Geoffrey J. Smith

Developing standards for dissolved iron in seawater

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.

Dissolved trace element cycles in the San Francisco Bay estuary

Dissolved trace element (copper, nickel, cadmium, zinc, cobalt, and iron) concentrations were measured in surface water samples collected from 27 stations in the San Francisco Bay and Sacramento—San Joaquin Delta during April, August and December of 1989. The trace element distributions were relatively similar for all three sampling periods, and evidenced two distinct biogeochemical regimes within the estuarine system. The two regimes were comprised of relatively typical trace element gradients in the northern reach and anthropogenically perturbed gradients in the southern reach of the estuary. These dichotomous trace element distributions were consistent with previous reports on the distributions of nutrients and some other constituents within the estuary. In the northern reach, trace element and dissolved phosphate concentrations were non-conservative. Simple estuarine mixing models indicated substantial internal sources of dissolved copper (46–150%), nickel (250–500%) and cadmium (630–780%) relative to riverine inputs in April and August, and sizable internal sinks for dissolved cobalt (> 99%) and iron (> 70%) during the same periods. Dissolved zinc fluxes varied temporally, with a relatively large (135%) internal source in April and a relatively small (29%) internal sink in August. Concentrations of many trace elements (copper, nickel, cadmium, zinc, and cobalt) in the southern reach were anomalously high relative to concentrations at comparable salinities in the northern reach. Mass balance calculations indicated that those excesses were primarily due to anthropogenic inputs (waste-water discharges and urban runoff) and diagenetic remobilization from benthic sediments. The magnitude of these excesses was amplified by the long hydraulic residence time of dissolved constituents within the South Bay. The influence of other factors was evident throughout the system. Notably, upwelling appeared to elevate substantially dissolved cadmium concentrations at the mouth of the estuary and authigenic flocculation appeared to dominate the cycling of dissolved iron in both the northern and southern reaches of the system. Biological scavenging, geochemical scavenging and diagenic remobilization were also found to be important in different parts of the estuary. Additional complementary information is required to quantify accurately these processes.

Collection and detection of natural iron-binding ligands from seawater

Abstract Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms, including phytoplankton, which are responsible for the primary productivity in the worlds oceans. However, due to the low solubility of Fe in seawater, phytoplankton are often limited by their inability to incorporate enough Fe to allow for optimal growth rates in regions with dissolved Fe concentrations below 1 nM. It has been postulated that certain phytoplankton may produce compounds to facilitate the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (>99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, origin, and chemical characteristics of these organic chelates are largely unknown. Although it has been implied that some components of natural Fe-binding ligands are siderophores, no direct analyses of such compounds from natural seawater have been conducted. Here, we present a simple solid-phase extraction technique employing Biobeads SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from large volumes (>200 l) of seawater. Additionally, we report on the first successful determination of molecular weight size classes and preliminary iron-binding functional group characterization within those size classes for isolates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affinities (with respect to inorganic iron—Fe′) of K FeL,Fe′ cond =10 11.5 –10 11.9 M −1 , similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In addition, 63% of the extracted compounds from surface-collected samples fall within the defined size range of siderophores (300–1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophores extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-binding functional groups consistent with biologically produced siderophores. These results provide further insight into characteristics of the Fe-binding ligands that are thought to be important in controlling the biological availability of Fe in the oceans.

The distribution of colloidal and particulate bioactive metals in Narragansett Bay, RI

Abstract The bioactive metals Fe, Mn, Cu, Zn and Ni in Narragansett Bay, RI, were partitioned into soluble, colloidal and particulate size fractions using a combination of conventional and cross-flow filtrations. Particulate samples (0.2–8.0 μm; >8 μm) were chemically fractionated into acetic-acid reactive and non-reactive metals. Conventional “dissolved” samples ( Mn>Zn>Cu>Ni with concentrations in the 0.2–0.8 μm fraction being generally higher than in the >8.0 μm fraction. The acid leachable fraction of the particulate phase increased from ∼32%–80% in the order Fe 8.0 μm) being generally less labile than small particulates (0.2–8.0 μm). The colloidal phase represented an average 4%–96% of the “dissolved” metals, ranging in importance from Fe (96%)>Cu (44%)>Ni (25%)>Zn (7%)>Mn (4%). Although generally small, the colloidal fraction of Zn and Mn was highest in a region of the bay where biomass typically is high. Changes in soluble and colloidal fractions along a transect through the bay indicate that a significant proportion of Fe, Cu and Ni were transferred from “dissolved” to particulate size fractions via colloid aggregation. Predicting colloidal metal concentrations from measurements of particulate mass ( C p ) and literature values of colloid metal partition coefficients ( K c ) underestimated the measured concentrations by 5–50×. Acetic acid leachable metal concentrations in the small particle (0.2–8 μm) phase correlated well with metal concentrations in the larger (8 kDa–0.2 μm) colloid fraction ( r =0.91–0.99). In contrast, metals in the smaller colloid fraction (1–8 kDa) were for the most part independent of any measured parameters. Metals were not distributed equally between colloidal size classes; colloidal Zn was associated with larger colloids (>90%), Fe and Ni were associated primarily with larger colloids (∼70–85%) but also with the smaller colloid fraction, while ≳70% of colloidal Cu was associated with smaller colloids. The non-uniform distribution of metals within colloidal size classes indicates that metal:colloid associations are regulated by specific interactions. These findings suggest that it is inappropriate to employ single, non-specific sorbing metal tracers (e.g. Th) to delimit the pathways and kinetics of bioactive metal interactions with marine colloids.

Silver in San Francisco Bay Estuarine Waters

Spatial gradients of silver concentrations in the surface waters of San Francisco Bay reveal substantial anthropogenic perturbations of the biogeochemical cycle of the element throughout the estuarine system. The most pronounced perturbations are in the south bay, where dissolved (<0.45 μm) silver concentrations are as high as 250 pM. This is more than one order-of-magnitude above baseline concentrations in the northern reach of the estuary (6 pM) and approximately two orders-of-magnitude above natural concentrations in adjacent coastal waters (3 pM). The excess silver is primarily attributed to wastewater discharges of industrial silver to the estuary on the order of 20 kg d−1. The contamination is most evident in the south bay, where wastewater discharges of silver are on the order of 10 kg d−1 and natural freshwater discharges are relatively insignificant. The limited amount of freshwater flushing in the south bay was exacerbated by persistent drought conditions during the study period. This extended the hydraulic residence time in the south bay (≥160 d), and revealed the apparent seasonal benthic fluxes of silver from anthropogenically contaminated sediments. These were conservatively estimated to average ≈16 nmol m−2 d−1 in the south bay, which is sufficient to replace all of the dissolved silver in the south bay within 22 d. Benthic fluxes of silver throughout the estuary were estimated to average ≈11 nmol m−2 d−1, with an annual input of approximately 540 kg yr−1 of silver to the system. This dwarfs the annual fluvial input of silver during the study period (12 kg yr−1) and is equivalent to approximately 10% of the annual anthropogenic input of silver to the estuary (3,700–7,200 kg yr−1). It is further speculated that benthic fluxes of silver may be greater than or equal to waste water fluxes of silver during periods of intense diagenic remobilization. However, all inputs of dissolved silver to the estuary are efficiently sorbed by suspended particulates, as evidenced by the relatively constant conditional distribution coefficient for silver throughout the estuary (Kd≈105).