Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

Abstract

Abstract. Active chlorophyll a fluorescence approaches, including fast repetition rate fluorometry (FRRF), have the potential to provide estimates of phytoplankton primary productivity at an unprecedented spatial and temporal resolution. FRRF-derived productivity rates are based on estimates of charge separation in reaction center II (ETR RCII ), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETR RCII and carbon fixation provides physiological insight into phytoplankton photosynthesis and is critical for the application of FRRF as a primary productivity measurement tool. In the present study, we simultaneously measured phytoplankton carbon fixation and ETR RCII in the iron-limited NE subarctic Pacific over the course of a diurnal cycle. We show that rates of ETR RCII are closely tied to the diurnal cycle in light availability, whereas rates of carbon fixation appear to be influenced by endogenous changes in metabolic energy allocation under iron-limited conditions. Unsynchronized diurnal oscillations of the two rates led to 3.5-fold changes in the conversion factor between ETR RCII and carbon fixation (K c /n PSII ). Consequently, diurnal variability in phytoplankton carbon fixation cannot be adequately captured with FRRF approaches if a constant conversion factor is applied. Utilizing several auxiliary photophysiological measurements, we observed that a high conversion factor is associated with conditions of excess light and correlates with the increased expression of non-photochemical quenching (NPQ) in the pigment antenna, as derived from FRRF measurements. The observed correlation between NPQ and K c /n PSII requires further validation but has the potential to improve estimates of phytoplankton carbon fixation rates from FRRF measurements alone.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Therefore, a spectral correction factor was derived and applied to correct values of r PSII (and consequently ETR RCII ) as well as short-term 14 C-uptake rates (Schuback et al 2016). The spectral distribution in all three was measured using a micro-spectrometer equipped with a fiber-optic probe (STS-VIS, Ocean Optics).…”

“…Spectrally corrected rates of 2-h 14 C-uptake and ETR RCII were used to derive the conversion factor J/n PSII (mol e -mol C -1 (mol Chl a) mol RCII -1 ) at light limitation (a RCII /a Chl a ) and light saturation (P RCII max / P Chl a max ), as described in more detail in Schuback et al (2015Schuback et al ( , 2016.…”

“…Fit parameters (a, E k , P max ) for FRRf-based ETR RCII and 14 C-uptake derived from P versus E curves as well as conversion factors between both and NPQ NSV observed at 880 lmol photons m -2 s -1 as described in Schuback et al (2015Schuback et al ( , 2016. Significant effects (p [ 0.05) of light, or interaction of light and CO 2 are indicated by a, or c, respectively…”

Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions

“…In the absence of iron limitation, this ratio can be used as a sensitive indicator of algal photosynthetic efficiency (Maxwell and Johnson, 2000;Schuback et al, 2016).…”

Thermal Performance Curves of Functional Traits Aid Understanding of Thermally Induced Changes in Diatom-Mediated Biogeochemical Fluxes

“…A second series of measurements was performed to parameterize the light-dependent response of electron transport kinetics and identify shifts in the light-saturated rates of electron transport and the light utilization efficiency. In the interest of minimizing diurnal impacts (Schuback et al, 2016), we conducted rapid light curves on a single replicate for each temperature to reduce the time period of measurements. Measurements of the light dependent electron transport rate (ETR) were achieved with a rapid light curve protocol where samples were illuminated by the FASTAct instrument chamber that supplied actinic white light at increasing light intensities of 0,10,28,47,66,85,104,123,160,251,330,402,550,756,905,1208 µmol photons m −2 s −1 .…”