Search

Phytoplankton blooms can cause acute effects on marine ecosystems due either to their production of endogenous toxins or to their enormous biomass leading to major impacts on local economies and public health. Despite years of effort, the causes of these Harmful Algal Blooms are still not fully understood. Our hypothesis is that bacteria that produce photoactive siderophores may provide a bioavailable source of iron for phytoplankton which could in turn stimulate algal growth and support bloom dynamics. Here we correlate iron concentrations, phytoplankton cell counts, bacterial cell abundance, and copy numbers for a photoactive siderophore vibrioferrin biosynthesis gene in water samples taken from 2017 cruises in the Gulf of California, and the Pacific Ocean off the coast of northern Baja California as well as during a multiyear sampling at Scripps Pier in San Diego, CA. We find that bacteria producing the photoactive siderophore vibrioferrin, make up a surprisingly high percentage of total bacteria in Pacific/Gulf of California coastal waters (up to 9%). Vibroferrin's unique properties and the widespread prevalence of its bacterial producers suggest that it may contribute significantly to generating bioavailability of iron via photoredox reactions.

The reactivity of the labile cluster Os3(CO)10(MeCN)2 (1) with the monofunctionalized heterocyclic ligands 6-R-2,2′-bipyridine (where R = Et, Ph) has been investigated. The alkyl-substituted heterocycle 6-Et-2,2′-bipyridine reacts with 1 in refluxing CH2Cl2 to give an isomeric mixture of HOs3(CO)9(N2C12H11) due to cyclometalation of the side-chain ethyl group (2) and ortho metalation of the unsubstituted bipyridine ring (3). The solid-state structure of the latter cluster, HOs3(CO)9(N2C10H6-6-Et) (3), has unequivocally established the site of the C-H bond activation in the product. Treatment of 1 with the aryl-substituted ligand 6-Ph-2,2′-bipyridine proceeds similarly with ortho metalation at the ancillary phenyl group and the C-6′ ortho site of the unsubstituted bipyridine ring, as verified by 1H NMR spectroscopy. The X-ray diffraction structure of the thermodynamically more stable bipyridine-metalated cluster HOs3(CO)9(N2C10H6-6-Ph) (5) has been determined. The course of these reactions is discussed with respect to our recent study involving the reaction of cluster 1 with the ligand 6-Me-2,2′-bipyridine. The reaction between the labile cluster Os3(CO)10(MeCN)2 (1) and the monofunctionalized heterocyclic ligand 6-Et-2,2′-bipyridine proceeds readily at room temperature to furnish an isomeric mixture of the cyclometalated and ortho-metalated hydride-bridged clusters HOs3(CO)9(N2C12H11) (2 and 3). Treatment of 1 with 6-Ph-2,2′-bipyridine also yields two distinct hydride-containing clusters that result from independent ortho-metalation paths involving the 6-phenyl substituent and unsubstituted bipyridine group. The bipyridine-derived ortho metalation attendant in the new clusters HOs3(CO)9(N2C10H6-6-Et) (3) and HOs3(CO)9(N2C10H6-6-Ph) (5) has been established by X-ray crystallography.

Reaction of the Zn, Cd, or Co nitrate salts with the deprotonated ligand (2-hydroxy-3-t-butyl-methylphenyl)bis(3,5-dimethylpyrazolyl)methane (L1O−) in methanol produced the following complexes: [(L1OH)Zn(NO3)2] in two isomorphs, a = 40.983(8) A, b = 9.571(2) A, c = 15.667(8) A, α = 90○, β = 106.38(1)○, γ = 90○, C2/c, and a = 13.027(3) A, b = 14.781(4) A, c = 16.107(3) A, α = 90○, β = 105.30(1)○, γ = 90○, P21/n; [(L1OH)Cd(pz)(NO3)2] a = 14.7476(2) A, b = 13.5411(2) A, c = 16.7223(2) A, α = 90○, β = 110.3840(10)○, γ = 90○, P21/c; and [(L1O)Co(pz)(NO3)] a = 11.4240(2) A, b = 13.4498(2) A, c = 13.8056(2) A, α = 105.2080(10)○, β = 105.8130(10)○, γ = 112.7470(10)○, P\({\bar 1}\). The Zn adopts a pseudotetrahedral four-coordinate geometry where the potentially tridentate ligand is actually bidentate with a protonated and uncoordinated phenoxy arm. The Co complex is pseudooctahedral six-coordinate where the phenoxy arm is deprotonated and coordinated. Finally the Cd complex is seven-coordinate but the metal is not coordinated through the phenoxy group that is again protonated.

