Sulfate inhibition of molybdenum-dependent nitrogen fixation by planktonic cyanobacteria under seawater conditions: a non-reversible effect

The trace element molybdenum is a central component of several enzymes essential to bacterial nitrogen metabolism, including nitrogen fixation. Despite reasonably high dissolved concentrations (for a trace metal) of molybdenum in seawater, evidence suggests that its biological reactivity and availability are lower in seawater than in freshwater. We have previously argued that this difference is related to an inhibition in the uptake of molybdate (the thermodynamically stable form of molybdenum in oxic natural waters) by sulfate, a stereochemically similar ion. Low molybdenum availability may slow the growth rate of nitrogen-fixing cyanobacteria, and in combination with an ecological control such as grazing by zooplankton, keep fixation rates very low in even strongly nitrogen-limited coastal marine ecosystems. Here we present results from a seawater mesocosm experiment where the molybdenum concentration was increased 10-fold under highly nitrogen-limited conditions. The observed effects on nitrogen-fixing cyanobacterial abundance and nitrogen-fixation inputs were much smaller than expected. A follow-up experiment with sulfate and molybdenum additions to freshwater microcosms showed that sulfate (at seawater concentrations) greatly reduced nitrogen fixation by cyanobacteria and that additions of molybdenum to the levels present in the seawater mesocosm experiment only slightly reversed this effect. In light of these results, we re-evaluated our previous work on the uptake of radio-labeled molybdenum by lake plankton and by cultures of heterocystic cyanobacteria. Our new interpretation indicates that sulfate at saline estuarine levels (>8–10 mM) up to seawater (28 mM) concentrations does inhibit molybdenum assimilation. However, the maximum molybdenum uptake rate (V _max) was a function of the sulfate concentration, with lower V _max values at higher sulfate levels. This indicates that this inhibition is not fully reversed at some saturating level of molybdenum, as assumed in a simple competitive inhibition model. A multi-enzyme, mixed kinetics model with two or more uptake enzyme systems activated in response to the environmental sulfate and molybdate conditions may better explain the repressive effect of sulfate on Mo-mediated processes such as nitrogen fixation.     

Models of regulation and accumulation of trace metals in marine invertebrates

1. General principles governing trace metal uptake and accumulation in marine invertebrates are identified.2. Key determinants of trace metal body concentrations are bioavailability from seawater and from food. However, the nature of the trace metal (essential vs non-essential, chemical properties, etc.) and the physiological state of the organism, strongly influence subsequent handling, distribution, tissue accumulation and excretion.3. The roles of metal-binding proteins (metallothioneins, transferrin-like proteins, etc.) and haemolymph cellular elements in metal transport and storage are described.4. Uptake of many trace metals from seawater generally conforms to Fick's Law of Diffusion, but is also influenced by non-specific binding to ligands in body fluids and cells, potential differences across body surfaces and, in some instances, by active transport processes involving ionic pumps and pinocytosis.5. Potential mechanisms underlying regulation of whole organism and tissue metal loads are outlined and compared with accumulation strategies. The significance of trace metal levels is discussed with regard to the well-being of marine invertebrates and their use in biomonitoring studies of trace metal pollution.     

Impact of recent coastal development and human activities on Nha Trang Bay, Vietnam: evidence from a Porites lutea geochemical record

Nha Trang Bay (NTB) is located on the Central Vietnam coast, western South China Sea. Recent coastal development of Nha Trang City has raised public concern over an increasing level of pollution within the bay and degradation of nearby coral reefs. In this study, multiple proxies (e.g., trace metals, rare earth elements (REEs), and Y/Ho) recorded in a massive Porites lutea coral colony were used to reconstruct changes in seawater conditions in the NTB from 1995 to 2009. A 14-year record of REEs and other trace metals revealed that the concentrations of terrestrial trace metals have increased dramatically in response to an increase in coastal development projects such as road, port, and resort constructions, port and river dredging, and dumping activities since 2000. The effects of such developmental processes are also evident in changes in REE patterns and Y/Ho ratios through time, suggesting that both parameters are critical proxies for marine pollution.     

