Hydrolysable carbohydrate data collected from the trace metal rosette in the Southern Ocean during the austral summer of 2016/2017, on board the Antarctic Circumnavigation Expedition. 

***** Dataset abstract *****
Hydrolysable carbohydrate (referred to as TPZT from the analytical methodology used) is part of the labile pool of dissolved organic carbon that is excreted by most (micro)organisms or released by continental margins/sediments. It is a carbon source for heterotrophic bacteria. These carbohydrates could also potentially bind iron and act as an iron binding ligand. 

This data is used to explore the nature of iron ligands and relate to biological and chemical oceanography.

***** Original data collection *****

Sample collection:
Filtered seawater (0.22 µm, PALL Acropack cartridge) samples were collected in duplicate from the trace metal rosette. Falcon 15 mL tubes were rinsed three times and filled to 11 mL, prior to be stored frozen upright until analysis. Samples from each station were gathered in a ziplock bag.

Method of sample processing and data collection:
Characterization of mono- and polysaccharides in seawater was done by TPTZ spectroscopy using the method of Myklestad et al. (1997). The reaction is based on the reduction of Fe3+ from potassium ferricyanide to Fe2+ (potassium ferrocyanide) by means of oxidation of the aldehyde group of the saccharides into a carboxyl group. The colorimetric reagent used herein was TPTZ (2,4,6-tripyridyl-s-triazine), which when bound to Fe2+, gives a blue colour complex absorbing at 595 nm. This made it possible to establish a calibration curve via spectrophotometry. The sugar used as a standard was D-Glucose. Since the analysed samples were to be mono-saccharides, hydrolysis was carried out beforehand.

The hydrolysis step was as follows: 4 ml of our samples (or standards) were placed in a 5 ml ampule to which 0.4 ml of 1M hydrochloric acid was added. The ampule was sealed under the flame and placed in an oven for one hour at 125 °C. Once the time had elapsed, samples were allowed to cool down to room temperature. The ampule was then opened, and 0.4 ml of 1M sodium hydroxide was added to neutralize the content.

One ml of seawater, hydrolysate or standard, was placed in a 5 ml glass tube. One ml of Reagent A was added, the tube was resealed and stirred and then placed in a thermostatic bath at 90-95 °C for 10 minutes. Immediately after these 10 minutes, the tube was opened and 1 ml of reagent B and 2 ml of Reagent C were added. All manipulations were done in dim light. The solution was stirred vigorously and allowed to stand in the dark for 30 minutes prior to being transferred into a 10-mm plastic cuvette. 
Absorbance was read at 595 nm against Milli-Q water using a PerkinElmer Lambda 365 spectrophotometer. 

Reagent A is a solution of potassium ferricyanide (0.7 mM) prepared as follows: 400 mg NaOH, 20 g Na2CO3 and 230 mg K3[Fe5(CN)6] per litre of solution in Milli-Q water. 
Reagent B is a solution of ferric chloride (2 mM) prepared as follows: 164 g sodium acetate anhydrous, 42 g citric acid and 300 g acetic acid per litre of solution in Milli-Q water. Then 32.4 mg of FeCl3 anhydrous is dissolved in 100 ml of this solution. This reagent is kept away from light for a maximum of two days. 

Reagent C is a solution of TPTZ (2,4,6-Tripyridyl-s-triazine) (2.5 mM) prepared as follows: 0.78 mg of TPTZ per millimeter in 3M acetic acid. This reagent is kept away from light for a maximum of one week.

To avoid contamination, carbon residual contamination from plastic material (bottle, tips) used was removed by an overnight soak in 1.2 M HCl, followed by five rinses in ultrapure water, drying and packing under a laminar flow. Glassware was pre-combusted at 550 °C for 5 hours.

Method of data analysis:
A six-point calibration curve (5, 10, 20, 30, 40 and 60 µmol/L carbon from D-Glucose) was performed twice per analytical run, with r2 > 0.99. The detection limit was determined at 2.46 µmol/L carbon.

All samples were analysed in triplicate, and the average value with standard deviation was reported. All standard deviations were less than 10 % of the average value. 

Reference for method:
Myklestad S., Skånøy E., Hestmann S. A sensitive and rapid method for analysis of dissolved mono- and polysaccharides in seawater. Mar Chem 56:279–286 (1997)

***** Quality checking ***** 

For each series of analysis at least two procedural blanks (using ultrapure water) and three to six standards (checking the analytical deviation) were performed. All samples are analysed in triplicate. If a significant deviation (> 10%) was measured, hydrolysis and analysis were repeated.

***** Dataset contents *****

ace_hydrolysable_carbohydrates_tpzt_data.csv, data file, comma-separated values 
ace_hydrolysable_carbohydrates_tpzt_data_visual_summary.png, metadata, portable network graphics
README.txt, metadata, text format
data_file_header.txt, metadata, text format
change_log.txt

NaN
All null values where no data point exists from lack of sample, have been set to NaN.

***** Dataset contact *****

Christel Hassler, University of Geneva, Switzerland. ORCID: 0000-0002-8976-5469. Email: christel.hassler@unige.ch

***** Dataset license *****

This hydrolysable carbohydrate dataset from ACE is made available under the Creative Commons Attribution 4.0 International License (CC BY 4.0) whose full text can be found at https://creativecommons.org/licenses/by/4.0/

***** Dataset citation *****

Please cite this dataset as:
Hassler, C. (2020). Hydrolysable carbohydrate data collected from the Trace Metal Rosette in the Southern Ocean during the austral summer of 2016/2017, on board the Antarctic Circumnavigation Expedition. (Version 1.1) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.3967034