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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 ETRPSII and gross O 2 evolution for all species and conditions tested; however, the ETRPSII 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 ETRPSII and CO 2 uptake. The highly variable nature of the ETRPSII: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. © Inter-Research 2009

aquatic microbial ecology

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