Kinetics of silicon uptake

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

---

Silicon uptake kinetics (Vmax, Ks)
Approach: 32Si incubations at multiple ambient silicic acid concentrations; Michaelis–Menten fitting
Context: incubation, in situ and lab
Spatial scale: point sample
Temporal scale: hours to days
Units: Vmax (d-1); Ks (µmol Si L-1 or µmol Si cell-1)
Community captured: bulk
Co-measurements: ambient [Si(OH)4], PAR, temperature, BSi

Method Overview

Silicon uptake kinetics are determined by conducting ³²Si tracer incubations across a range of silicic acid concentrations spanning from near-ambient to saturating levels. At each substrate concentration, the uptake rate is measured as for the standard silicon uptake assay, and the resulting uptake rate vs. [Si(OH)₄] relationship is fitted to the Michaelis–Menten equation to derive the maximum specific uptake rate (V_max, d^-1) and the half-saturation constant for silicic acid (K_s, µmol Si L^-1 or per-cell equivalent). These kinetic parameters characterize the community's capacity to access silicic acid at varying concentrations and indicate whether the community is silicon-limited.

Scale of measurement

Point sample following hours to days of incubation across multiple substrate concentrations.

Data generated

V_max (d^-1) and K_s (µmol Si L^-1); these kinetic parameters characterize the community's affinity for silicic acid and maximum uptake capacity. When ambient [Si(OH)₄] < K_s, the community is silicon-limited.

Units & currency

Units are V_max (d^-1) and K_s (µmol Si L^-1 or µmol Si cell^-1). The currency is Si.

Sample size

Typical samples are 100–300 mL per substrate concentration, with multiple replicates.

Limitations

Kinetic parameters derived from short incubations may not represent steady-state values; acclimation to substrate additions can shift V_max and K_s over longer timescales. Normalization to BSi introduces the same uncertainty as for bulk uptake (inclusion of dead frustules). V_max and K_s represent the integrated response of all silicifying organisms in the community and cannot be attributed to individual species without additional fractionation.

Example Applications & Protocols

Classic examples

• Brzezinski & Phillips (1997) Evaluation of ³²Si as a tracer for measuring silica production rates in marine waters

Recent applications

• Giesbrecht & Varela (2021) Summertime biogenic silica production and silicon limitation in the Pacific Arctic Region

Common calculations/conversions

• V_Si = V_max × [Si(OH)₄] / (K_s + [Si(OH)₄]); rearranged as a Lineweaver–Burk or Eadie–Hofstee plot to obtain V_max and K_s.