Silicon uptake
| Silicon uptake (32Si) |
|---|
| Approach: radiotracer (32Si) incubation |
| Context: incubation, in situ and lab |
| Spatial scale: point sample |
| Temporal scale: hours to days |
| Units: µmol Si L-1 h-1; µmol Si cell-1 h-1 |
| Community captured: bulk (primarily diatoms) |
| Co-measurements: ambient silicic acid [Si(OH)4], PAR, temperature, biogenic silica (BSi) |
Method Overview
Silicon uptake rates are measured using 32Si, a low-energy beta-emitting radiotracer with a half-life of ~140 years. Silicic acid (32Si(OH)4) is added at tracer concentrations to seawater samples incubated under simulated in situ or in situ light and temperature conditions. Diatoms and other silicifying organisms incorporate the labelled silicic acid into their frustules at rates proportional to the ambient silicic acid uptake rate. At the end of the incubation, cells are filtered onto polycarbonate membranes; the filter is placed on a scintillator-coated Mylar film, and the beta emission from 32Si incorporated into BSi is counted by a low-energy beta counter. The uptake rate is calculated from the fraction of added 32Si incorporated relative to total available silicic acid[1].
Scale of measurement
Point samples after incubations of hours to days depending on ambient Si uptake rates. Depth profiles require incubations at each target depth or under the appropriate simulated irradiance.
Data generated
Silicon uptake rates (µmol Si L-1 h-1), equivalent to biogenic silica production when steady state is assumed. Normalization to BSi concentration yields the specific uptake rate (VSi, d-1), which reflects the community's silicification rate relative to standing stock.
Units & currency
Units are µmol Si L-1 h-1 or µmol Si cell-1 h-1 (cell-specific). The currency is Si.
Sample size
Typical samples are 100–300 mL in volume.
Repositories & databases
Limitations
The method assumes Michaelis–Menten kinetics and that the tracer is taken up at the same rate as ambient silicic acid. Bottle confinement and changes in light or mixing can alter uptake rates relative to in situ conditions (non-steady state). When uptake rates are normalized to BSi, bulk BSi measurements include both living cells and dead frustules, inflating the denominator and underestimating specific rates.
Example Applications & Protocols
Classic examples
- Brzezinski & Phillips (1997) Evaluation of 32Si as a tracer for measuring silica production rates in marine waters [1]
Recent applications
- Krause et al. (2011) Application of low-level beta counting of 32Si for the measurement of silica production rates in aquatic environments [2]
- Giesbrecht & Varela (2021) Summertime biogenic silica production and silicon limitation in the Pacific Arctic Region [3]
Common calculations/conversions
- Uptake rate ρSi (µmol Si L-1 h-1) = (32Siincorporated / 32Siadded) × [Si(OH)4] / incubation time.
- Specific uptake rate VSi (d-1) = ρSi / [BSi].
References
- ↑ 1.0 1.1 Brzezinski, M. A., & Phillips, D. R. (1997). Evaluation of 32Si as a tracer for measuring silica production rates in marine waters. Limnology and Oceanography, 42(5), 856–865. https://doi.org/10.4319/lo.1997.42.5.0856
- ↑ Krause, J. W., Brzezinski, M. A., Landry, M. R., Baines, S. B., Nelson, D. M., Selph, K. E., Taylor, A. G., & Twining, B. S. (2011). Application of low-level beta counting of 32Si for the measurement of silica production rates in aquatic environments. Marine Chemistry, 127, 40–47. https://doi.org/10.1016/j.marchem.2011.07.001
- ↑ Giesbrecht, K. E., & Varela, D. E. (2021). Summertime biogenic silica production and silicon limitation in the Pacific Arctic Region from 2006 to 2016. Global Biogeochemical Cycles, 35, e2020GB006629. https://doi.org/10.1029/2020GB006629