photosynthesis and dark respiration of laboratory isolates of Asterionella formosa are shown in Fig. 3.6. The normalised, Arrhenius coefficient is -18.88 x 10-3 per reciprocal kelvin. In the more familiar, if incorrect, terms, Qjo is 2.18. Almost all quoted values, applying both to named species and to phytoplankton in general, fall in the range 1.8-2.25 (Eppley, 1972; Harris, 1978). Some variation is probable, not least because photosynthesis is a complex of many individual reactions. However, its maximum, light-saturated rate is primarily a function of temperature (Morel, 1991), even though the several descriptive equations fitted to experimental data differ mutually (Eppley, 1972; Megard, 1972) and have been shown to be underestimates against the maximum growth rates that have been observed (Brush et al., 2002). Whereas photon capture has a Q10 close to 2 (see Section 3.2.1), protein assembly and internal transport have greater temperature sensitivity (Tamiya et al., 1953; see also Section 5.3.2 for the influence of temperature on growth). The Qj0 of steady, dark respiration rates of healthy phyto-plankters is similarly close to 2.

The second comment is that all these deductions are subject to uncertainties about the precision of the radiocarbon method. Interpretational difficulties were recognised early on (includ ing by Steemann Nielsen himself) and, perplex-ingly, these persist to the present day. The most important concerns the metabolic exchanges and cycling of carbon, in which the labelled carbon participates relatively freely. At first, labelled carbon moves in only one direction, from solution to photosynthate; it is a manifestation of gross photosynthesis. As the experiment proceeds, some of the 14C-labelled carbohydrate may be assimilated but it may just as easily be used in basal respiration, or it may well be subject to photorespiration or excretion (see Section 3.2.3). This means that, as the incubation proceeds, the method is ostensibly measuring something closer to net photosynthesis. Long incubations may determine only net photosynthetic 14C incorporation (Steemann Nielsen, 1955; Dring and Jewson, 1979). Comparing net 14C assimilation with net oxygen production over 24-h incubations, by which time respired 14CO2 is being refixed, takes PQ closer to 1.4 (see Marra, 2002).

The switches towards this more balanced state of exchanges will be approached at different rates, depending upon temperature and the irradiance to which the incubating material is exposed and on the physiological condition of the alga at the outset. The behaviour has been expressed through various descriptive equations (notably those formulated by Hobson et al., 1976; Dring and Jewson, 1982; Marra et al., 1988; Williams and Lefevre, 1996). A probabilistic outcome is that the largest proportion of the gross uptake of 14C is assimilated into new protein and biomass in healthy cells when they are simultaneously exposed to sub-saturating light intensities. Conversely, with approaching light saturation of the growth-assimilatory demand for photo-synthate (recognising that this may be constrained by factors other than the supply of photo-synthate), then more of the excess is vented or metabolised in other ways. Thus, the ratio of net photosynthetic carbon fixation to gross photosynthetic carbon fixation (Pn : Pg) is in the proportion of the fraction of the gross photo-synthate that can be assimilated, i.e. (Pg — R)/Pg (Marra, 2002). Here, R may represent not just the basal, autotrophic respiration (Ra) but, in addition, all metabolic elimination of excess photosynthate (Rh). Even under the optimal conditions envisaged, Ra never disappears but is always finite, being, at least, about 4% of Pmax (or the maximum sustainable Pg at the same temperature), and typically 7-10% (Talling, 1957b, 1971; Reynolds, 1984a). Reynolds' (1997a) best predictor of basal respiration in a number of named freshwater phytoplankters at 20 °C is related to the surface-to-volume ratio [Ra20 = 0.079 (s/v)0 325]. On the other hand, the sum of physiological losses in light-saturated and/or nutrient-deficient cells, including that vented as DOC, can be extremely high, approaching 100% of the fixation rate: the quotient (Pg - R - Rh)/Pg and the ratio Pn : Pg both fall toward zero.

This is a plausible way of explaining difficulties experienced in accounting for the fate of the carbon fixed in photosynthesis (Talling, 1984; Tilzer, 1984; Reynolds et al., 1985) and the sometimes very large gaps between primary carbon fixation and net biomass accumulation (Jassby and Goldman, 1974a; Forsberg, 1985). Thus, beyond gaining a broad perspective on the constraints acting on chlorophyll-specific photo-synthetic rates, it is necessary also for the physiological ecologist to grasp the manner in which the environmental conditions mould the deployment of fixed carbon into the population dynamics of phytoplankton.

0 0

Post a comment