Single Turnover Chlorophyll Fluorescence
Photosynthetic primary productivity is the light-driven process of extracting reducing power from water to drive CO2 reduction to carbohydrates (i.e. CO2 ‘fixation’). Global primary productivity is a critical source of O2 for the atmosphere and oceans. Marine primary productivity also plays an important role in carbon sequestration to the deep ocean through the biological pump, while providing a critical source of organic matter to support aquatic food webs and metabolism.
| What is being measured in 1 - 3 words |
|---|
| Photosynthetic electron transport |
| Approach: active chlorophyll fluorescence (single turnover), optical photophysiology |
| Context: in situ, underway, incubation, autonomous platforms |
| Spatial scale: mL to km2 (point measurements to transects) |
| Temporal scale: milliseconds to seasons (instantaneous to long-term monitoring) |
| Units: electrons m-3 s-1, e- PSII-1 s-1, derived C or O2 units |
| Community captured: target species culture experiments, bulk phytoplankton community; taxon-weighted signal |
| Co-measurements: irradiance, temperature, salinity, nutrient concentrations, chlorophyll a, light history |
Method Overview
Single Turnover Active Chlorophyll Fluorescence (ST-ChlF) is an optical technique used to quantify photosynthetic activity in aquatic primary producers by resolving rapid fluorescence induction and relaxation transients following short excitation flashes. These transients reflect the opening and closure of Photosystem II (PSII) reaction centres and downstream electron transport processes.
By fitting photophysiological models to fluorescence transients, ST-ChlF enables estimation of photosynthetic electron transfer rates (ETR), which scale stoichiometrically with gross oxygen evolution and are closely linked to primary productivity. The method provides rapid, non-invasive measurements and can be applied across laboratory, ship-based, and autonomous observing platforms.
Output
Scale of measurement
- Cellular to community-integrated photosynthesis
- Short- and long temporal scales
Data generated
- Raw fluorescence induction-relaxation transients
- Primary photophysiological parameters (e.g. Fo, Fm, σPSII, τ)
- Electron transfer rates (ETR)
- Light-response (ETR–E) curves and derived parameters (α, Ek, ETRmax)
- Ancillary stress and efficiency metrics (e.g. NPQ-related indices)
Units & currency
- ETR reported as electrons per unit volume or per PSII reaction centre per second
- Can be converted to O2 or carbon-based primary productivity using stoichiometric or empirical relationships
- Represents gross photosynthetic activity on instantaneous time scales
Sample size
- Typically mL per measurement
- Not necessarily needing sample destruction
- Compatible with continuous flow-through systems
Repositories & databases
Limitations
- ETR is not a direct measure of carbon fixation and requires conversion assumptions
- Relationships between ETR, O2, and carbon vary with taxonomy and environmental conditions
- Sensitive to non-photochemical quenching (NPQ) and light history
- Requires careful calibration, spectral correction, and baseline fluorescence treatment
- Community-weighted signal may mask taxon-specific responses
Example Applications & Protocols
Classic examples
Recent applications
Common calculations/conversions
- Conversion of ETR to gross O2 production (4 e- per O2)
- Empirical ETR–C relationships calibrated against 14C incubations
- Light-response curve fitting (ETR vs irradiance)
References
SCOR Working Group 156 (2021). A User Guide for the Application of Single Turnover Active Chlorophyll Fluorescence for Phytoplankton Productivity Measurements. Version 1.0.