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In-Depth Information
the medium. The converse correlation observed between the pH and soluble P
concentration indicated that organic acid production by these PS strains might have
played a significant role in the acidification of the medium facilitating the P
solubilization as also reported by Hwangbo et al. (
2003
).
3.2.2 Temperature
Temperature is one of the important factors that immediately affect the interior of
the cell and the biological activity of soils. Generally, normal temperature has been
employed in soil biological research. On the contrary, the soil temperature remains
seldom constant under field conditions because of the diurnal temperature and
seasonal fluctuations. Therefore, bacteria responds differently to varying tempera-
tures and hence are capable of growing at elevated temperature (thermophiles,
thermotolerant) and also at extremely downshifted temperatures such as at or
below 0, 15, and 20
C (psychrophiles, psychrotolerant) by synthesizing a group
of heat and cold shock proteins, respectively. These temperature-induced proteins
are highly significant for the survival of bacteria at higher or lower temperatures
(Negi et al.
2009
). Moreover, temperature has bewildering effects onto the
“P”-solubilizing abilities of naturally occurring thermotolerant PSB, for example,
B. subtilis
(Moussa et al.
2013
),
Acidithiobacillus caldus
(Xiao et al.
2011
), and
B. smithii
(Chang and Yang
2009
), and psychrotolerant strains of bacteria, for
instance,
Pseudomonas fragi
(Selvakumar et al.
2009
). At low temperature, the
biological activity remains low which, however, improves further with increasing
temperature towards optimum range, beyond which microorganisms are either
desiccated or show variable responses. Generally, the PS microbes identified and
considered so far belong to mesophilic group (Khan et al.
2007
,
2010
), suggesting
that they could only be utilized under mesophilic environment. However, a few
thermotolerant (Chang and Yang
2009
; Rao et al.
2009
; Maheswar and Sathiyavani
2012
; Panda et al.
2013
) and psychrophilic PS organisms have also been reported
exhibiting P-solubilizing activity (Katiyar and Goel
2003
; Negi et al.
2009
; Pallavi
and Gupta
2013
). In this regard, bacterial cultures particularly
B. subtilis
and
B. circulans
showed consistent P solubilization even at 45
C which was due to
the ability of their enzyme systems to tolerate higher temperatures. Such a situation
of higher soil temperature during summer is generally found in tropics where
temperature may reach up to 50
C. Therefore, the inoculant designed for growth
and survival at higher temperatures in soil and happening during storage and
transport is one of the most craving characteristics. It is thus urgently needed to
isolate PS organisms which can tolerate high temperatures of tropics. To fulfil such
demands, Gaind and Gaur (
1991
) tested several PSM for their P-solubilizing
efficiency while growing them at 35, 40, and 45
C temperatures. Even though
there was a marked variation in PS activity of PSM, the effect was more pro-
nounced at 45
C. The thermotolerant bacterial strains exhibiting PS activity at
higher temperatures were identified as
B. subtilis
and
B. circulans,
while fungal
