No abstract

In this review, we briefly describe the two main techniques used to measure variable fluorescence in the aquatic environment, and show how the parameters derived from this techmque can be used to estimate the rate of photosynthesis. The methods estimate the photochemical efficiency of photosystem II from ratios of fluorescence levels. Flashes of light that are transiently saturating for photochemistry (i.e. they are sufficiently bright to close all PSII reaction centres) are used to obtain the maximum fluorescence level. The type of saturating flash differs between methods. In one approach, single turnover (ST) flashes are applied. This allows only one charge separation during the flash and reduces only the primary acceptor of PS II, raising fluorescence to a level F mr STj' In a second approach the flashes are multiple turnover (MT), which allow repeated charge separation processes until all electron acceptors of PS II are reduced. A relaxation of quenching is induced by the longer flash, and this raises the maximum fluorescence to a higher level, F m ( MT} Application of the different approaches to an algal sample will result In differing F m values and, as a result, different values for the photochemical efficiency of PS II, with the MT method giving higher values than ST. Several designs of equipment, based on MT or ST techniques, are available for use with phytoplankton or benthic algae. Both techniques measure variable fluorescence, but there are a number of important differences in the methods used to calculate photosynthetic rates. In our view, this necessitates the use of a different terminology in order to avoid confusion, until the underlying physiological differences are resolved. An example IS given showing that combining terminology from the different approaches will result in calculation of erroneous photosynthetic electron transport rates.Key words: absorption, fluorescence, PAM, FRRF, photosynthesis, photosynthetic electron transport, variable fluorescence, optical cross-section, functional cross-sectIOn Introduction Understanding the function of aquatic ecosystems requires knowledge of carbon cycles, which in turn requires a detailed knowledge of biological CO 2 fixation (Geider et 01., 200 I). The process of gross photosynthetic carbon fixation entails the absorption of light energy by antenna pigments, followed by the process of charge separation, which produces reducing equivalents (NADPH 2 ). These are used together with photosynthetically-generated ATP to fuel the C02-fixing enzymatic reactions of the Calvin cycle. There are many techniques available to measure photosynthesis or primary production, and the two terms are often used interchangeably. A discussion of the different Correspondence to: lacco Kromkamp. e-mail: j.kromkamp(llmoo.knaw.nl photosynthesis and production terms is outside the scope of this review; the reader is referred to Williams (1993), Platt & Sathyendranath (1993) and Sakshaug et 01. (1997) for more information. Here, we define photosynthesis as gross carbon fixa...

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.

A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of m~crophytobenthos on an Intertidal mudflat Spectral ~rradiance measurements Indicate that 7500 of the signal detectable by the PAM onginates In the upper 150 pm of the s e d~m e n t From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season Photoacclimation to lower irrad~ance was ind~cated by changes in the maximum ETR and the saturating photon irradiance parameter I, When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra The data also showed that the benthic algae did not seem to expenence photoinhib~tion or COZ limitation To explain this, ~t is hypothes~sed that there is a continuous vertical migration in the top layer of the s e d~m e n t , whele algae can a v o~d photoinhibition due to prolonged periods of high irrad~ance and lack of CO2 by m~grating downwards whlle others migrate upwards KEY WORDS Microphytobenthos Chlorophyll fluorescence Photosynthesis Vert~cal I-rugratlon C-limitation

Marine phytoplankton account for about 50% of all global net primary productivity (NPP). Active fluorometry, mainly Fast Repetition Rate fluorometry (FRRf), has been advocated as means of providing high resolution estimates of NPP. However, not measuring CO2-fixation directly, FRRf instead provides photosynthetic quantum efficiency estimates from which electron transfer rates (ETR) and ultimately CO2-fixation rates can be derived. Consequently, conversions of ETRs to CO2-fixation requires knowledge of the electron requirement for carbon fixation (Φe,C, ETR/CO2 uptake rate) and its dependence on environmental gradients. Such knowledge is critical for large scale implementation of active fluorescence to better characterise CO2-uptake. Here we examine the variability of experimentally determined Φe,C values in relation to key environmental variables with the aim of developing new working algorithms for the calculation of Φe,C from environmental variables. Coincident FRRf and 14C-uptake and environmental data from 14 studies covering 12 marine regions were analysed via a meta-analytical, non-parametric, multivariate approach. Combining all studies, Φe,C varied between 1.15 and 54.2 mol e− (mol C)−1 with a mean of 10.9±6.91 mol e− mol C)−1. Although variability of Φe,C was related to environmental gradients at global scales, region-specific analyses provided far improved predictive capability. However, use of regional Φ e,C algorithms requires objective means of defining regions of interest, which remains challenging. Considering individual studies and specific small-scale regions, temperature, nutrient and light availability were correlated with Φ e,C albeit to varying degrees and depending on the study/region and the composition of the extant phytoplankton community. At the level of large biogeographic regions and distinct water masses, Φ e,C was related to nutrient availability, chlorophyll, as well as temperature and/or salinity in most regions, while light availability was also important in Baltic Sea and shelf waters. The novel Φ e,C algorithms provide a major step forward for widespread fluorometry-based NPP estimates and highlight the need for further studying the natural variability of Φe,C to verify and develop algorithms with improved accuracy.