Phytoplankton-mediated redox cycle of iron in the epilimnion of Lake Kinneret

Iron Isotopic Composition of Suspended Particulate Matter in Hongfeng Lake

The geochemical study of iron isotopes is of great significance to deeply understand the surface material circulation process and its environmental effects in surface and subsurface environments. Eutrophication lakes are an important part of the surface and subsurface environment; however, knowledge of the geochemical behavior and fractionation mechanism of iron isotopes in the biogeochemical cycling of eutrophication lakes is still scarce. In this study, a eutrophic lake with seasonal anaerobic characteristics (Hongfeng Lake) was selected as the study object to systematically analyze the iron isotope composition of suspended particles in lake water in different seasons as well as examining suspended particles in the main tributaries, sediments, pore water, planktonic algae, and other samples. The results show that the value of δ56Fe in Hongfeng Lake is between −0.85‰ and +0.14‰, and the value of δ56Fe has a high linear correlation with Fe/Al, indicating that the continental source material carried by the main inflow tributaries of the lake has an important influence on the source of iron in the lake. At the same time, Hongfeng lake is a medium eutrophication lake. Algal bloom and the content of chlorophyll a (Chl-a) is high, combined with the high correlation between Chl-a and the value of δ56Fe, which indicate that the growth of algae has an important influence on the change of iron isotope composition of suspended particles matter (SPM) in lake water and the adsorption and growth absorption of Fe by algae is the main reason for the change of the value of δ56Fe, so Fe isotope can be used to trace the lake’s biological action. For the lake and its inflow tributaries, δ56Fe values are higher in summer than those in winter. And the δ56Fe value of SPM in lake that varies with depth is more obvious in summer than in winter. In addition, there is an obvious thermocline in summer, which leads to hydrochemical stratification. Moreover, according to a linear correlation analysis, the content of DOC (dissolved organic matter) in Hongfeng Lake’s upper and lower waters, respectively, has a high correlation with the value of δ56Fe. Additionally, in the upper water, it is positively correlated, while on the bottom, there is a negative correlation relationship, which indicates that the difference in algae metabolism patterns between the upper and lower water bodies of Hongfeng Lake plays an important role in the iron isotope composition of suspended particulate matters (SPM). The composition of the Fe isotope in SPM is changed by organic adsorption and growth absorption of algae in upper water. With an increase in depth, the degradation process becomes the main one. In addition, the value of δ56Fe is low and Fe/Al is high in the water bottom, which indicates that “ferrous-wheel” cycle form at the bottom of the water.

Variochelins, Lipopeptide Siderophores from Variovorax boronicumulans Discovered by Genome Mining

Photoreactive siderophores have a major impact on the growth of planktonic organisms. To date, these molecules have mainly been reported from marine bacteria, although evidence is now accumulating that some terrestrial bacteria also harbor the biosynthetic potential for their production. In this paper, we describe the genomics-driven discovery and characterization of variochelins, lipopeptide siderophores from the bacterium Variovorax boronicumulans, which thrives in soil and freshwater habitats. Variochelins are different from most other lipopeptide siderophores in that their biosynthesis involves a polyketide synthase. We demonstrate that the ferric iron complex of variochelin A possesses photoreactive properties and present the MS-derived structures of two degradation products that emerge upon light exposure.

Disassembling iron availability to phytoplankton

The bioavailability of iron to microorganisms and its underlying mechanisms have far reaching repercussions to many natural systems and diverse fields of research, including ocean biogeochemistry, carbon cycling and climate, harmful algal blooms, soil and plant research, bioremediation, pathogenesis, and medicine. Within the framework of ocean sciences, short supply and restricted bioavailability of Fe to phytoplankton is thought to limit primary production and curtail atmospheric CO₂ drawdown in vast ocean regions. Yet a clear-cut definition of bioavailability remains elusive, with elements of iron speciation and kinetics, phytoplankton physiology, light, temperature, and microbial interactions, to name a few, all intricately intertwined into this concept. Here, in a synthesis of published and new data, we attempt to disassemble the complex concept of iron bioavailability to phytoplankton by individually exploring some of its facets. We distinguish between the fundamentals of bioavailability – the acquisition of Fe-substrate by phytoplankton – and added levels of complexity involving interactions among organisms, iron, and ecosystem processes. We first examine how phytoplankton acquire free and organically bound iron, drawing attention to the pervasiveness of the reductive uptake pathway in both prokaryotic and eukaryotic autotrophs. Turning to acquisition rates, we propose to view the availability of various Fe-substrates to phytoplankton as a spectrum rather than an absolute “all or nothing.” We then demonstrate the use of uptake rate constants to make comparisons across different studies, organisms, Fe-compounds, and environments, and for gaging the contribution of various Fe-substrates to phytoplankton growth in situ. Last, we describe the influence of aquatic microorganisms on iron chemistry and fate by way of organic complexation and bio-mediated redox transformations and examine the bioavailability of these bio-modified Fe species.