Interactions between toxic (As, Cd, Hg and Pb) and nutritional essential (Ca, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, Zn) elements in the tissues of cattle from NW Spain

Since the toxicity of one metal or metalloid can be dramatically modulated by the interaction with other toxic or essential metals, studies addressing the chemical interactions between trace elements are increasingly important. In this study correlations between the main toxic (As, Cd, Hg and Pb) and nutritional essential (Ca, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, Zn) elements were evaluated in the tissues (liver, kidney and muscle) of 120 cattle from NW Spain, using Spearman rank correlation analysis based on analytical data obtained by ICP-AES. Although accumulation of toxic elements in cattle in this study is very low and trace essential metals are generally within the adequate ranges, there were significant associations between toxic and essential metals. Cd was positively correlated with most of the essential metals in the kidney, and with Ca, Co and Zn in the liver. Pb was significantly correlated with Co and Cu in the liver. A large number of significant associations between essential metals were found in the different tissues, these correlations being very strong between Ca, Cu, Fe, Mn, Mo and Zn in the kidney. Co was moderately correlated with most of the essential metals in the liver. In general, interactions between trace elements in this study were similar to those found in polluted areas or in experimental studies in animals receiving diets containing high levels of toxic metals or inadequate levels of nutritional essential elements. These interactions probably indicate that mineral balance in the body is regulated by important homeostatic mechanisms in which toxic elements compete with the essential metals, even at low levels of metal exposure. The knowledge of these correlations may be essential to understand the kinetic interactions of metals and their implications in the trace metal metabolism.     

The role of trace metals in photosynthetic electron transport in O2-evolving organisms

Iron is the quantitatively most important trace metal involved in thylakoid reactions of all oxygenic organisms since linear (= non-cyclic) electron flow from H₂O to NADP⁺ involves PS II (2–3 Fe), cytochrome b₆-f (5 Fe), PS I (12 Fe), and ferredoxin (2 Fe); (replaceable by metal-free flavodoxin in certain cyanobacteria and algae under iron deficiency). Cytochrome c₆ (1 Fe) is the only redox catalyst linking the cytochrome b₆-f complex to PS I in most algae; in many cyanobacteria and Chlorophyta cytochrome c₆ and the copper-containing plastocyanin are alternatives, with the availability of iron and copper regulating their relative expression, while higher plants only have plastocyanin. Iron, copper and zinc occur in enzymes that remove active oxygen species and that are in part bound to the thylakoid membrane. These enzymes are ascorbate peroxidase (Fe) and iron-(cyanobacteria, and most al gae) and copper-zinc- (some algae; higher plants) superoxide dismutase. Iron-containing NAD(P)H-PQ oxidoreductase in thylakoids of cyanobacteria and many eukaryotes may be involved in cyclic electron transport around PS I and in chlororespiration. Manganese is second to iron in its quantitative role in the thylakoids, with four Mn (and 1 Ca) per PS II involved in O₂ evolution. The roles of the transition metals in redox catalysts can in broad terms be related to their redox chemistry and to their availability to organisms at the time when the pathways evolved. The quantitative roles of these trace metals varies genotypically (e.g. the greater need for iron in thylakoid reactions of cyanobacteria and rhodophytes than in other O₂-evolvers as a result of their lower PS II:PS I ratio) and phenotypically (e.g. as a result of variations in PS II:PS I ratio with the spectral quality of incident radiation).     

The co-evolution of phytoplankton and trace element cycles in the oceans

The composition of the oceans and of its biota have influenced each other through Earth's history. Of all the biologically essential elements, nitrogen is the only one whose seawater concentration is clearly controlled biologically; this is presumably the main reason why the stoichiometry of nitrogen (defined as its mol ratio to phosphorus), but not that of the trace nutrients manganese, iron, cobalt, nickel, copper, zinc and cadmium, is the same in seawater and in the plankton. Like the major nutrients, the trace nutrients are depleted in surface seawater as a result of quasi-complete utilization by the biota. This is made possible in part by the ability of marine phytoplankton to replace one trace metal by another in various biochemical functions. This replacement also results in an equalization of the availability of most essential trace metals in surface seawater. The difference in the stoichiometric composition of the plankton and of deep seawater, which is the dominant source of new nutrients to the surface, indicates that some nutrients are likely recycled with different efficiencies in the photic zone. The difference in the composition of the ocean and its biota provides insight into the coupling of biochemistry and biogeochemistry in seawater.     

