Page 108 - The Biochemistry of Inorganic Polyphosphates

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Functions of polyphosphate and polyphosphate-dependent enzymes
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Kaltwasser, 1962; Lichko et al., 1982; Kulaev and Vagabov, 1983). The same effect is ob-
served when free P i accumulates in a cell as a result of degradation processes, especially
the degradation of nucleic acids (Harold, 1962b). Phosphate overplus (hypercompensation
effect) is observed for both prokaryotes and eukaryotes. Phosphorus starvation results in
de-repression of phosphatases localized on cell surfaces and of phosphate uptake systems
(Harold, 1966; Nesmeyanova et al., 1974a,b, 1975a). Both processes, the cleavage of or-
ganic phosphorus compounds and P i uptake from the medium, may increase the P i level
when phosphorus-starved cells are placed in a medium containing this element. In order
to maintain a sensibly constant level of P i , it is converted into PolyP (Ehrenberg, 1960;
Harold, 1962b, 1966).
The importance of maintaining a constant low level of P i in cells is relevant on ac-
count of several considerations. First, P i concentration is, in turn, a powerful controlling
factor of biochemical processes. Secondly, accumulation of any significant amounts of P i
in cells would result in a considerable change of its osmotic pressure and pH. It is also
possible that free P i in high concentrations is toxic for cells. One example of such toxicity
was observed during P i accumulation by the halophilic archae Halobacterium salinar-
ium (Smirnov et al., 2002a,b). This archaeon is able to take up about 90 % of P i from
culture medium but unable to synthesize PolyPs in large amounts. As a result, massive
P i uptake leads to an accumulation of magnesium phosphate in cells, a change in cell
morphology, and the death of some part of the population. In fact, it is likely that the
large amounts of PolyPs, which are accumulated in cells of microorganisms under cer-
tain culture conditions, are a detoxification product of P i entering the cells. Some features
of the PolyP function as a P i reserve in prokaryotes and eukaryotes will be described
below.

7.1.1 In Prokaryotes
Many bacteria are able to accumulate PolyPs if the P i content in the medium is high.
In Acinetobacter johnsonii, these polymers make up to 30 % of dry biomass (Deinema
et al., 1985). Large amounts of PolyPs are characteristic of the bacteria from wastewaters
with a high phosphate content. A. johnsonii (Deinema et al., 1985; Kortstee et al., 1994,
2000), Microlunatus phosphovorus (Nakamura et al., 1995), Microthrix parvicella (Erhardt
et al., 1997) and Rhodocyclus sp. (Keasling et al., 2000), isolated from activated sludge, are
examples of such bacteria. The biotechnology of ‘enhanced biological phosphate removal’
(EBPR) has become a field of rapid development. This is based on the ability of bacterial
communities of activated sludge to remove P i from waste and to accumulate PolyPs in
sludge biomass. Many reviews describe the biochemical and biotechnological aspects of
this process (Kortstee et al., 1994, 2000; Van Loosdrecht et al., 1997; Ohtake et al., 1999;
Keasling et al., 2000; Mino, 2000; McGrath and Quinn, 2003). We will return to this topic
in Chapter 9.
In E. coli, the level of PolyP drops drastically under phosphate starvation, and the
subsequent addition of orthophosphate to the medium restores the initial phosphate level
(Nesmeyanova et al., 1973, 1974a,b; Nesmeyanova, 2000). Some genetic manipulations
increased the ability of E. coli to accumulate PolyP (Kato et al., 1993a; Hardoyo et al., 1994;
Ohtake et al., 1994; Sharfstein et al., 1996). High levels of accumulation were achieved