Page 162 - The Biochemistry of Inorganic Polyphosphates

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Peculiarities of polyphosphate metabolism
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PolyP ( µmol P i (g biomass) −1 ) 30
40

20

10

0
3
2
4
Time of growth (d) 5 6
Figure 8.13 Changes in PolyP content during growth of Halobacterium salinarium on a medium
with 2.3 mM of P i (Smirnov et al., 2002a): ( ) acid-soluble PolyP; ( ) alkali-soluble PolyP.

Methanosarcina frisia accumulates phosphate up to a level of 14 % of its dry weigh (Rud-
31
nick et al., 1990). The phosphate is stored as PolyP, as shown by P NMR spectroscopy. This
archaeonaccumulatemorephosphateinthepresenceofmethanolasthecarbonsource,when
compared with CO 2 and H 2 as the only carbon and energy sources (Rudnick et al., 1990).
Halobacterium salinarium and Halorubrum distributum, extremely halophilic archae,
were capable of consuming up to 95 % of the phosphate from the culture medium at
P i concentrations of 2.3 and 11.5 mM. These archae possess PolyPs of acid-soluble and
alkali-soluble fractions, and the contents of these PolyPs changed abruptly during growth
(Andreeva et al., 2000; Smirnov et al., 2002a,b). The dynamics of the PolyP content during
the growth of H. salinarium on the medium with 2.3 mM P i is shown on Figure 8.13.
However, the phosphorus of the PolyP was no more than 10 % of all of the phosphorus
accumulated in the culture (Table 8.1). The greater part of the P i was present in biomass as
magnesium orthophosphate, the amount of which by the early stationary phase might have
reached nearly 90 % of the phosphate consumed by the cells (Smirnov et al., 2002a,b).
The excess accumulation of P i evoked changes in cell morphology, and a part of the cell
population lost viability. The accumulation of phosphate as its inorganic soluble salt is an
unfavourable factor for the vital functions of individual cells but may be useful for the sur-
vival of a population as a whole at further growth on a phosphate-deficient medium. It was
shown that H. salinarium cells grown on a medium with P i excess can use the P i -phosphate
reserve in a P i -limited medium (Smirnov et al., 2002a,b). Obviously, H. salinarium has
insufficiently developed regulatory mechanisms, which might regulate phosphate utiliza-
tion and reservation as PolyP. Although this organism had an appreciable PolyP pool, its
exopolyphosphatase activity was very low (Andreeva et al., 2000). This activity did not
depend on the phosphate content in the medium, the amount of PolyP, or the culture age.
Thus, in contrast to yeast and a number of bacteria where PolyPs play an important role in
maintaining homeostasis of phosphorus compounds in a cell under unfavorable conditions,
the PolyP function and metabolism in H. salinarium are not directly connected with P i reser-
vation. H. salinarium obviously shows a rather ancient and primitive form of phosphate
reservation as inorganic P i .