On Earth, the crustal abundance of vanadium (0.019%; 20th most abundant element) compares to that of zinc; however, vanadium is more dispersed than Zn. The common redox states in the minerals of the Earth’s crust are +V, +IV and +III ^[1]. Crude oil (asphaltenes) can obtain up to 5 g V per litre; the high concentrations here originate from the coordinative incorporation of vanadium (VO²⁺) into, essentially, porphinogens. The main sources of atmospheric vanadium are marine aerosols and continental dust originating from geological processes, and due to anthropogenic input, including emissions—VO₂, V₂O₅ and vanadium carbide—from combustion engines. In aqueous media, such as seawater, lakes and rivers, vanadium is present as H₂VO₄⁻/HVO₄²⁻ (depending on the pH); common average concentrations in marine environments (pH~7; where the dominant species is H₂VO₄⁻ (and HV₂O₇³⁻ at higher concentrations)) amount to 1.8 µg/L, in rivers to 0.7 µg/L. The average amount of vanadium in sediments is 150 mg/kg ^[2]. In mining areas, vanadium concentrations in aqueous environments can go up by a factor of 10⁶. In human blood plasma, vanadium concentrations are higher by a factor of ~10 with respect to seawater, pointing towards a (possibly essential) role of vanadium in life: In living organisms, vanadate H₂VO₄⁻ acts as an antagonist/competitor/enhancer of phosphate but tends to be toxic at non-physiological concentrations. Vanadate also forms binary and ternary compounds with carbonate, viz., HVO₃CO₃²⁻ and VO₂(CO₃)₂³⁻, with logK values of 1.09 and 0.17, respectively. At higher concentrations, oligomers such as the dimeric V₂O₇⁴⁻ can form; reducing conditions are responsible for the generation of V^IVO²⁺·aq (which precipitates to form VO(OH)₂ or remains dissolved due to coordination to (organic) substrates) and V^III·aq (present in, e.g., ascidians). The redox interconversion of vanadium (V^V ↔ V^IV ↔ V^III) in a natural environment is often monitored or exerted by specialised bacteria, such as Pseudomonas vanadiumreducens.

Besides various strains of bacteria, several multicellular organisms resort to vanadium as an essential element, the latter occasionally also in cooperation with bacteria. Noteworthy among these higher organisms are ascidians, marine brown algae and certain mosses, amavadin in Amanita mushrooms and the nodules in the roots of legumes.

