Comparing electron transport with gas exchange: parameterising exchange rates between alternative photosynthetic currencies for eukaryotic phytoplankton

Abstract

Estimates of aquatic primary productivity derived from in situ active chl a fluorescence have rapidly gained popularity over the past 2 decades. This trend has been driven primarily by the need to improve upon 'conventional' carbon (C) uptake-or oxygen (O 2 ) evolution-based productivity estimates that require water samples to be incubated ex situ. Unlike the conventional approaches to measuring productivity, chlorophyll fluorescence measurements inherently describe only the activity of photosystem II (PSII) in the light reactions; thus, the photosynthetic 'currency' of the fluorescencebased approach is an electron turnover rate for PSII (ETR PSII ). A photosynthetic currency of electrons has limited ecological relevance but can be converted to a currency of carbon if an 'exchange rate', i.e. a value or factor of equivalence for any single time point, is applied. We used fast repetition rate fluorometry (FRRf), mass inlet membrane spectrometry (MIMS) and 14 C uptake to determine ETR PSII , gross and net O 2 evolution and C fixation measured simultaneously for 6 microalgal species and for different steady-state growth conditions. Quantifying the PSII reaction centre (RCII) concentration and the spectral dependency of the effective absorption cross section yielded an FRRf approach that provided a robust estimate of the ETR PSII and gross O 2 evolution for all species and conditions tested; however, the ETR PSII exceeded carbon dioxide (CO 2 ) uptake by a factor of ~5.4 to 11.6. At least 3 species exhibited substantial light-dependent O 2 cycling to account for ~40 to 60% of the difference between the ETR PSII and CO 2 uptake. The highly variable nature of the ETR PSII :CO 2 uptake 'exchange rate' observed here highlights the need for future studies that rely on active fluorescence to examine aquatic productivity to focus towards a systematic description of how electrons are coupled to C fixation in nature.KEY WORDS: Fast repetition rate fluorescence · Electron transport · Photosynthesis · Oxygen evolution · Carbon uptake · Phytoplankton Resale or republication not permitted without written consent of the publisherContribution to AME Special 2 'Progress and perspectives in aquatic primary productivity' OPEN PEN ACCESS CCESS

Primary Production in the Ocean

“…This effect may be compounded in the algae due to their taxonomic and resultant physiological diversity (Suggett, 2011). Suggett et al, (2009) used an FRR fluorometer to measure ETR PSII and examined the relationship between ETR PSII and photosynthesis measured by either gross O 2 production or 14 CO 2 fixation. Measurements were made using six species of eukaryotic phytoplankton, representing a diversity of taxonomic groups.…”

The influence of phytoplankton pigments composition and dominant cell size on fluorescence-derived photophysiological parameters and implications for primary production rates

“…even so, we know that фe:C varies as a function of taxa (Suggett et al 2009b) as well as with different oceanographic provinces and environmental conditions (Lawrenz et al 2013). Indeed, further measures of фe:C are clearly warranted to verify the impacts of the dominant cells' size on this ratio and how this potentially varies across the highly dynamic nutrient-phytoplankton conditions.…”

“…The latter is an approximate ratio found for natural communities dominated by cyanobacteria and small flagellates (Suggett et al 2009b), which was chosen due to the dominance of small cells found in our study, based on the fractionated chlorophyll-a concentration.…”

“…a. ETRPSII light curves modeled from the parameterization according to the dominant size of the phytoplankton community. b. Carbon-based primary production estimates from ETRPSII modeled according to the community size, using fixed values for nPSII=0.002 and фe:c=8 mol e -/mol CO2 (sensu Suggett et al 2009b).…”

“…Besides the impacts of environmental condition on both Fv/Fm and σPSII, several studies have demonstrated the influence of phytoplankton cell size and taxonomy on measurements of variable fluorescence, and hence on their photosynthetic rates (Suggett et al 2004;Suggett et al 2009b;Rattan et al 2012;Moore et al 2005). Specifically, whilst lower Fv/Fm and increased σPSII occurs via nutrient starvation, the same response also occurs via taxonomic changes under nutrient limitation: the small cells that are favored in low-nutrient environments exhibit lower Fv/Fm and higher σPSII values than larger cells (Fishwick et al 2006;Suggett et al 2009a).…”