Eike Breitbarth

Considerations for environmental fate and ecotoxicity testing to support environmental risk assessments for engineered nanoparticles

There is an increasing concern over the safety of engineered nanoparticles (ENPs) to humans and the environment and it is likely that the environmental risks of these particles will have to be tested under regulatory schemes such as REACH. Due to their unique properties and the fact that their detection and characterisation in complex matrices is challenging, existing analytical methods and test approaches for assessing environmental risk may not be appropriate for ENPs. In this article we discuss the challenges associated with the testing of ENPs to generate data on persistence, mobility, bioavailability and ecotoxicity in the environment. It is essential that careful consideration is given to the selection of the test material, the test system (including test vessels and study media) and the test exposure conditions. During a study it is critical that not only the concentration of the ENP is determined but also its characteristics (e.g. size, shape, degree of aggregation and dissolution). A range of analytical techniques is available including microscopy-based approaches (e.g transmission and scanning electron microscopy), dynamic light scattering, and size separation approaches (e.g. field flow fractionation and hydrodynamic chromatography) coupled to detection methods such as inductively coupled plasma MS. All of these have their disadvantages: some are unable to distinguish between ENPs and natural interferences; some techniques require sample preparation approaches that can introduce artefacts; and others are complex and time-consuming. A combination of techniques is therefore needed. Our knowledge in this area is still limited, and co-ordinated research is required to gain a better understanding of the factors and processes affecting ENP fate and effects in the environment as well as to develop more usable, robust and sensitive methods for characterisation and detection of ENPs in environmental systems.

Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current

Significance Silica-shelled diatoms dominate marine phytoplankton blooms and play a key role in ocean ecology and the global carbon cycle. We show how differences in ecological traits of dominant Southern Ocean diatom species, observed during the in situ European Iron Fertilization Experiment (EIFEX), can influence ocean carbon and silicon cycles. We argue that the ecology of thick-shelled diatom species, selected for by heavy copepod grazing, sequesters silicon relative to other nutrients in the deep Southern Ocean and underlying sediments to the detriment of diatom growth elsewhere. This evolutionary arms race provides a framework to link ecology with biogeochemistry of the ocean. Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.

Regeneration of Fe(II) during EIFeX and SOFeX

Investigations into Fe(II) cycling during two Southern Ocean mesoscale iron enrichment experiments, SOFeX and EIFeX, clearly show the importance of Fe(II) to iron speciation during these experiments. In both cases the added Fe(II) persisted significantly longer than its expected oxidation time indicating a significant Fe reduction process at work. During EIFeX diel studies showed a strong photochemically induced cycle in Fe(II) production in sunlit surface waters. Our results suggest that the photochemical cycling of iron may also be important in unfertilized waters of the Southern Ocean.

Influence of salinity and organic matter on the toxicity of Cu to a brackish water and marine clone of the red macroalga Ceramium tenuicorne

Cu is a major active component in anti-fouling paints, which may reach toxic levels in areas with intense boat traffic and therefore is a metal of environmental concern. The bioavailability of metals is influenced by factors such as salinity and organic matter measured as total organic carbon (TOC). The influence of these two factors was studied, with a focus on brackish water conditions, by exposing a marine and a brackish water clone of the red macroalga Ceramium tenuicorne to Cu in different combinations of artificial seawater (salinity 5-15‰) and TOC (0-4 mg/L) in the form of fulvic acid (FA). In addition, the toxicity of Cu to both clones was compared in salinity 10‰ and 15‰. The results show that by increasing TOC from 0 to 2 and 4 mg/L, Cu was in general less toxic to both algal clones at all salinities tested (p<0.05). The effect of salinity on Cu toxicity was not as apparent, both a positive and negative effect was observed. The brackish water clone showed generally to be more sensitive to Cu in salinity 10‰ and 15‰ than the marine counterpart. In conclusion, FA reduced the Cu toxicity overall. The Cu tolerance of both strains at different salinities may reflect their origin and their adaptations to marine and brackish water.

The Growth Response of Two Diatom Species to Atmospheric Dust from the Last Glacial Maximum

Relief of iron (Fe) limitation in the surface Southern Ocean has been suggested as one driver of the regular glacial-interglacial cycles in atmospheric carbon dioxide (CO2). The proposed cause is enhanced deposition of Fe-bearing atmospheric dust to the oceans during glacial intervals, with consequent effects on export production and the carbon cycle. However, understanding the role of enhanced atmospheric Fe supply in biogeochemical cycles is limited by knowledge of the fluxes and ‘bioavailability’ of atmospheric Fe during glacial intervals. Here, we assess the effect of Fe fertilization by dust, dry-extracted from the Last Glacial Maximum portion of the EPICA Dome C Antarctic ice core, on the Antarctic diatom species Eucampia antarctica and Proboscia inermis. Both species showed strong but differing reactions to dust addition. E. antarctica increased cell number (3880 vs. 786 cells mL-1), chlorophyll a (51 vs. 3.9 μg mL-1) and particulate organic carbon (POC; 1.68 vs. 0.28 μg mL-1) production in response to dust compared to controls. P. inermis did not increase cell number in response to dust, but chlorophyll a and POC per cell both strongly increased compared to controls (39 vs. 15 and 2.13 vs. 0.95 ng cell-1 respectively). The net result of both responses was a greater production of POC and chlorophyll a, as well as decreased Si:C and Si:N incorporation ratios within cells. However, E, antarctica decreased silicate uptake for the same nitrate and carbon uptake, while P. inermis increased carbon and nitrate uptake for the same silicate uptake. This suggests that nutrient utilization changes in response to Fe addition could be driven by different underlying mechanisms between different diatom species. Enhanced supply of atmospheric dust to the surface ocean during glacial intervals could therefore have driven nutrient-utilization changes which could permit greater carbon fixation for lower silica utilization. Additionally, both species responded more strongly to lower amounts of direct Fe chloride addition than they did to dust, suggesting that not all the Fe released from dust was in a bioavailable form available for uptake by diatoms.