Evaluation of biophysical and optical determinations of light absorption by photosystem II in phytoplankton

Abstract

Fast repetition rate (FRR) fluorescence can provide highly resolved estimates of light absorption by photosystem II (PSII), a variable that is critical to bio-optical determinations of phytoplankton productivity. We compared estimates of chlorophyll a-specific light absorption by PSII, a chl PSII , using both biophysical (FRR) and optical (chlorophyll a-specific light absorption coefficient, a chl ) techniques on cultures of phytoplankton from diverse taxa. Biophysical determinations of a chl PSII were obtained from the product of the effective light absorption cross-section of PSII (σ PSII ), measured by FRR fluorescence, and the ratio of PSII reaction centers to chlorophyll a (n PSII ), measured by oxygen flash yields. Both parameters were highly variable between individual taxa. In mixtures of algae, the estimates of σ PSII were largely determined by the taxon that dominated fluorescence intensity. Independent optical estimates of a chl PSII were obtained by weighting the light absorption spectra of photosynthetic pigments by the fluorescence excitation spectra. The biophysical and optical estimates of a chl PSII were highly correlated (r 2 = 0.94) with a slope that was not significantly different from 1 and an intercept of 0. Estimates of productivity using biophysical or optical absorption measurements should therefore be comparable when the latter are adjusted to account for the proportion of light provided for photochemistry into both photosystem I and II. Finally, we show how simultaneous measurements of σ PSII and a chl PSII can be used to derive n PSII where flash-yield measurements are impractical, as is almost universally the case in field measurements.

Photosynthetic electron turnover in the tropical and subtropical Atlantic Ocean

“…Discrete samples from the pre-dawn casts during AMT 11 were held in the 'enclosed' chamber and exposed to 50 individual sequences each separated by 1-s intervals. These 50 acquisitions were logged internally and subsequently averaged into a single, high signal: noise transient (see Suggett et al, 2004). Similarly, all logged fluorescence transients from each vertical cast per station were averaged into 5-10 m depth bins to increase the signal to noise ratio.…”

“…Throughout all cruises, the FRR fluorometer was programmed to deliver and record single photochemical turnover excitation and emission sequences as described by Moore et al (2003Moore et al ( , 2005 and Suggett et al (2003Suggett et al ( , 2004. Specifically, saturating-chain sequences of 100 1.1 ms flashes were applied at 2.8 ms intervals.…”

“…An IRF was only determined following the final cruise, AMT 11 using a chlorophyll a extract and deionised water (Laney, 2003;Suggett et al, 2004;Moore et al, 2005) for all gains and both FRR chambers. We subsequently applied this IRF to fluorescence transients obtained from all prior cruises.…”

“…Also, we can correct for the spectral weighting of FRRexcitation Suggett et al, 2001;Raateoja et al, 2004) and obtain some knowledge of the RCII concentration indirectly (Suggett et al, 2001(Suggett et al, , 2004Moore et al, 2005) provided additional bio-optical and/or biophysical properties of the PSII antenna-reaction centre complex are measured.…”

Monitoring Ocean Change in the 21st Century

“…This technology has high potential due to its non--destructive nature and rapid analysis, but needs further calibration and assessment relative to the tracer methods. Furthermore, this technique is highly dependent on the phytoplankton species in the samples, since it measures the variable fluorescence in Photosystem II, and different algae have different arrangements of photosystems (Lutz et al, 2001;Suggett et al, 2004;Johnsen and Sakshaug, 2007). Subsequently, the sequence of flashes may have to be adjusted for resident phytoplankton assemblages and/or different types of oceanic waters.…”