Use of H2(18)O2 to measure absolute rates of dark H2O2 production in freshwater systems

Photochemical production is usually considered to be the main source of H2O2 in freshwater systems; here we show that significant dark production also occurs. We used isotope-labeled H2O2 as a tracer to simultaneously determine H2O2 production and decay rates in incubations of unfiltered water samples. Our new technique for H2(18)O2 analysis, requiring only small sample volumes and simple field equipment, allows for preservation of samples in remote locations, followed by gas chromatography mass spectrometry (GCMS) analysis up to six days later. Dark H2O2 production rates of 29-122 nM/h were observed in several lakewater samples. Measured production and decay rates were consistent with pseudo steady-state, early morning [H2O2] measurements made in each water body. Dark H2O2 production is likely to be more important than photochemical production for the total H2O2 budget over 24 h in the freshwater systems we examined. Our results imply that processes usually assumed to be photochemically induced in freshwaters, such as metal redox cycling mediated by H2O2 and O2(-), and production of strong oxidants from the reaction of H2O2 with Fe(II) (Fenton's reaction) could also be occurring at significant rates in the absence of light.

Bioavailability of trace metals to aquatic microorganisms: importance of chemical, biological and physical processes on biouptake

An important challenge in environmental biogeochemistry is the determination of the bioavailability of toxic and essential trace compounds in natural media. For trace metals, it is now clear that chemical speciation must be taken into account when predicting bioavailability. Over the past 20 years, equilibrium models (free ion activity model (FIAM), biotic ligand model (BLM)) have been increasingly developed to describe metal bioavailability in environmental systems, despite the fact that environmental systems are always dynamic and rarely at equilibrium. In these simple (relatively successful) models, any reduction in the available, reactive species of the metal due to competition, complexation or other reactions will reduce metal bioaccumulation and thus biological effects. Recently, it has become clear that biological, physical and chemical reactions occurring in the immediate proximity of the biological surface also play an important role in controlling trace metal bioavailability through shifts in the limiting biouptake fluxes. Indeed, for microorganisms, examples of biological (transport across membrane), chemical (dissociation kinetics of metal complexes) and physical (diffusion) limitation can be demonstrated. Furthermore, the organism can employ a number of biological internalization strategies to get around limitations that are imposed on it by the physicochemistry of the medium. The use of a single transport site by several metals or the use of several transport sites by a single metal further complicates the prediction of uptake or effects using the simple chemical models. Finally, once inside the microorganism the cell is able to employ a large number of strategies including complexation, compartmentalization, efflux or the production of extracellular ligands to minimize or optimize the reactivity of the metal. The prediction of trace metal bioavailability will thus require multidisciplinary advances in our understanding of the reactions occurring at and near the biological interface. By taking into account medium constraints and biological adaptability, future bioavailability modeling will certainly become more robust.

Bioavailability of iron sensed by a phytoplanktonic Fe-bioreporter

This study describes a short-term (12 h) evaluation of iron (Fe) bioavailability to an Fe-dependent cyanobacterial bioreporter derived from Synechococcus PCC 7942. Several synthetic ligands with variable conditional stability constants for Fe(lll) (K* of 10(19.8) to 10(30.9)), in addition to several defined natural Fe-binding ligands and a fulvic acid of aquatic origin (Suwannee River), were used to elucidate the forms of Fe that are discerned by this phytoplanktonic microbe: Fe-HEBD (log conditional stability constant, K*, = 28.1, HEBD = N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid monohydrochloride hydrate), Fe-HDFB (K* = 30.9, DFB = desferroxamine B), Fe-ferrichrome (K* = 23.2), Fe-DTPA (K* = 21.1, DTPA = diethylenetrinitrilopentaacetic acid), Fe-(8HQS)2 (K* = 20.4, 8HQS = 8-hydroxyquinoline-5-sulfonic acid), Fe-CDTA (K* = 19.8, CDTA = trans-1,2-cyclohexylenedinitrilotetraacetic acid), and Fe-EDTA (K* = 19.2). Iron bioavailability sensed by the bioreporter was related to diffusion limitation and activity of high-affinity transporters rather than by siderophore secretion. Iron complexed with a K* < 23.2 contributes to the bioavailable pool; bioavailability could be explained by disjunctive ligand exchange considerations and fully, partially, and nonbioavailable complexes could be distinguished according to their conditional stability constant. The use of Fe-bioreporters provides a relevant measurement of bioavailability to an important group of primary producers in freshwaters (cyanobacteria) and is thus a promising technique for understanding Fe cycling in aquatic systems.