PARENT SESSION

Symposium S2D Marine photosynthesis and production
Monday August 30th, 2004 2:40 PM-4:40 PM Room 510B
Chair: John Raven
Co-Chair: Doug Campbell

Estimating phytoplankton primary productivity by means of PAM, FRRF, oxygen-exchange and C-fixation. Jacco Kromkamp^*,1, Nicole Dijkman¹, ¹ NIOO-KNAW, Yerseke, Netherlands

ABSTRACT- Understanding how aquatic ecosystems function requires knowledge of primary production. In the marine environment phytoplankton this is mainly measured using C-fixation. Because this technique is difficult to use in standard monitoring practices a new approach is required. Variable fluorescence techniques are generally quick and can, in principle, be used in-situ. This also prevents bottle effects. However, because variable fluorescence techniques measure the initial steps of the light reaction, the calculated PSII electron transport rates (ETR) are not equal to rates of C-fixation or net or gross oxygen evolution. In this paper we compare photosynthesis measurements made on different unicellular algae using two variable fluorescence techniques (FRRF, using a single turnover protocol) and the PAM (using a multiple turnover protocol to measure Fm) with more classical techniques to measure photosynthesis: C-fixation and net- and gross oxygen evolution (¹⁸O₂ as tracer to measure oxygen uptake in the light with a membrane inlet mass spectrometer). Calculation of C-fixation from ETR requires knowledge of the absorption properties of the cells, the relative antenna sizes, the photosynthetic quotient and the electron yield, i.e. how many electrons are needed to produce an oxygen or fix a CO₂ molecule. We demonstrate that gross O2-evolution is generally linearly related to ETR estimated by both the PAM and the FRRF, but that variability between the estimates seem larger with the FRRF. In contrast to gross O₂-exchange, a comparison between ETR and C-fixation showed some degree of non-linearity at higher irradiances. Our results also demonstrate that estimation of C-fixation or O2-evolution requires good knowledge of the absorption properties of the cells and the electron yield. Nevertheless these uncertainties, we think that using applying variable fluorescence techniques in standard monitoring programs of aquatic ecosystems help to understand the way the ecosystem functions.

KEY WORDS: FRRF, PAM, photosynthesis