly decomposed plant material can be given at present. The data demonstrate, however, that information on only the total amount of a radionuclide in the soil solution will not be sufficient to interpret or predict its fate adequately in the soil. Received: 13 February 1998 / Accepted in revised form: 14 July 1998