PARTICULATE BIOGENIC SILICA

SUMMARY: Seawater samples are collected at discrete depths using CTD/rosette sampling procedures. A known volume is concentrated onto a 0.8 µm Nuclepore filter and stored at -20°C. In the laboratory, particulate Si is dissolved in a basic solution, with heat, over time.

1. Rationale and Assay Principle

Silicon (Si), present in seawater primarily as Si(OH)4, is a macronutrient required by certain algae (diatoms) and protozoans (radiolarians) and is a required trace nutrient for all organisms. The uptake of Si(OH)4 from seawater during the growth of diatoms results in the formation of biogenic particulate Si (opal).

Silicious particles dissolve in basic solutions, with heat, over time. Biogenic-Si dissolves more readily than Si compounds of mineral origin and the two can be separated by dissolution time and lithogenic Si concentrations can be determined by further dissolution with hydrofluoric acid. Only the procedure for the determination of biogenic Si is routinely performed in the HOT program. Aliquots derived from a controlled reaction mixture are reacted with an ammonium molybdate-sulfuric acid solution, chemically reduced, developed and the absorbance read at 810 nm.

2. Precautions

Glass, a Si product, is the most likely contamination source. Therefore, samples are collected in plastic bottles, filtered in plastic units onto PC filters and stored in polypropylene (PP), polystyrene (PS) tubes, or plastic petri dishes.

3. Sampling, Standards, Spikes, Blanks and Replicates

3.1. Samples for biogenic-Si are collected from both suspended and sinking particle sources. Therefore, see the appropriate processing section for specific sampling, filtration and storage protocols. The sample volume processed is also dependent on sample source.
3.2. Samples are filtered onto 0.8 µm polycarbonate (e.g. Nuclepore) filters.
3.3. When filtration is complete, the filter is placed in a labeled 50 ml PP-tube, or plastic petri-dish, and stored in the freezer. Filters are dried ~12 hr at 60°C in a drying oven prior to chemical analysis.

4. Analysis

4.1. 8 ml of 0.2 N NaOH solution is added to each tube, submerging the filter completely. Filter blanks, a positive control (diatomaceous earth), and matrix samples spiked with a Na2SiF6 standard, are included in each sample run. A negative control (quartz sand) is optional.
4.2. The samples are vortexed, placed into a water bath (95°C) and incubated for 2 hours.
4.3. After 2 hours the tubes are rapidly cooled in an ice bath. Once cool, 2 ml of 1 N HCl is added to each tube to neutralize the NaOH and halt any further dissolution. The samples are vortexed and centrifuged at 1000 xg for 10 minutes.
4.4. 1.5 ml of the supernatant is transferred into a new 15 ml PP tube containing 3.5 ml of distilled water. Alternatively, sample dilution may not be necessary if run on the Auto Analyzer.
4.5. Analysis of dissolved Si is by the method of Koroleff (1983). Briefly, a 1:1 H2SO4:ammonium molybdate solution (200 µl/5 ml sample), is added, mixed and allowed to react for 10 min before adding oxalic acid (200 µl/5 ml sample) and ascorbic acid (100 µl/5 ml sample) in rapid succession. The addition of these reagents result in the formation of a blue silicomolybdate compound. After an incubation period of 60 min the absorbance is read at 810 nm in a 1 cm cuvette cell. Alternatively, the samples are run on the Auto Analyzer – make sure that the proper sample matrix is used for the standardization.

5. Calculations

Slope is determined from the regression of the absorbances obtained from a dilution series of an in-house Na2SiF6 standard into the same matrix as the samples. Check standards are included and diluted from commercially available Si-solution (1mM, OSIL). Unknowns are corrected for dilution if necessary. Filter blanks are subtracted from appropriate samples. Unknown absorbances are divided by slope.

6. Equipment/Supplies

7. Reagents

For AA analysis – see SRSi protocol for reagents.

8. References

9. HOT Program Analytical Summary