2. Vanadium in Haloperoxidases, Nitrogenases, Amanita Mushrooms, Ascidians and Fan Worms

Along with the potentiality of vanadium in many catalytic applications carried out in the frame of industrial processes (examples are oxidation reactions, carbon–carbon bond formation, hydrogenation and dehydrogenation and cyanation [7]^[3]), the role of vanadium in naturally occurring catalytic processes is noteworthy. These functions include the oxidation of halides catalysed by haloperoxidases (particularly in seawater estuaries) on the one hand, and the reductive conversion of aerial dinitrogen into ammonia/ammonium ions (and hence nitrogen in a form utilisable in bio-physiological processes), catalysed by nitrogenases on the other hand. An additional naturally occurring vanadium compound is amavadin, present in the fly agaric mushroom. Specific brown and red algae, fungi and bacteria—such as the marine brown alga Ascophyllum nodosum, the marine flavobacterium Zobiella galactanivorans [8]^[4] and the cyanobacterium Synechocosccus (associated with macroalgae and specialised in the oxidation of iodide)—can have available vanadate-dependent haloperoxidases, one of the six known families of halogenating enzymes [9]^[5]. Actually, the VHPOs apparently derive from a species closely related to bacterial acid phosphatase [8]^[4], well in agreement with the structural similarity between phosphate and vanadate, and the competitive/comparable behaviour of these two anions in life processes. Iodide, bromide, chloride and cyanide may be subject to oxidation to hypohalite/hypohalous acid and cyanate, respectively. The halide specificity essentially reflects differences in hydrogen-bonding interaction between the active centre and the extended (second) coordination sphere. The oxidising agent commonly is H₂O₂/peroxide. Hypohalite in turn is involved in the biosynthesis of halogenated organic compounds, such as bromoform, which is released into the atmosphere. Figure 1 (left) shows the active centre of the vanadate-dependent iodo/bromoperoxidase of the marine brown alga Ascophyllum nodosum (Figure 1, right) from which this enzyme had been originally isolated [10]^[6] and characterised with respect to its reactivity [11]^[7]. The active centres of the enzyme present in other algal and bacterial species are essentially identical, i.e., they only differ in the second sphere amino acid surroundings of the central penta-coordinated {VO(OH)(O⁻)His} unit, and in H-bonding interaction between the first and the second coordination sphere. In summary, halogenases are involved in the oxidation of halides (to hypohalous acids) and, in such a way, in the halogenation of organic compounds and the oxidative elimination of (invasive) bacteria via the intermediate formation of reactive oxygen species [12]^[8]. A second vanadium-based enzyme is the vanadium nitrogenase VNase (Figure 2), which—along with the more prominent molybdenum variant and an additional “alternative” nitrogenase based on iron only—is present in nitrogen-fixing (soil) bacteria, such as the obligate anaerobic Azotobacter vinelandii [13]^[9], facultative anaerobes (e.g., Klebsiella oxytoca) [13]^[9] and the cyanobacterium Anabaena variabilis [14]^[10]; VNase can also be expressed in, e.g., symbionts of lichens [15]^[11]. In all of these diazotrophic organisms, V-Nases are predominantly activated under conditions of Mo limitation. The bacterium Rhodopseudomonas palustris, present—inter alia—in marine coastal sediments, has available all three (Mo, V and Fe-only) nitrogenases. Most N₂-fixing bacteria are obligatory anaerobes. The dominant function of this enzyme is the reductive conversion of inert dinitrogen to ammonium ions, Equation (1), thus making nitrogen accessible for (alimentary) utilisation by plants. The energy afforded for this reductive process, required to overcome a high activation energy barrier, is provided by adenosine triphosphate, ATP. The reduction of N₂ progresses via HN=NH and H₂N-NH₂. In the absence of N₂, H₂ is generated. Other substrates, such as carbon monoxide, are equally reduced, eventually forming alkenes and alkanes, Equation (2) [16]^[12]. Furthermore, CO₂ can be reduced to CH₄, Equation (3) [17]^[13]. The fly agaric mushroom Amanita muscaria (Figure 3) accumulates vanadium at concentrations of up to several hundred mg kg⁻¹ dry mass [18]^[14], with a maximum concentration found in the bulb. Other species of the genus Amanita can also store vanadium; examples are A. regalis and A. velatipes. Vanadium is present as the non-oxido V(IV) coordination compound amavadin (Figure 3) in the Ʌ and Δ isomeric forms. Its biological function has so far not yet been established with certainty. Due to the reversible oxidation/reduction (V^IV ⇆ V^V + e⁻), a role for amavadin as a redox catalyst is assumed: Amavadin homologues have been shown to mediate the reduction of NO₂⁻ to N₂O and the oxidation of water to O₂ [19]^[15]. Sea dwellers such as ascidians (e.g., Ascidia sydneiensis samea) and polychaeta fan worms (Pseudopotamilla [20]^[16], Perkinsiana [21]^[17]) accumulate vanadium from seawater. The fan worm P. occelata (Figure 4, left) contains vanadium bound to a nucleoside diphosphate kinase homolog, located in the epidermis of the branchial crown. The function of vanadium may be associated with the suppression of the activity of a kinase by V^IV (VO²⁺); the kinase otherwise remains unaffected by V^V (H₂VO₄⁻) [20]^[16]. Ascidians (also known as tunicates or sea squirts) such as Ascidia gemmata [21]^[17] and A. sydneiensis samea [22]^[18] accumulate vanadium in blood cells (the so-called vanadocytes; Figure 4, right). The actual vanadium concentration amounts to 350 mM, hence 10⁷ times that of vanadium (H₂VO₄⁻) in seawater. Bacterial genera (Pseudomonas and Ralstonia) in the branchial sac of the ascidia, and Treponema and Borelia in the intestinal content, appear to contribute to vanadium accumulation by absorption of vanadium through the epithelium of the branchial sac and reduction of vanadate to oxidovanadium(IV) and V³⁺ [23]^[19], the latter being stored in the cytoplasm and vacuoles termed vanadocytes. Vanadium is thus also involved in the redox interconversion of disulphide/dithiolate. In its non-oxido +IV state, vanadium binds to proteins rich in cysteine residues, referred to as vanabins. Vanabins can differ in the number of amino acids. For a biomimetic system modelling the reduction of vanadate (V) and coordination of non-oxido-V^IV to vanabin, see ref. [24]^[20].

3. Bacterial Issues

Matters concerning bacteria have already briefly been mentioned in ch. 3 in the context of N₂ fixation (see also cyanobacteria in rice fields [25]^[21]), haloperoxidases, amanita mushrooms and ascidians. Other bacterial activities aiming at redox interconversion (essentially reduction of vanadium(V)) concern several strains of bacteria—belonging to the eucarya—that are able to reduce vanadate H₂VO₄⁻ to oxidovanadium(IV) VO²⁺ (which commonly precipitates in the form of VO(OH)₂) and thus can contribute to the removal of vanadate(V) from drinking water. Vanadate—being a phosphate antagonist—is toxic at higher, non-physiological concentrations. Examples of these bacteria are Pseudomonas vanadiumreductans, Shewanella oneidensis, Geobacter metallireducens and Saccharomyces cerevisiae [26]^[22]. Reduction of vanadate to VO²⁺ can also be achieved by mesophilic bacteria (an archaeal bacterial strain) such as Methanosarcina mazei, which is active at 37 °C, and the thermophile Methanothermobacter termautropicus, which is thriving at an optimum temperature of 65 °C [27]^[23] and further—at concentrations < 5 mM, by Thiobacillus thiooxidans [25]^[21]. This bio-reduction of V^V to V^IV (VO(OH)₂↓) in a special anoxic growth medium inhibits methanogenesis (CO₂/acetate →→ CH₄, the reductant usually is H₂), the otherwise common domain of activity for these bacteria.