Spatial variability of aerosol iron mineralogy and oxidation states over the Arctic Ocean.

The mineralogy and oxidation state of aerosol iron (Fe) play important roles in controlling aerosol Fe solubility and consequent bioavailability in seawater. In this study, the spatial variability of Fe mineralogy and oxidation states in aerosols collected during the US GEOTRACES Western Arctic cruise (GN01) were determined using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy. Both Fe(II) minerals (biotite, ilmenite) and Fe(III) minerals (ferrihydrite, hematite, Fe(III) phosphate) were found in these samples. However, aerosol Fe mineralogy and solubility observed during this cruise varied spatially and can be grouped into three clusters based on the air masses that affected aerosols collected in different regions: (1) biotite-enriched particles (87 % biotite, 13 % hematite) with the air masses passing over Alaska, showing relatively low Fe solubility (4.0 ± 1.7 %); (2) ferrihydrite-enriched particles (82 % ferrihydrite, 18 % ilmenite) collected in the remote Arctic air, showing relatively high Fe solubility (9.6 ± 3.3 %); (3) the fresh dust derived from North America and Siberia, primarily dominated by hematite (41 % hematite, 25 % Fe(III) phosphate, 20 % biotite, 13 % ferrihydrite), showing relatively low Fe solubility (5.1 ± 3.5). A significant positive correlation was found between Fe oxidation state and Fe fractional solubility, suggesting that long-range transport could modify iron (hydr) oxide such as ferrihydrite through atmospheric processing, influencing aerosol Fe solubility and consequently Fe bioavailability in the remote Arctic Ocean.

Influence of Atmospheric Processes on the Solubility and Composition of Iron in Saharan Dust.

Aerosol iron was examined in Saharan dust plumes using a combination of iron near-edge X-ray absorption spectroscopy and wet-chemical techniques. Aerosol samples were collected at three sites located in the Mediterranean, the Atlantic, and Bermuda to characterize iron at different atmospheric transport lengths and time scales. Iron(III) oxides were a component of aerosols at all sampling sites and dominated the aerosol iron in Mediterranean samples. In Atlantic samples, iron(II and III) sulfate, iron(III) phosphate, and iron(II) silicates were also contributors to aerosol composition. With increased atmospheric transport time, iron(II) sulfates are found to become more abundant, aerosol iron oxidation state became more reduced, and aerosol acidity increased. Atmospheric processing including acidic reactions and photoreduction likely influence the form of iron minerals and oxidation state in Saharan dust aerosols and contribute to increases in aerosol-iron solubility.

Role of biogenic silica in the removal of iron from the Antarctic seas.

Iron has a key role in controlling biological production in the Southern Ocean, yet the mechanisms regulating iron availability in this and other ocean regions are not completely understood. Here, based on analysis of living phytoplankton in the coastal seas of West Antarctica, we present a new pathway for iron removal from marine systems involving structural incorporation of reduced, organic iron into biogenic silica. Export of iron incorporated into biogenic silica may represent a substantial unaccounted loss of iron from marine systems. For example, in the Ross Sea, burial of iron incorporated into biogenic silica is conservatively estimated as 11 µmol m⁻² per year, which is in the same range as the major bioavailable iron inputs to this region. As a major sink of bioavailable iron, incorporation of iron into biogenic silica may shift microbial population structure towards taxa with relatively lower iron requirements, and may reduce ecosystem productivity and associated carbon sequestration.

Fluorometric quantification of natural inorganic polyphosphate.

Polyphosphate, a linear polymer of orthophosphate, is abundant in the environment and a key component in wastewater treatment and many bioremediation processes. Despite the broad relevance of polyphosphate, current methods to quantify it possess significant disadvantages. Here, we describe a new approach for the direct quantification of inorganic polyphosphate in complex natural samples. The protocol relies on the interaction between the fluorochrome 4',6-diamidino-2-phenylindole (DAPI) and dissolved polyphosphate. With the DAPI-based approach we describe, polyphosphate can be quantified at concentrations ranging from 0.5-3 microM P in a neutral-buffered freshwater matrix with an accuracy of +/-0.03 microM P. The patterns of polyphosphate concentration versus fluorescence yielded by standards exhibit no chain length dependence across polyphosphates ranging from 15-130 phosphorus units in size. Shorter length polyphosphate molecules (e.g., polyphosphate of three and five phosphorus units in length) contribute little to no signal in this approach, as these molecules react only slightly or not at all with DAPI in the concentration range tested. The presence of salt suppresses fluorescence from intermediate polyphosphate chain lengths (e.g., 15 phosphorus units) at polyphosphate concentrations ranging from 0.5-3 microM P. For longer chain lengths (e.g., 45-130 phosphorus units), this salt interference is not evident at conductivities up to approximately 10mS/cm. Our results indicate that standard polyphosphates should be stored frozen for no longer than 10-15 days to avoid inconsistent results associated with standard degradation. We have applied the fluorometric protocol to the analysis of five well-characterized natural samples to demonstrate the use of the method.