Metal–organic complexation in the marine environment

We discuss the voltammetric methods that are used to assess metal–organic complexation in seawater. These consist of titration methods using anodic stripping voltammetry (ASV) and cathodic stripping voltammetry competitive ligand experiments (CSV-CLE). These approaches and a kinetic approach using CSV-CLE give similar information on the amount of excess ligand to metal in a sample and the conditional metal ligand stability constant for the excess ligand bound to the metal. CSV-CLE data using different ligands to measure Fe(III) organic complexes are similar. All these methods give conditional stability constants for which the side reaction coefficient for the metal can be corrected but not that for the ligand. Another approach, pseudovoltammetry, provides information on the actual metal–ligand complex(es) in a sample by doing ASV experiments where the deposition potential is varied more negatively in order to destroy the metal–ligand complex. This latter approach gives concentration information on each actual ligand bound to the metal as well as the thermodynamic stability constant of each complex in solution when compared to known metal–ligand complexes. In this case the side reaction coefficients for the metal and ligand are corrected. Thus, this method may not give identical information to the titration methods because the excess ligand in the sample may not be identical to some of the actual ligands binding the metal in the sample

Evolutionary genomics of a cold-adapted diatom: Fragilariopsis cylindrus

The Southern Ocean houses a diverse and productive community of organisms1, 2. Unicellular eukaryotic diatoms are the main primary producers in this environment, where photosynthesis is limited by low concentrations of dissolved iron and large seasonal fluctuations in light, temperature and the extent of sea ice3, 4, 5, 6, 7. How diatoms have adapted to this extreme environment is largely unknown. Here we present insights into the genome evolution of a cold-adapted diatom from the Southern Ocean, Fragilariopsis cylindrus8, 9, based on a comparison with temperate diatoms. We find that approximately 24.7 per cent of the diploid F. cylindrus genome consists of genetic loci with alleles that are highly divergent (15.1 megabases of the total genome size of 61.1 megabases). These divergent alleles were differentially expressed across environmental conditions, including darkness, low iron, freezing, elevated temperature and increased CO2. Alleles with the largest ratio of non-synonymous to synonymous nucleotide substitutions also show the most pronounced condition-dependent expression, suggesting a correlation between diversifying selection and allelic differentiation. Divergent alleles may be involved in adaptation to environmental fluctuations in the Southern Ocean

Atmospheric and marine controls on aerosol iron solubility in seawater

The fraction of atmospherically deposited iron which dissolves in seawater, or becomes available to phytoplankton for growth, is a key determinant of primary productivity in many open ocean regions. As such this parameter plays an important part in the global oceanic cycles of iron and carbon, and yet the factors that control iron dissolution from aerosol are very poorly understood. In this manuscript we seek to synthesise the available knowledge of these factors, which operate in the atmosphere and in seawater. A conceptual model of the overall aerosol iron solubility is presented, in which we liken the various controls on iron solubility to sets of parallel electrical resistors. We also discuss experimental methods for the determination of iron solubility and make recommendations for future studies in this area

Effects of iron surface adsorption and sample handling on iron solubility measurements

Seawater samples from two separate cruises in the Southern Ocean (ANTXXI/3 (EIFeX) and ANTXXIII/9) were collected for measurements of iron solubility by 55Fe addition. For both sets of samples, a significant loss of the dissolved portion of the added Fe was observed during the 72 hour duration of each Fe solubility measurement incubation. The decrease in dissolved Fe was related to Fe precipitation and adsorption onto bottle walls. The dissolved Fe data can be successfully modeled assuming that two colloidal Fe species (organically complexed Fe and inorganic Fe) were quickly formed following the addition of dissolved Fe(III) to the seawater. Model results indicate that Fe dissociated from weak organic complexes was the main contributor to wall sorption during the first 6 h following Fe addition, and that most of the Fe deposited after the first 6 h arose from the dissociation of colloidal inorganic species. Effects of sample freezing on Fe solubility measurements are also discussed