PROTOCOL FOR ORTHOSILICIC ACID MEASUREMENTS

 

Sampling protocol :

Water was collected from the Rosette bottles in polyethylene (PE) scintillation vials, beforehand rinsed three times with seawater. Samples were fixed with 25 µL HgCl₂ and stored at ambiant temperature onboard and in a refrigerated room at the laboratory. Orthosilicic acid was measured colorimetrically following Mullin and Riley’s (1965) protocol modified by Strickland and Parsons (1972).

Precision: ± 70 nmol L^-1 for POMME 1 and 2.

 

PROTOCOL FOR BIOGENIC (BSi) AND LITHOGENIC SILICA (LSi) MEASUREMENTS IN THE WATER COLUMN

Sampling protocol :

One liter seawater was collected at discrete depths and then filtered on polycarbonate membranes (0.6 µm, 47 mm). The filters were stored in plastic petri dishes at ambiant temperature until BSi/LSi analysis.

Laboratory analysis :

BSi and LSi were measured using Paasche’s method (1973) as modified by Nelson et al. (1989): BSi and LSi were sequentially extracted on the same filter after successive basic and acid treatment. BSi was extracted during a hot sodium hydroxide (NaOH 0.2 N) attack (45 min) which converts BSi into the dissolved orthosilicic acid form (Si(OH)₄). Si(OH)₄ is then quantified using Strickland and Parsons (1972) spectrophotometric method. After the first basic attack, filters were rinsed free of remaining Si(OH)₄ and dried again at 60°C. LSi, preserved in the sample, was then treated with hydrofluoric acid (HF 2.9 N) for 24h. As for BSi, LSi was then measured through quantification of the dissolved Si(OH)₄ form.

Precision: ± 16 nmol L^-1 for BSi, ± 48 nmol L^-1 for LSi, for POMME 1 and 2.

PROTOCOL FOR MEASURING SILICA PRODUCTION RATES

Protocol :

Dawn-to-dawn in situ uptake experiments were realized using an immersed production line, at 8 incubation depths. 275 mL seawater, sampled before dawn, were spiked with 0.022 µCi (800 Bq) of the radioactive isotope silicon-32 (³²Si) .The specific activity of the ³²Si solution was 15.5 kBq µg Si^–1. After incubation, samples were filtered on polycarbonate Nucleopore membrane (0.6 µm, 47 mm). Filters were rinsed with filtered (0.2 µm) seawater, and placed in scintillation vials. The ³²Si uptake was measured in a Packard 1600-TR scintillation counter by Cerenkov effect, following the method described by Tréguer et al., (1991) and Leynaert (1993). Precision of the method averages 10%. Si specific uptake rates VSi (in d^-1) have been derived from BSi and rSi measurements using the following equation : VSi = rSi/BSi.

Si kinetics (Ks and Vmax)

Two kinetic uptake experiments were made at two depths (usually 20 and 40 m) at each production stations. Samples used were issued from the same Niskin bottles as those used for in situ silicon uptake measurements. 6 samples from each depth received non radioactive Si(OH₄) additions so that concentrations increased respectively by 0, 1, 2, 5, 15 and 40 µM. Bottles were incubated on board in a deck incubator for 8h using neutral nickel screens. Samples were thereafter treated as described for in situ samples. Kinetic parameters V_max and K_S were calculated by fitting the data to a Michaelis-Menten curve using the procedure described by Wilkinson (1961).

 

References :

Leblanc, K. (2002). Variabilité spatiale et temporelle du cycle du silicium dans divers milieux oligotrophes et mésotrophes. Marseille. Université de la Méditerranée. Thesis. 269 pp.

Leynaert, A. (1993). La production de silice biogénique dans l'océan : de la mer de Weddell à l'océan Antarctique. Paris. Université Pierre et Marie Curie (Paris VI). Thesis. 99 pp.

Mullin, J.B. and Riley, J.P. (1965). The spectrophotometric determination of silicate-silicon in natural waters with special reference to seawater. Anal. Chim. Acta, 46, 491-501.

Nelson, D.M., Walker, O.S.Jr., Muench, R.D., Gordon, L.I., Sullivan, C.W., Husby, D.M. (1989). Particulate matter and nutrient distributions in the ice-edge zone of the Weddell Sea : relationship to hydrography during late summer. Deep-Sea Research, 36, 191-209.

Paasche, E. (1973). Silicon and the ecology of marine plankton diatoms. I. Thalassiosira pseudonana (Cyclotella nana) grown in a chemostat with silicate as limiting nutrient. Marine Biology, 19, 117-126.

Strickland, J. D. H. and Parsons, T. R. (1972). A practical handbook of seawater analysis. Fish. Res. Bd Canada Bull., 1, 155-164.

Tréguer, P., Lindner, L., Van Bennekom, A. J., Leynaert, A., Panouse, M. and Jacques, M. (1991). Production of biogenic silica in the Weddell-Scotia Seas measured with ³²Si. Limnol. Oceanogr., 36, (6), 1217-1227.

Wilkinson G.N. (1961). Statistical estimations in enzyme kinetics. Biochem. J., 80, 324-332.