Isolation and identification of siderophores produced by cyanobacteria

Cyanobacteria are one of the most successful and oldest forms of life that are present on Earth. They are prokaryotic photoautotrophic microorganisms that colonize so diverse environments as soil, seawater, and freshwater, but also stones, plants, or extreme habitats such as snow and ice as well as hot springs. This diversity in the type of environment they live in requires a successful adaptation to completely different conditions. For this reason, cyanobacteria form a wide range of different secondary metabolites. In particular, the cyanobacteria living in both freshwater and sea produce many metabolites that have biological activity. In this review, we focus on metabolites called siderophores, which are low molecular weight chemical compounds specifically binding iron ions. They have a relatively low molecular weight and are produced by bacteria and also by fungi. The main role of siderophores is to obtain iron from the environment and to create a soluble complex available to microbial cells. Siderophores play an important role in microbial ecology; for example, in agriculture they support the growth of many plants and increase their production by increasing the availability of Fe in plants. The aim of this review is to demonstrate the modern use of physico-chemical methods for the detection of siderophores in cyanobacteria and the use of these methods for the detection and characterization of the siderophore-producing microorganisms. Using high-performance liquid chromatography-mass spectrometry (LC-MS), it is possible not only to discover new chemical structures but also to identify potential interactions between microorganisms. Based on tandem mass spectrometry (MS/MS) analyses, previous siderophore knowledge can be used to interpret MS/MS data to examine both known and new siderophores.     

Allelopathy of Aquatic Autotrophs

Allelopathy in aquatic environments may provide a competitive advantage to angiosperms, algae, or cyanobacteria in their interaction with other primary producers. Allelopathy can influence the competition between different photoautotrophs for resources and change the succession of species, for example, in phytoplankton communities. Field evidence and laboratory studies indicate that allelopathy occurs in all aquatic habitats (marine and freshwater), and that all primary producing organisms (cyanobacteria, micro- and macroalgae as well as angiosperms) are capable of producing and releasing allelopathically active compounds. Although allelopathy also includes positive (stimulating) interactions, the majority of studies describe the inhibitory activity of allelopathically active compounds. Different mechanisms operate depending on whether allelopathy takes place in the open water (pelagic zone) or is substrate associated (benthic habitats). Allelopathical interactions are especially common in fully aquatic species, such as submersed macrophytes or benthic algae and cyanobacteria. The prevention of shading by epiphytic and planktonic primary producers and the competition for space may be the ultimate cause for allelopathical interactions. Aquatic allelochemicals often target multiple physiological processes. The inhibition of photosynthesis of competing primary producers seems to be a frequent mode of action. Multiple biotic and abiotic factors determine the strength of allelopathic interactions. Bacteria associated with the donor or target organism can metabolize excreted allelochemicals. Frequently, the impact of surplus or limiting nutrients has been shown to affect the overall production of allelochemicals and their effect on target species. Similarities and differences of allelopathic interactions in marine and freshwater habitats as well as between the different types of producing organisms are discussed. Referee: Dr. Friedrich Jüttner, Universität Zürich-Limnologische Station, Institut für Pflanzen biologie, Universität Zürich, Seestrasse 187, Ch-8802 Klichberg ZH, Switzerland     

Exopolysaccharide-producing cyanobacteria in heavy metal removal from water: molecular basis and practical applicability of the biosorption process

Microorganisms can remove metals from the surrounding environment with various mechanisms, either as metabolically mediated processes or as a passive adsorption of metals on the charged macromolecules of the cell envelope. Owing to the presence of a large number of negative charges on the external cell layers, exopolysaccharides (EPS)-producing cyanobacteria have been considered very promising as chelating agents for the removal of positively charged heavy metal ions from water solutions, and an increasing number of studies on their use in metal biosorption have been published in recent years. In this review, the attention was mainly focused on the studies aimed at defining the molecular mechanisms of the metal binding to the polysaccharidic exocellular layers. Moreover, the few attempts done in the use of EPS-producing cyanobacteria for metal biosorption at pilot scale and with real wastewaters are here reviewed, discussing the main positive issues and the drawbacks so far emerging from these experiments.     

Levels, distribution and chemical forms of trace elements in food plants

The content of trace elements in plants can vary widely, depending upon the composition of the soil in which they grow, other environmental factors, and the species or cultivar of the plant. A high growth rate of the plant may cause internal 'dilution' of trace elements. Complex formation with soil organic colloids and compounds, cell wall material and ligands in and inside the cell membranes are of critical importance in uptake, though most evidence shows that it is the free metal ion in the external solution that is absorbed; the detailed mechanisms are still unknown. Other processes such as excretion of organic compounds, reductants and hydrogen ions from the root greatly alter availability of trace metals, and iron has to be reduced to the ferrous form before uptake. The mean composition of plant shoots is affected by age and season; element mobility in the xylem and phloem determines translocation, and hence concentrations in individual parts of the plant. The rate of retranslocation can be strongly affected by the abundance of the element. Symptoms of deficiency or excess are well documented, but are often not dependable. The essentiality of the trace metals depends upon their function as part of enzymes, and these are briefly reviewed, with stress on processes in plants. Only a small fraction of the total amount of an element is bound in the enzyme; of the remainder, some is present as the free metal ion (Mn) or as complexes of small molecular mass (Cu, Zn, Ni, Fe), the rest being bound to cell wall material. Certain species or genotypes have resistance against high levels of some elements in the soil. Several mechanisms may be involved, one being very strong binding to root cell walls. There are also large genetic differences in susceptibility to trace element deficiencies.     

