Articles | Volume 13, issue 4
Biogeosciences, 13, 1019–1035, 2016
https://doi.org/10.5194/bg-13-1019-2016
Biogeosciences, 13, 1019–1035, 2016
https://doi.org/10.5194/bg-13-1019-2016

Research article 23 Feb 2016

Research article | 23 Feb 2016

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

Nina Schuback1, Mirkko Flecken2, Maria T. Maldonado1, and Philippe D. Tortell1,3 Nina Schuback et al.
  • 1Departement of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada
  • 2RWTH Aachen University, Aachen, Germany
  • 3Department of Botany, University of British Columbia, Vancouver, BC, Canada

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 (ETRRCII), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETRRCII 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 ETRRCII in the iron-limited NE subarctic Pacific over the course of a diurnal cycle. We show that rates of ETRRCII 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 ETRRCII and carbon fixation (Kc / nPSII). 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 Kc / nPSII requires further validation but has the potential to improve estimates of phytoplankton carbon fixation rates from FRRF measurements alone.

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Short summary
We measured rates of phytoplankton photosynthesis at the level of initial charge separation in PSII and at the level of carbon uptake in iron-limited phytoplankton field assemblages over a diurnal cycle. Our results show significant diurnal uncoupling of the two rates. The uncoupling between rates complicates the derivation of carbon-based estimates of phytoplankton production from rates of charge separation. However, it can be explained as evolutionary adaption to iron limited conditions.
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