Iron recycling and export from Antarctic krill faecal pellets

Brief description

The aim of this project was to estimate the iron recycling and export potential of Antarctic krill faecal pellets. To determine this, we firstly determined the sinking rate of the faecal pellets, characteristics which may influence the sinking rate (e.g., density, length and diameter), and then determined the portion of the total iron in the faecal pellets that is leached over a 12 h period under a continuous flow of seawater. The data sets that can be accessed here are: 1. Faecal pellet characteristics, including the raw and analysed data 2. The dry weight and length of faecal pellets used for total Fe estimation 3. Raw and final iron leachate data

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Firstly, faecal pellets were collected from Antarctic krill at the Australian Antarctic Division using a plastic pipette with the tip enlarged to avoid fragmentation. Krill were fed a diet of 75% of the diatom Phaeodactylum triconutum and 25% of the flagellate Pyramimonas gelidicola. Faecal pellets were photographed for analysis of length, diameter and area on Image J. Once collected, faecal pellets were placed in a 1 L glass cylinder filled with filtered seawater, and the time the faecal pellets took to sink within a 10 cm range was recorded. To determine the relationship between faecal pellet length and dry weight, faecal pellets were photographed, rinsed in milliQ water to remove any salt, dried in an oven at 60°C for 12 h (Clarke et al., 1988), and weighed dry (Mettler Toledo AG285, measurement error of ± 0.03 mg). This allowed an estimation of the dry weight of our faecal pellets used during the dissolution experiment to determine the total Fe concentrations (in mg Fe g-1). For the leaching experiment, fragmented and intact faecal pellets were placed on a 0.2um acid-washed filter paper, and then placed into a filter holder on a continuous flow leachate system. This system allowed for the continuous flow of Southern Ocean seawater over the particles to avoid saturation and hence is more representative of natural conditions. Seawater was collected for dissolved Fe analysis continuously between 0 mins and 72 mins, and then again at 372 minutes, and lastly at 732 minutes. After 732 minutes, the filter papers with the particles were collected and stored for the analysis of particulate Fe. Dissolved Fe samples were immediately acidified to pH 2 with ultrapure HCl, stored at room temperature and analysed after pre-concentration on an ICP-MS. Following the final sampling period, filters were frozen at -20° C prior to analysis for particulate Fe. Seawater sub-samples were analysed for dFe (<0.2µm) using an offline seaFAST preconcentration system (SC-4 DX seaFAST 32/pico, ESI, USA). Methods followed Wuttig et al., (2019), with samples pre-concentrated 10 times. Samples were organised from expected lowest concentrations (i.e., 12 h timestep) to highest concentrations (i.e., 12 min timestep) to minimise sample carry-over. Briefly, metals were isolated from the seawater matrix through a Nobias PA1 resin column, followed by elution in 1000 µL of 1.6 M ultrapure HNO3 (Seastar Baseline). Reference samples (NASS-7 and GSC), as well as external and internal standards, were used throughout analysis to ensure analytical precision and accuracy (Table S1). Following preconcentration, dFe concentrations were determined within 12 h using a Thermo Fisher Scientific ELEMENT 2 sector field inductively coupled plasma mass spectrometer (SF-ICP-MS, Central Science Laboratory, CSL, Tasmania) in medium resolution (MR) mode. The measured detection limit for dFe was 0.015 nM (3 σ of blank UHP). Concentrations of pFe on the filters (>0.2µm) from the dissolution experiment were quantified following Bowie et al., (2010). Sample filters, three blank filters and three filters with certified reference material (BCR-414) were analysed for pFe. Briefly, filters were digested in 15 mL trace-metal clean teflon PFA vials with the addition of 750 µL of 12 M HCl, 250 µL of 16 M HNO3 and 250 µL of 29 M hydrofluoric acid (HF) to ensure the digestion of refractory particles (Bowie et al., 2010). Vials were capped, heated at 90℃ for 12 h on a hotplate, and then dry evaporated at 95℃ for 4 h. Samples and reference materials were resuspended with 10 mL of 2% HNO3, diluted at 10x with 2% HNO3 to make up 10 mL, with Indium added as an internal standard. Samples were then analysed using SF-ICP-MS. The measured average recovery for Fe in BCR-414 was 91% (n=3). Initial (i.e., total) faecal pellet Fe concentration was calculated as the sum of all dFe leached from each dissolution timestep added to the final pFe measured on the filters. To determine the dFe leached in between sampling periods (i.e.,72 min and 360 min, and 372 min and 720 min) a linear interpolation was applied at 12 min intervals, as each data point represents 12 mins of dFe dissolution. The interpolated data was then added to the measured dFe and pFe concentrations to determine the total Fe concentration (in μg Fe; Corkill et al., in review). Data from each 12 min interval of experiments (measured and interpolated) were converted to percentages of the total Fe. These were converted to cumulative data by summing each timestep with all prior timesteps.

Notes

Credit
AJRS, LR and DL received grant funding from the Australian Government as part of the Antarctic Science Collaboration Initiative program. This research was also supported by the Australian Research Council Special Research Initiative, Australian Centre for Excellence in Antarctic Science (Project Number SR200100008). DL is supported by an Australian Research Council (ARC) Future Fellowship FT190100688. Access to ICP-MS instrumentation was supported through ARC LIEF funding (LE0989539). We would like to thank and acknowledge all participants of the TEMPO voyage and the CSIRO Marine National Facility for its support in the form of sea time on RV Investigator, support personnel, scientific equipment, and data management. The authors would also like to thank Dr. Robert Strzepek and Dr. Pauline Latour for the SOLACE seawater for the leaching experiment, Anita Butterley for the sinking rate experiment advice and Blair Smith, Anton Rocconi and Tasha Waller for their support and advice with the maintenance of the Antarctic krill.