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Degradation of dissolved organic phosphorus (P-monoesters): Alkaline phosphatase activity (APA)

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Alkaline phosphatase activity (APA)
Approach: fluorescent MUF-phosphate substrate assay
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: mol fluorescent tag L-1 h-1
Community captured: heterotrophs, microalgae
Co-measurements: cell abundance

Method Overview

Alkaline phosphatase (AP) is an ectoenzyme that cleaves phosphate monoesters from dissolved organic phosphorus (DOP) compounds, releasing inorganic phosphate and making it available for uptake. APA is measured using 4-methylumbelliferyl phosphate (MUF-P) as a fluorogenic substrate: AP cleaves the phosphate group, releasing 4-methylumbelliferone (MUF), which fluoresces strongly (excitation ~365 nm, emission ~450 nm). The rate of MUF release over a time course is measured fluorimetrically in a flow-through fluorimeter or plate reader, and calibrated against a MUF standard curve to yield APA at saturating substrate concentration (Vmax). APA is a widely used indicator of phosphorus stress in marine phytoplankton and heterotrophic bacteria, as AP expression is strongly induced under inorganic phosphate (Pi) limitation.

The assay is commonly applied in both bulk mode (measuring community APA) or can reach single-cell resolution by flow cytometry using fluorescent MUF substrates added directly to seawater samples.

Scale of measurement

Sampling incubations after hours to days at in situ or controlled temperature.

Data generated

Potential APA rate at Vmax (mol MUF L-1 h-1), reflecting the maximum enzymatic capacity for phosphate monoester hydrolysis. High APA is indicative of phosphorus stress.

Units & currency

Units are mol fluorescent tag L-1 h-1. The currency is phosphorus.

Sample size

Typical samples are < 1 L in volume.

Repositories & databases

Limitations

Rates represent Vmax at saturating substrate and may substantially exceed in situ rates under sub-saturating ambient DOP concentrations. MUF-P is a proxy substrate for the full range of phosphate monoesters in natural waters. The assay does not distinguish between AP expressed by bacteria, phytoplankton, or released as dissolved extracellular enzymes.

Example Applications & Protocols

Classic examples

  • Hoppe (1983) Significance of exoenzymatic activities in the ecology of brackish water: measurements by means of methylumbelliferyl-substrates [1]

Recent applications

  • Su et al. (2023) A dataset of global ocean alkaline phosphatase activity [2]
  • Thomson et al. (2020) Relative importance of phosphodiesterase vs. phosphomonoesterase activities for DOP hydrolysis in epi- and mesopelagic waters [3]

Common calculations/conversions

  • APA (nmol MUF L-1 h-1) = ΔRFU / incubation time / calibration slope; where calibration slope is from an MUF standard curve.

References

  1. ↑ Hoppe, H.-G. (1983). Significance of exoenzymatic activities in the ecology of brackish water: measurements by means of methylumbelliferyl-substrates. Marine Ecology Progress Series, 11, 299–308.
  2. ↑ Su, B., Song, X., Duhamel, S., Mahaffey, C., Davis, C., Ivančić, I., & Liu, J. (2023). A dataset of global ocean alkaline phosphatase activity. Scientific Data, 10, 205. https://doi.org/10.1038/s41597-023-02081-7
  3. ↑ Thomson, B., Wenley, J., Lockwood, S., Twigg, I., Currie, K., Herndl, G. J., Hepburn, C. D., & Baltar, F. (2020). Relative importance of phosphodiesterase vs. phosphomonoesterase (alkaline phosphatase) activities for dissolved organic phosphorus hydrolysis in epi- and mesopelagic waters. Frontiers in Earth Science, 8, 560893. https://doi.org/10.3389/feart.2020.560893
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