Agriculture Reference
In-Depth Information
medium towards alkalinity, which inhibits the growth of bacteria, resulting in little
solubilization (Halder et al.
1990
). Aeration is yet another important factor that
contributes hugely to P solubilization by microbes. Strains of
P. striata
and
A. awamori
improve the P solubilization in shake culture as compared to stationary
cultures. The increased P solubilization in shake culture has been attributed to
enhanced aeration following shaking of cultures. Under aeration, the P concen-
tration has been reported to increase from 349 ppm to 1,675 ppm, while in the
non-aerated environment, it increases from 242 ppm to 1,164 ppm (Jung
et al.
2002
). However, the P removal by
Burkholderia
spp. from sediments
containing mineral phosphate is facilitated in the absence of aeration (Kim
et al.
2005
).
3.2.6 Kinds of Microorganisms and Incubation Periods
The extent of P solubilization, naturally by microbes, also depends on the compo-
sition of microorganisms since heterogeneously distributed microbial communities
exhibit varying capacity to solubilize P (Khan et al.
2007
). Among microbial
communities, the genera
Bacillus
(
B. polymyxa
) and
Pseudomonas
(
P. striata
)
have shown maximum P-solubilizing activity (Khan et al.
2009
), followed by
Penicillium
and
Aspergillus
(Khan et al.
2010
), while
Streptomyces
is the least
effective one. Generally, fungi have more PS activity in liquid media than bacteria,
actinomycetes, and yeast (Khan et al.
2010
). P-solubilizing fungi show greater
P-solubilizing activity both in precipitated agar and in liquid media than do bacteria
because the hyphae of fungi remain attached to P mineral particles and fungi in soil
are able to traverse longer distances more easily than bacteria and are thus more
important to P solubilization in soils. Even there is difference in P solubilization
among pigment-producing and pigment-non producing bacterial strains (Jayashree
et al.
2011
). As an example, pink-pigmented facultative methylotrophic (PPFM)
strains isolated from Adyar and Cooum rivers in Chennai and forest soils in Tamil
Nadu, India, along with
Methylobacterium extorquens
,
M. organophilum
,
M. gregans
, and
M. komagatae
showed phosphate solubilization activity on
NBRIP-BPB plates after 7 days of growth. The growth of PPFMs in
TCP-amended medium has been reported to be directly proportional to the concen-
tration of glucose oxidized. Higher P solubilization has been observed in four
strains MSF 32 (415 mg/l), MDW 80 (301 mg/l),
M. komagatae
(279 mg/l), and
MSF 34 (202 mg/l), after 7 days of incubation. A drop in pH from 6.6 to 3.4 has
been found coupled with an increase in titratable acidity. Furthermore, acid phos-
phatase activity has been found to be more pronounced in the culture filtrate than
alkaline phosphatase activity (Jayashree et al.
2011
). Many P-solubilizing bacteria
lose their ability to solubilize P on regular sub-culturing (Khan et al.
2007
), while
fungi in contrast retain their P-solubilizing activity even after several sub-culturing
and could continue actively solubilizing P for many years (Khan et al.
2010
).