We report here on a new class of siderophores isolated from Rhodococcus erythropolis IGTS8, the first structurally characterized from any species of Rhodococcus and for which we suggest the name heterobactins. These siderophores consist of a tripeptide of sequence (N-OH)-L-Orn-Gly-D-Orn-(δ-N-dihydroyxbenzoate). The alpha amino group of the D-Orn is derivatized either as a 2-hydroxybenzoxazolate in heterobactin A or remains free in heterobactin B. The structures were determined by a combination of amino acid analysis, mass spectrometry and NMR methods. The two new compounds are true siderophores in that they relieve iron limited growth in the producing strain. The heterobactins are also transported by other non-producing bacteria. Growth promotion tests using various transport mutants revealed that in E. coli heterobactin A is only recognized by the catecholate receptor Cir while heterobactin B is taken up in both E. coli and A. flavescens JG9 via a hydroxamate transport system.

The photoreactivep-azidobenzoyl analog of ferrioxamine B was used to show that ferrioxamine-B-mediated iron transport is separate and distinct from coprogen-mediated iron transport inEscherichia coli. Photolysis of this analog inhibited uptake of [59Fe]ferrioxamine B but not [59Fe]coprogen or [59Fe]ferrichrome. Conversely, photolysis of thep-azidobenzoyl analog of coprogen B inhibited uptake of [59Fe]coprogen but not [59Fe]ferrioxamine B or [59Fe]ferrichrome. Photolabeling of outer membranes withp-azidobenzoyl-[59Fe]ferrioxamine B resulted in the labeling of two iron-regulated peptides with molecular masses of about 66 and 26 kDa. Expression of these peptides was increased when ferrioxamine B was the sole iron source. Both peptides were present in outer membrane preparations of thefhuF mutant H1717, but the 66 kDa peptide was not inducible. These results are evidence for an outer membrane receptor inE. coli unique for linear ferrioxamines.

Rhizoferrin-mediated iron uptake was studied in two different classes of organisms: a rhizoferrin producing fungus, Absidia spinosa (Zygomycetes), and a ferric rhizoferrin utilizing bacterium, Morganella morganii (Enterobacteriaceae). The uptake of iron rhizoferrin and some of its metal analogs (chromium, rhodium, gallium), was followed and kinetic parameters measured in A. spinosa. These metal ion complexes were taken up in a concentration- and energy-dependent manner indicative of an active transport system. The uptake of the kinetically inert chromium and rhodium and reductively inert gallium complexes suggests a variation of the so called ‘shuttle’ mechanism may be operative. The recognition of one geometrical isomer of chromium-rhizoferrin but not another argues for a degree of stereospecificity in the uptake process. A growth promotion plate assay was used to examine metal-rhizoferrin uptake in M. morganii. The results indicate that a number of factors including the nature of the chelating agent (e.g. bipyridyl or EDDHA) used to induce iron deficiency need to be considered before these simple plate assays can be reliably used to indicate the presence or absence of a particular siderophore uptake system.

Several derivatives of the coprogen and ferrichrysin classes of siderophores were synthesized as potential affinity labels of the iron uptake system inNeurospora crassa. While only one of these compounds has proved useful as an affinity label, all were recognized and transported byNeurospora crassa. One derivative, chloroacetyl-ferrichrysin, proved to be an unexpectedly potent reversible inhibitor (K1=0.4 μM) of both ferrichrysin and coprogen uptake, similar to the natural siderophore, ferrirubin. The reported results provide further understanding of the steric and electronic requirements of siderophores for the iron uptake system inNeurospora crassa.