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Source: Generalised values synthesised from the literature (Harris, 1978; Reynolds, 1984a, 1990, 1994a; Padisak 2003).

for measuring the photosynthetic incorporation of carbon dioxide labelled with the radioactive isotope. Still using darkened and undarkened bottles suspended in the light field, the essential principle is that the natural carbon source is augmented by a dose of radio-labelled NaH14CO3 in solution, which carbon source is exploited and taken up and fixed into the photosynthetic algae. At the end of the exposure, the flask contents are filtered and the residues are submitted to Geiger counting (for a recent methodological guide, see Howarth and Michaels, 2000), and the quantity thus assimilated is calculated. It is supposed that 14C will be fixed in photosynthesis in the same proportion to 12C that is available in the pool at the start of the exposure. Then,

12CO2 uptake/12CO2 available

= 14CO2 uptake/14CO2 available (3.10)

Since its introduction, the method has been improved in detail (the liquid-scintillation and gas-phase counting technique is nowadays preferred) but, in essence, the original method has survived intact (Sondergaard, 2002). Providing proper licensing and handling protocols are followed meticulously, the method is easy to apply and yields reproducible results. Comparisons with simultaneous measurements of oxygen evolution normally give tolerable agreement, if allowance is made for a PQof ~1.15 (Kirk, 1994). A large number of results have been published in the literature and these have been the subject of a series of syntheses (including Harris, 1978, 1986; Fogg and Thake, 1987; Kirk, 1994; Padisak, 2003). It is sufficient in the present context simply to summarise the key characteristics that have been reported (maximum measured chlorophyll-specific rates of light-saturated carbon incorporation and the chlorophyll-specific photosynthetic efficiencies under sub-saturating light intensities (see Table 3.1).

Two comments are important to make, however. One is the positive linkage of light saturation with temperature, at least within the range of the majority of observations - between 5 and 25 °C. As with many other cellular processes, activity increases with increasing temperatures, up to maximal levels varying between 25 and 40 °C. There is a plain dependence for the photosynthetic rates to accelerate with higher temperatures and, so, for there to be a higher threshold flux of photons necessary to saturate it. With no change in the strongly light-limited rates of photosynthesis, a may vary little (i.e. light-limited photosynthesis is not temperature-constrained; left-hand plot in Fig. 3.5). Thus, Ik increases with temperature, in broadly similar proportion to the increase in Pmax.

As a function of customary temperature, Pmax increases non-linearly, roughly doubling with each 10 °C rise in temperature. This multiple, formalised as the Qjo factor, is now used infrequently as a physiological index: preference is now accorded to the slope of reactivity on the Arrhenius scale, which expresses absolute temperature (in kelvins) as a reciprocal scale, 1/(temperature in K). As an example, the measured temperature sensitivity of light-saturated h a

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