Evaluation of ³²Si as a tracer for measuring silica production rates in marine waters

Abstract

The radioisotope 32Si was evaluated as a tracer for measuring silica production rates in marine waters. j2Si is a weak p-emitter that decays to 32P allowing the isotope to be used to determine simultaneously the production rate of biogenic silica and organic phosphorus. High specific activity '*Si, 42,000-52,000 Bq (pug Si)-I, was produced by the proton spallation of KCl. Reasonable tracer additions result in only nanomolar increases in the ambient silicic acid and orthophosphate concentrations during production rate experiments. Protocols for quantifying the amount of 32Si and 32P in samples by liquid scintillation counting (LSC) and gas-flow proportional counting (GFPC) are presented. Both protocols eliminate the 4-month wait for secular equilibrium between 32Si and 72P required by earlier methods. GFPC is especially useful as the instrumentation is lightweight and portable allowing real-time analysis of samples at sea. Real-time analysis by GFPC at sea and postcruise analysis by LSC gave results that agreed to within 17 and 7% for 12Si and 32e respectively. Subsequent intercalibrations of the LSC and GFPC methods indicated that agreement to within O.l-4% for both isotopes can be achieved. Parallel incubations of samples from Monterey Bay, California, using the radioisotope 32Si and the stable isotope 'OSi gave silica production rates that generally agreed to within 30%. No significant bias was observed between rates obtained with the two tracers. The greater sensitivity and ease of analysis of the radioisotope makes 32Si the tracer of choice for future studies of silica production in the ocean.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Laboratory studies indicate a reduction of biogenic silica content with increasing pCO 2 levels (Milligan et al 2004;Hoppe et al 2015). To our knowledge, our study is the first to apply the 32 Si uptake technique (Brzezinski and Phillips, 1997) to assess pCO 2 effects on natural phytoplankton assemblages. Our results showed that OA had no effect on either Si uptake (Table 2) or bSi:C ratios (Table 3).…”

“…The activity of 32 Si in the samples was determined by gas-flow proportional counting at secular equilibrium using a Risø 25-5 low-level beta GM multicounter (Krause et al 2011). Biogenic silica production rates (lmol Si L -1 d -1 ) were calculated as in Brzezinski and Phillips (1997).…”

Iron and zinc effects on silicic acid and nitrate uptake kinetics in three high-nutrient, low-chlorophyll (HNLC) regions

“…NO 3 -uptake rates were calculated using equations in Dugdale & Goering (1967). These values compared well with NO 3 -uptake rates calculated using equations adapted from Brzezinski & Phillips (1997).…”

“…After incubation, samples were vacuum-filtered (<150 mm Hg) onto 0.6 µm polycarbonate filters and allowed to air dry ( 32 Si) or filtered onto pre-combusted Whatman GF/F glass fiber filters (~0.7 mm) and stored frozen in pre-combusted aluminum foil packets ( 15 N). Samples with 32 Si were stored until 32 Si and 32 P reached secular equilibrium (> 3 mo) and 32 Si activity was assayed according to Brzezinski & Phillips (1997). Si(OH) 4 uptake rates were calculated as described therein.…”

Biogeochemical responses to late-winter storms in the Sargasso Sea, III—Estimates of export production using 234Th:238U disequilibria and sediment traps

“…Krause et al (2008) measured bSiO 2 silica production rates during this study using the 32 Si tracer and incubation method described by Brzezinski and Phillips (1997) and Brzezinski et al (2001). Depth integrated (0 -140 m) bSiO 2 production rates averaged 1.03 ± 0.96 (n = 17) and 2.28 ± 2.06 (n = 9) mmol Si m -2 d -1 for the West and East Transects, respectively, a substantial increase over previous Sargasso Sea measurements (Brzezinski and Kosman, 1996;Brzezinski and Nelson, 1995;Nelson and Brzezinski, 1997).…”