Manganese co-limitation of phytoplankton growth and major nutrient drawdown in the Southern Ocean.

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. Variability in utilization alters oceanic CO2 sequestration at glacial-interglacial timescales. The role for insufficient iron has been examined in detail, but manganese also has an essential function in photosynthesis and dissolved concentrations in the Southern Ocean can be strongly depleted. However, clear evidence for or against manganese limitation in this system is lacking. Here we present results from ten experiments distributed across Drake Passage. We found manganese (co-)limited phytoplankton growth and macronutrient consumption in central Drake Passage, whilst iron limitation was widespread nearer the South American and Antarctic continental shelves. Spatial patterns were reconciled with the different rates and timescales for removal of each element from seawater. Our results suggest an important role for manganese in modelling Southern Ocean productivity and understanding major nutrient drawdown in glacial periods.

Collision-induced dissociation of three groups of hydroxamate siderophores: ferrioxamines, ferrichromes and coprogens/fusigens.

The behaviour of a series of hydroxamate siderophores--microbially produced iron complexes - was investigated using electrospray ionisation mass spectrometry (ESI-MS). Three groups of iron hydroxamate siderophores, namely the ferrioxamines, ferrichromes and coprogens/fusigens, were separated by high-performance liquid chromatography (HPLC) prior to ESI and MS(2) fragmentation. For the majority of the siderophores, both protonated molecules and sodium adducts were observed. The most abundant ion was selected for collision-induced fragmentation. Potential fragmentation mechanisms are postulated and discussed. Fragmentation patterns differed between siderophore groups; however, common fragmentation patterns were observed for siderophore ions within the groups examined. Cleavage frequently occurred at carbon-nitrogen or carbon-oxygen bonds. Fragmentation of the ions also involved cleavage of iron-oxygen bonds and transfer of the charge to iron.

Hydroxamate siderophores: occurrence and importance in the Atlantic Ocean.

Siderophores are chelates produced by bacteria as part of a highly specific iron uptake mechanism. They are thought to be important in the bacterial acquisition of iron in seawater and to influence iron biogeochemistry in the ocean. We have identified and quantified two types of siderophores in seawater samples collected from the Atlantic Ocean. These siderophores were identified as hydroxamate siderophores, both ferrioxamine species representative of the more soluble marine siderophores characterized to date. Ferrioxamine G was widely distributed in surface waters throughout the Atlantic Ocean, while ferrioxamine E had a more varied distribution. Total concentrations of the two siderophores were between 3 and 20 pM in the euphotic zone. If these compounds are fully complexed in seawater, they represent approximately 0.2-4.6% of the <0.2 microm iron pool. Our data confirm that siderophore-mediated iron acquisition is important for marine heterotrophic bacteria and indicate that siderophores play an important role in the oceanic biogeochemical cycling of iron.

Octanol-solubility of dissolved and particulate trace metals in contaminated rivers: implications for metal reactivity and availability.

The lipid-like, amphiphilic solvent, n-octanol, has been used to determine a hydrophobic fraction of dissolved and particulate trace metals (Al, Cd, Co, Cu, Mn, Ni, Pb, Zn) in contaminated rivers. In a sample from the River Clyde, southwest Scotland, octanol-solubility was detected for all dissolved metals except Co, with conditional octanol-water partition coefficients, D(ow), ranging from about 0.2 (Al and Cu) to 1.25 (Pb). In a sample taken from the River Mersey, northwest England, octanol-solubility was detected for dissolved Al and Pb, but only after sample aliquots had been spiked with individual ionic metal standards and equilibrated. Spiking of the River Clyde sample revealed competition among different metals for hydrophobic ligands. Metal displacement from hydrophobic complexes was generally most significant following the addition of ionic Al or Pb, although the addition of either of these metals had little effect on the octanol-solubility of the other. In both river water samples hydrophobic metals were detected on the suspended particles retained by filtration following their extraction in n-octanol. In general, particulate Cu and Zn (up to 40%) were most available, and Al, Co and Pb most resistant (<1%) to octanol extraction. Distribution coefficients defining the concentration ratio of octanol-soluble particle-bound metal to octanol-soluble dissolved metal were in the range 10(3.3)-10(5.3)mlg(-1). The presence of hydrophobic dissolved and particulate metal species has implications for our understanding of the biogeochemical behaviour of metals in aquatic environments. Specifically, such species are predicted to exhibit characteristics of non-polar organic contaminants, including the potential to penetrate the lipid bilayer. Current strategies for assessing the bioavailability and toxicity of dissolved and particulate trace metals in natural waters may, therefore, require revision.

Hydrophobicity and octanol-water partitioning of trace metals in natural waters.

The hydrophobicities of dissolved Al, Cu, Mn, and Pb have been determined in various contaminated natural water samples by 1-octanol extraction and C18 column retention. Octanol extraction varied among the metals studied and between the environments sampled but, in general, was greatest for Pb, whose conditional octanol-water partition coefficient, Dow, exceeded unit value in some samples. In most cases, metal partition into octanol either increased with increasing pH or exhibited a maximum under near-neutral conditions. Although the order and pH-dependence of metal retention by the C18 columns was consistent with these observations, the extent of retention was generally greater than the extent of metal extraction by octanol, possibly because of interferences effected by the C18 column matrix. Speciation calculations and results of controlled experiments employing metals in the presence of model ligands suggest that metals may become hydrophobic either by neutralizing relatively hydrophilic ligands or by combining with ligands that are intrinsically hydrophobic themselves. Given that octanol solubility affords an upper estimate of lipophilicity, the results of this investigation may have important implications regarding our understanding of metal bioavailability and toxicity in natural waters.