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

Nina Schuback,Philippe D. Tortell,Mirkko Flecken,Maria T. Maldonado

doi:10.5194/bg-13-1019-2016

Nina Schuback, Philippe D. Tortell + Show 2 more

Open Access

https://doi.org/10.5194/bg-13-1019-2016

Copy DOI

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 (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.

Highlights

Marine phytoplankton account for ∼ 50 % of global carbon fixation (Field et al, 1998) and play a key role in Earth’s biogeochemical cycles
We expect that surface hydrography and phytoplankton characteristics are sufficiently homogeneous in this oceanic region, such that minor water mass advection would not have significantly influenced primary productivity or photophysiological parameters measured over the diurnal cycle
Chlorophyll a concentrations were homogeneously distributed throughout the mixed layer (0.26 ± 0.03 mg m−3; eight depths sampled on one cast at 12:30 local time (LT)), while temperature was nearly invariant (10.4 ± 0.07 ◦C) during our sampling period

Summary

Introduction

Marine phytoplankton account for ∼ 50 % of global carbon fixation (Field et al, 1998) and play a key role in Earth’s biogeochemical cycles. Rates of phytoplankton primary production have been measured using incubation-based assays, tracing the evolution of oxygen or the assimilation of CO2 (Williams et al, 2008). Over the past 2 decades, biooptical approaches based on measurements of active chlorophyll a fluorescence (ChlF) yields (Kolber and Falkowski, 1993; Schreiber, 2004) have emerged as an attractive alternative, avoiding artifacts related to bottle containment and achieving unparalleled spatial and temporal resolution. The method most prominently applied to measure ChlF yields in field assemblages of marine phytoplankton is fast repetition rate fluorometry (FRRF) (Kolber et al, 1998). ChlF yields, as measured by FRRF, can be used to estimate electron transport in photosystem II (ETRRCII, mol e− mol RCII−1 s−1), and these rates can be converted to carbon units based on theoretical calculations.

Methods

Results

Discussion

Conclusion