MRPuptake kinetics

Phosphorus uptake and transport in microorganisms are thought to depend on two separate uptake mechanisms. In the Enterobacteria, such as Escherichia coli, which can normally experience a much higher external concentration than a free-living phytoplankter, a low-affinity membrane transport system normally operates (Rao and Torriani, 1990). If external phosphorus concentration falls, however, to <20 |M (~0.6 mg P l-1), the second, high-affinity system is activated. This one is ATP-driven and is linked directly to periplasmic phosphate-binding sites. Working with a cultured strain of the marine cyanobac-terium, Synechococcus sp. WH7803, Scanlan et al. (1993) demonstrated the accelerated synthesis of several intracellular polypeptides as their cultures became increasingly depleted of soluble phosphorus. One of these, a 32-kDa polypeptide, was localised in the cell wall, linked to an intracellular 100-kDa polypeptide. Together, these conform to the typical structure of a receptor transport system (cf. Fig. 4.3). These polypeptides showed 35% identity and 52% similarity with those of E. coli. They also showed that the encoding genes were almost identical to those isolated from other Synechococcus strains, which had already been linked to the induction of phosphatase activity in Synechococcus PCC7942 (Ray et al., 1991). Alkaline phosphatases are a well-known group of zinc-based enzymes which break phosphate ions from organic polymers in the external medium close to the cell. These are also said to be produced only under conditions of declining external orthophosphate concentrations. Ihlen-feldt and Gibson (1975) noted phosphatase production in a freshwater Synechococcus at external concentrations of <4 |M P.

Eukaryotic phytoplankton has not been investigated to this level of biochemical detail. However, it seems likely that analogous mechanisms and similar sensitivities apply among the many phytoplankters that inhabit aquatic environments in which phosphate concentrations are frequently <1 |M P and, often, an order of magnitude less again (<0.1 |M P, i.e. <10-7 mol L-1, <3 |g P L-1).

Most of our present knowledge of the phosphorus-uptake kinetics of phytoplankton comes from the numerous laboratory studies on named species, carried out mainly in the middle years of the last century. Some of these have been used in the compilation of compendia and reviews (Reynolds, 1988a, 1993a; Padisak, 2003). In order to make valid interspecific comparisons (such as those in Fig. 4.5), it is necessary to convert often disparate measurements to appropriate common scales. Only volume- or carbon-specific uptake rates of planktic cells lend themselves to some generalisations. One of these is that the maximum phosphorus-uptake rates (VUmJ of a range of freshwater phytoplankton are comparable, at least within 2 orders of magnitude, when expressed per unit area of algal unit surface (Fig. 4.5b: 0.5 to 35 x 10-19 mol P |im-2 s-1). When normalised to cell carbon, the same data translate to maximum uptake rates

Figure 4.5

(a) Absolute maximal phosphorus uptake rates of phytoplankton cells and colonies as reported in the literature reviewed by Reynolds (1988a) and expressed on a common scale. (b) The same data normalised to the surface areas of the cells or colonies as appropriate. (c) The same data normalised to cell carbon (the shaded part of the histograms correspond to the fastest carbon-specific rate of P assimilation in growth; the balance represents spare capacity (note the logarithmic scales). The algae are Ana, Anabaena flos-aquae; Ast, Asterionella formosa; Chla, Chlamydomonas sp.; Chlo, Chlorella; Din, Dinobryon divergens; Eud, Eudorina unicocca; Mic, Microcystis aeruginosa; Per, Peridinium cinctum; Plaa, Planktothrix agardhii; Scq, Scenedesmus quadricauda: Vol, Volvox aureus. Data presented in Reynolds (1993a) and redrawn from Reynolds (1997a) with permission.

Figure 4.5

(a) Absolute maximal phosphorus uptake rates of phytoplankton cells and colonies as reported in the literature reviewed by Reynolds (1988a) and expressed on a common scale. (b) The same data normalised to the surface areas of the cells or colonies as appropriate. (c) The same data normalised to cell carbon (the shaded part of the histograms correspond to the fastest carbon-specific rate of P assimilation in growth; the balance represents spare capacity (note the logarithmic scales). The algae are Ana, Anabaena flos-aquae; Ast, Asterionella formosa; Chla, Chlamydomonas sp.; Chlo, Chlorella; Din, Dinobryon divergens; Eud, Eudorina unicocca; Mic, Microcystis aeruginosa; Per, Peridinium cinctum; Plaa, Planktothrix agardhii; Scq, Scenedesmus quadricauda: Vol, Volvox aureus. Data presented in Reynolds (1993a) and redrawn from Reynolds (1997a) with permission.

of between 0.1 and 21 x 10-6 mol P (mol cell C)-1 s-1. Against the theoretical requirement for phosphorus to sustain a doubling of the cell carbon (9.4 x 10-3 mol P (mol cell C)-1), these maximal P-uptake rates (VUmax) would be sufficient to meet the growth demand in from 440 to 94 000 s (~7 minutes to 26 h), supposing a saturating concentration and a constant rate of uptake. The steady-state phosphorus requirements of the same planktic species growing at their respective maximal cellular growth rates (from Chapter 5) are inserted in Fig. 4.5c to emphasise a second generalisation. It is that we need not be too concerned about phosphorus-limited uptake rates that remain capable of saturating growth rates down to external concentrations (reading from Fig. 4.4) of the order of 0.1 x 10-6 mol P L-1 (see also Section 5.4.4).

Nevertheless, ambient concentrations of MRP in some natural waters may be chronically constrained to this order and, in many others, be frequently drawn down to such levels. We may readily accept that phytoplankton tolerant of such conditions must invoke high-affinity mechanisms for phosphorus uptake. Our interest should be sharply focused on the shape of the uptake curve at the extreme left-hand side of Fig. 4.4 and the beneficial distortion represented by a relatively low half-saturation concentration (Ku). Indeed, the lower is the concentration required to half-saturate the uptake of phosphorus, then the greater is the likely ability of the alga to fulfil its requirements at chronically low external concentrations. The faster is the uptake capacity at the low, markedly sub-saturating resource levels, then the greater is the alga's affinity for phosphorus and the greater is its ability to compete for scarce resources.

Pursuing this reasoning further, Sommer's (1984) experiments distinguished several differing adaptive strategies among freshwater phyto-plankton for contending with variable supplies of

Table 4.2 Some species-specific values of maximum phosphorus uptake rate (VUmax) at ~20 °C and the external concentration of MRP required to half-saturate the uptake rate (KU, being the concentration required to sustain °.5 VUmax )

(mol cell C)-1 s-1

Ku |xmol P L-1

Referencesa

Chlamydomonas reinhardtii

0 0

Post a comment