Bioactive Compounds from Cyanobacteria and Microalgae: An Overview

ABSTRACT

Cyanobacteria (blue-green algae) are photosynthetic prokaryotes used as food by humans. They have also been recognized as an excellent source of vitamins and proteins and as such are found in health food stores throughout the world. They are also reported to be a source of fine chemicals, renewable fuel and bioactive compounds. This potential is being realized as data from research in the areas of the physiology and chemistry of these organisms are gathered and the knowledge of cyanobacterial genetics and genetic engineering increased. Their role as antiviral, anti-tumour, antibacterial, anti-HIV and a food additive have been well established. The production of cyanobacteria in artificial and natural environments has been fully exploited. In this review the use of cyanobacteria and microalgae, production processes and biosynthesis of pigments, colorants and certain bioactive compounds are discussed in detail. The genetic manipulation of cyanobacteria and microalgae to improve their quality are also described at length.     

Partitioning of trace metals in sediments: Relationships with bioavailability

As a result of complex physical, chemical and biological processes, a major fraction of the trace metals introduced into the aquatic environment is found associated with the bottom sediments, distributed among a variety of physico-chemical forms. As these different metal forms will generally exhibit different chemical reactivities, the measurement of the total concentration of a particular metal provides little indication of potential interactions with the abiotic or biotic components present in the environment. In principle, the partitioning of sediment-bound metals could be determined both by thermodynamic calculations (provided equilibrium conditions prevail) and by experimental techniques. The modelling of sediment-bound metals is far less advanced than is that of dissolved species, primarily because the thermodynamic data needed for handling sediment-interstitial water systems are not yet available. The partitioning of a metal among various fractions obtained by experimental techniques (e.g., sequential extraction procedures) is necessarily operationally defined. These methods have, however, provided significant insight into the physico-chemical factors influencing the bioavailability of particulate trace metals; some of these factors are discussed.     

Cell composition and metal tolerance in cyanobacteria

This review examines interactions between cyanobacteria and metals with an emphasis on metal tolerance in these organisms. Aspects of metal toxicity and accumulation in various cyanobacteria species as related to cell composition will also be reviewed.     

Seasonal Variation of Metals in Seawater, Sediment, and Manila Clam Ruditapes philippinarum from China

Concentrations of trace metals (Cu, Pb, Zn, Cd, Cr, Hg, and As) were determined for the first time in seawater, sediment, and Manila clam from Deer Island, Liaoning Province, China. The seawater, sediment, and clam samples were collected seasonally at three clam farming sites around Deer Island during 2010–2011. The average concentrations of Cu, Pb, Zn, Cd, Cr, Hg, and As in the seawater samples were 4.16, 0.72, 5.88, 0.45, 2.51, 0.03, and 1.02 μg/l, respectively. The seasonal variations of trace metals in seawater showed a significant difference in the concentrations of Cu, Pb, Zn, Hg, and As among seasons. The average concentrations of Cu, Pb, Zn, Cd, Cr, Hg, and As in the sediment samples were 6.43, 13.80, 53.08, 1.10, 36.40, 0.05, and 4.78 mg/kg dry weight, respectively. Trace metal concentrations in sediment seasonally varied significantly except for Cd and Hg. The average concentrations of Cu, Pb, Zn, Cd, Cr, Hg, and As in the clam samples were 11.28, 0.61, 92.50, 0.58, 3.98, 0.03, and 1.98 mg/kg dry weight, respectively. Concentrations of Cu, Zn, Cd, Cr, and As in Manila clam showed marked seasonal fluctuations with significant difference. Cu and Zn were the metals with the highest mean biosediment accumulation factor values in Manila clam. Besides, significant correlations for the concentrations of Cu and Zn relative to their concentrations in sediment were also found. Such differences in regression analyzes may be explained by differential bioaccumulation of essential and xenobiotic metals. Concentrations of trace metals in Manila clam did not exceed the maximum established regulatory concentrations for human consumption. Moreover, the calculations revealed that the estimated daily intake values for the examined clam samples were below the internationally accepted dietary guidelines and the calculated hazard quotient values were well less than 1, thus strongly indicating that health risk associated with the intake studied metals through the consumption of Manila clam from Deer Island was absent.     

Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an active biosphere. The Middle Island Sinkhole (MIS) in Lake Huron is an analogue for shallow Proterozoic waters due to its low oxygen water chemistry and microbial communities that exhibit diverse metabolic functions at the sediment–water interface. This study uses sediment trace metal contents and microbial abundances in MIS sediments and an oxygenated Lake Huron control site (LH) to infer mechanisms for trace metal burial. The adsorption of trace metals to Mn‐oxyhydroxides is a critical burial pathway for metals in oxic LH sediments, but not for the MIS mat and sediments, consistent with conventional understanding of Mn cycling. Micronutrient trace metals (e.g., Zn) are associated with organic matter regardless of oxygen and sulfide availability. Although U and V are conventionally considered to be organically complexed in suboxic and anoxic conditions, U and organic covary in oxic LH sediments, and Mn‐oxyhydroxide cycling dominates V deposition in the anoxic MIS sediments. Significant correlations between Mo and organic matter across all redox regimes have major implications for our interpretations of Mo isotope systematics in the geologic record. Finally, while microbial groups vary between the sampling locales (e.g., the cyanobacteria in the MIS microbial mat are not present in LH sediments), LH and MIS ultimately have similar relationships between microbial assemblages and metal burial, making it difficult to link trace metal burial to microbial metabolisms. Together, these results indicate that bulk sediment trace metal composition does not capture microbiological processes; more robust trace metal geochemistry such as isotopes and speciation may be critical for understanding the intersections between microbiology and sediment geochemistry.     

Sediments as a source for contaminants?

This review article covers the processes affecting trace metals in deposited sediments. The sediment-water system can be divided in three parts: the oxic layer, the anoxic layer and the oxic-anoxic interface. Available data show that trace metals like Cu, Zn and Cd occur as sulphides in marine and estuarine anoxic sediments. Calculations show that organic complexation is unlikely and the dominant species are sulphide and bisulphide complexes. Chromium and arsenic are probably present as adsorbed species on the sediments; their concentrations in the pore waters, therefore depend on the concentrations in the sediments. The oxic-anoxic interface plays the major role in the potential flux of trace metals from the sediments, however this interface is not well studied at present. Changes from an anoxic to an oxic environment as occurs during dredging and land disposal of contaminanted sediments may cause a remobilization of some trace metals.     

Selectivity in the heavy metal removal by exopolysaccharide-producing cyanobacteria

Aims:  The aim of this study was to assess the selective removal of Cu(II), Cr(III) and Ni(II) by strains of exopolysaccharide (EPS)-producing cyanobacteria, and to investigate the interaction of sorption in solutions with multiple-metals.
Methods and Results:  Nine EPS-producing cyanobacteria were tested for their ability to remove Cr, Cu and Ni in both single- and multiple-metal solutions. In the single-metal solutions, some of the strains showed very high values of metal uptake, however, only two of them showed the capability to selectively remove one or two of the metals present in the multiple-metal solutions. In multi-metal systems, the binding process was either noninteractive, synergistic or competitive between metal ions for different strains of cyanobacteria.
Conclusions:  Cyanothece 16Som 2 showed significantly greater sorptive capacity for Cu (1·5–20×) and Cr (2–50×) than all other strains tested. The Nostoc PCC 7936 strain showed high specific and almost exclusive selectivity towards Cu, which suggests its use aimed at recovering this metal from multiple-metal solutions.
Significance and Impact of the Study:  To find out microbial sorbents with good metal selectivity may be very useful for building up processes aimed to recover valuable metals from industrial wastewaters.     

Bioactive compounds from cyanobacteria and microalgae: an overview

Cyanobacteria (blue-green algae) are photosynthetic prokaryotes used as food by humans. They have also been recognized as an excellent source of vitamins and proteins and as such are found in health food stores throughout the world. They are also reported to be a source of fine chemicals, renewable fuel and bioactive compounds. This potential is being realized as data from research in the areas of the physiology and chemistry of these organisms are gathered and the knowledge of cyanobacterial genetics and genetic engineering increased. Their role as antiviral, anti-tumour, antibacterial, anti-HIV and a food additive have been well established. The production of cyanobacteria in artificial and natural environments has been fully exploited. In this review the use of cyanobacteria and microalgae, production processes and biosynthesis of pigments, colorants and certain bioactive compounds are discussed in detail. The genetic manipulation of cyanobacteria and microalgae to improve their quality are also described at length.     

Differences in energy transfer of a cyanobacterium,Synechococcus sp. PCC 7002, grown in different cultivation media

Photosynthesis Research - Currently, cyanobacteria are regarded as potential biofuel sources. Large-scale cultivation of cyanobacteria in seawater is of particular interest because seawater is a...