Given the relatively high half-saturation constants for RUBISCO carboxylation (12-60 |M) (see Section 3.2.3), photosynthetic carbon fixation is plainly vulnerable to rate limitation by the low aquatic concentrations to which CO2 may be drawn (<10 |M) (see Section 3.4.1). Even with relatively plentiful supplies of carbon dioxide, the harvesting mechanisms of aquatic plants need to be well developed (Raven, 1991). In the first place, satisfaction of the principal requirements of planktic cells embedded in the viscous range is subject to Fick's laws of diffusion. The number of moles of a solute (n) that will diffuse across an area (a) in unit time, t, is a function of the gradient in solute concentration, Co, (i.e. dCo/dx) and the coefficient of molecular diffusion of the substance (m):
Reynolds (1997a) used data for the single, spherical cell of Chlorella (diameter <4 x 10-6m, approximate surface area <50.3 x 10-12m2) to illustrate the limits of diffusion dependence. Given (i) that, for an average small-sized solute molecule (such as carbon dioxide), m ~ 10-9m2s-1, that (ii) the thickness of the adjacent water layer from which nutrients may be absorbed is equal to the cell radius and (iii) the concentration of carbon dioxide molecules beyond is at air equilibrium (11 |molCO2L-1, or 11 x 10-3molm-3), then Eq. (3.19) is solved to deliver 275 x 10-18 mol s-1. Now, let us assume the volume of the cell (v) is 33.5 x 10-18 m3
and contains 0.63 x10-12 mol carbon (Tables 1.2, 1.3). If we also assume every molecule of carbon dioxide so encountered is successfully taken into the cell, then the requirement to sustain the doubling of biomass without change in the internal carbon concentration is a further 0.63 x 10-12 mol C (cell C)-1. While the concentration gradient is maintained, the diffusion rate calculated from Eq. (3.19) is capable of delivering the entire carbon requirement to the cell in ~2300 s (i.e. just over 38 minutes).
For proportionately lower concentrations of carbon dioxide, Eq. (3.19) delivers smaller amounts per carbon per unit time and the time to accumulate the material for the next doubling is correspondingly extended. A concentration of 0.3 |imol CO2 L-1 could not sustain a doubling in less than 1 day, when growth rate would be considered to be carbon limited. With pH already close to 8.3, the (uncatalysed) dissociation of bicarbonate would support a continuing supply of carbon dioxide. When that too became exhausted, pH drifts quickly upwards as carbonate becomes the dominant form of inorganic carbon.
Many planktic algae avoid (or at least delay) carbon-dioxide limitation and slow bicarbonate dissociation through resort to a carbon-concentration mechanism, or CCM. Although the kinetic characteristics of the key RUBISCO enzyme (especially its high half-saturation requirement for CO2) cannot be modified, the CCM provides the means to maintain its activity by concentrating CO2 at the sites of car-boxylation. Since their function was first recognised (Badger et al., 1980, Allen and Spence, 1981; Lucas and Berry, 1985), the mechanisms assisting survival of low-CO2 conditions have continued to be intensively investigated. Progress has been reported in several helpful reviews (Raven, 1991, 1997; Badger et al., 1998; Moroney and Chen, 1998). Working with the green flagellate Chlamydomonas reinhardtii, Sultemeyer et al. (1991) showed that the algal CCM involves a series of ATP-mediated cross-membrane transfers - at the cell wall, the plasma membrane and the chloro-plast membrane - which transport and concentrate bicarbonate ions as well as carbon dioxide. Breakdown of the bicarbonate is accelerated through the action of carbonic anhydrase. The carbon dioxide thus available to the carboxyla-tion of RuBP is effectively concentrated by a factor of 40.
The more intensively studied CCM of the Cyanobacterium Synechococcus also transports and accumulates carbon dioxide and bicarbonate ions, achieving concentration factors in the order of 4000-fold (Badger and Gallacher, 1987). Recent work has revealed the mechanism and genetic control of each of four separate uptake pathways in Synechococcus PCC7942 (Omata et al., 2002: Price et al., 2002). Two take up CO2 at a relatively low affinity, one constitutive and the other inducible, involving thylakoid-based dehydroge-nase complexes. There is a third, inducible, high-affinity bicarbonate transporter (known as BCT-1) that is activated by a cAMP receptor protein at times of carbon starvation (a fuller discussion is included in Section 5.2.1). The fourth mechanism is a constitutive Na+-dependent bicarbonate transport system that is selectively activated (perhaps by phosphorylation).
CCMs represent a remarkable adaptation of some (but not all) photosynthetic microorganisms to the onset of carbon limitation of production rates. They are energetically expensive to operate and, not surprisingly, are invoked to assist survival and maintenance only under severe conditions of DIC depletion. The photon cost of fixation of CO2 concentrated by the cell as opposed to the harvest at equilibration is roughly doubled (>16 mol photon per mol C fixed) and the compensation point is raised to around 10 |imol photons m-2 s-1 (Raven et al., 2000).
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