Page 200 - The Biochemistry of Inorganic Polyphosphates

WU095/Kulaev
WU095-09
Applied aspects of polyphosphate biochemistry
184 March 9, 2004 15:44 Char Count= 0
2000; Ohtake and Kuroda, 2000; Mino, 2000; Keasling et al., 2000; Blackall et al., 2002;
Seviour et al., 2003; McGrath and Quinn, 2003).
EBPR involves the cycling of microbial biomass and influent wastewater through anaero-
bic and aerobic zones to achieve selection of microorganisms with high capacities for PolyP
accumulation in cells in the aerobic period. In the anaerobic zone of the treatment system,
the cells are electron-acceptor deficient and carbon-rich. It has been proposed that PolyP
is degraded to P i , which is excreted from the cell to increase the transmembrane proton
gradient. Carbon is then taken up via a proton-symport pump and stored inside the bacterial
cell as polyhydroxyalkanoate (PHA). In the subsequent aerobic zone of the treatment sys-
tem, the environment is electron-acceptor-rich but carbon-deficient. It has therefore been
proposed that PHA is degraded and PolyP is synthesized from ATP generated from PHA
metabolism. Since more P i is taken up during the aerobic phase than is secreted during the
anaerobic phase, there is a net removal of P i from the wastewater.
Although the anaerobic–aerobic process for EBPR is an established process from the
engineering point of view, it has not been clearly defined in microbiological terms. For
example, the phylogenetic or taxonomic groups responsible for EBPR and general structures
of the EBPR microbial community have not been described once and for all. Very few pure
cultures have been isolated as candidates for playing the key role in EBPR processes. Studies
of the metabolic aspects of EBPR have been based mainly on enriched mixed cultures but
not on pure cultures. Polyphosphate kinase activity and the occurrence of ppk genes have
been directly investigated in activated sludge performing enhanced biological phosphorus
removal (Bolesch and Keasling, 2000b).
There are two groups of microorganisms involved in EBPR. These are the PolyP-
accumulating organisms and their supposed ‘carbon competitors’, known as glycogen-
accumulating organisms. Acinetobacter spp. was first proposed as the bacteria responsible
for EBPR (Fuhs and Chen, 1975) and intensive studies of their physiology, genetics and
PolyP-metabolizing enzymes were carried out (Deinema et al., 1980, 1985; Van Groen-
estijn et al., 1989; Bonting et al., 1991, 1993a,b; Van Veen et al., 1994; Geissd¨orfer et al.,
1998). These data are summarized in a number of reviews (Kortstee and Van Veen, 1999;
Kortstee et al., 2000). Many strains of Acinetobacter were isolated from activated sludge
(Vasiliadis et al., 1990; Kim et al., 1997). However, Acinetobacter could produce only a
small proportion of cells in some sludges, where other bacteria prevailed (Auling et al.,
1991; Bond et al., 1999).
Nakamura et al. (1995) isolated a new PolyP-accumulating bacterium Microlunatus
phosphovorus by a laboratory-scale EBPR process. M. phosphovorus accumulated large
amount of PolyPs under aerobic conditions, which was then consumed along with the
anaerobic uptake of carbon sources such as glucose. However, it lacks the key metabolic
characteristics of EBPR; it neither takes up acetate nor accumulates PHA under anaerobic
conditions. Using the 16Sr RNA-targeted probe, M. phosphovorus was found to be about
2.7 % of the total bacteria, while the percentage of PolyP-accumulating bacteria detected
by the DAPI stain for PolyP was about 9 % of the total bacteria (Kawaharasaki et al., 1999).
Many authors believe that bacteria phylogenetically related to the Rhodocyclus group
are responsible for EBPR in activated sludge communities (Hesselmann et al., 1999; Mino,
2000; Keasling et al., 2000; Jeon et al., 2003). Using fluorescent in situ hybridization tech-
niques, the communities of Rhodocyclus-related organisms in two full-scale wastewater
treatment plants were estimated to be between 13 and 18 % of the total bacterial population