Isolation and identification of PSB
Bacterial colonies which produced clear transparent zones were isolated and purified by subculturing. Thirteen PSB isolates were obtained and maintained during the study period. PCR amplification of 16S rDNA and sequencing analysis showed the identity of the PSB and the details are provided in Table 2. PSB isolated in this study were identified as
Rhodococcus sp,
Arthrobacter sp,
B. pumilus,
B. subtilis,
B. paramycoides,
B. aryabhattai,
B. anthracis, B. altitudinis, Enterococcus sp
, Staphylococcus sp (2 nos) and
Pseudomonas sp (2 nos).
Vazquez et al., (2000) reported that PSB species such as
B. amyloliquefaciens, B. licheniformis, B. atrophaeus, Paenibacillus macerans, Vibrio proteolutics, Xanthobacter agilis, Enterobacter sp.,
Kluyvera cryocrescens, Pseudomonas stutzeri and
Chryseomonas luteola from the rhizospheric sediment of black and white mangroves in Mexico.
Ravikumar et al., (2009) screened eight PSB isolates namely,
B. megaterium, Micrococcus roseus, B. subtilis, Proteus mirabilis, Streptococcus aureus, Micrococcus luteus, Athrobacter illicius and
Enterobacter aerogenes from mangrove sediments.
Armandeh et al., (2022) isolated 11 isolates belonging to
Pseudomonas and
Acinetobacter from the sediments of aquaculture pond.
Measurement of P- solubilizing efficiency of PSB isolates in solid medium
Maximum clear zone of 9 mm thickness observed in
B. altitudinis.
Rhodococcus sp and
Arthrobacter sp showed 4 mm thickness of clear zone. Though the clear zone formation of other isolates such as
B. aryabhattai and
B. paramycoides were lower than that of
B. altitudinis, the solubilizing activity in liquid media was comparatively high.
The PSB isolates that produced very mild clear zones on solid medium, could also solubilize various types of insoluble inorganic phosphate in liquid medium (
Das, 1963). This may be because of varying diffusion rates of different organic acids secreted by an organism (
Johnson, 1959). Many researchers
(El-Tarabily et al., 2008; Park et al., 2012) reported that the P-solubilization by bacterial inoculation results in drop in pH of culture media, which facilitate transformation of insoluble-P into soluble-P
(Selvi et al., 2017; Behera et al., 2017a). In the present study on P - solubilizing efficiency, the liquid medium showed a reduction in pH (7.0 to 4.2) within 3 days of incubation. The reduction in pH of culture media is associated with the production of different organic acids.
P-solubilizing activity and ACPase activity of PSB isolates in liquid medium
The ACPase activity and P-Solubilizing activity of PSB isolates in broth after 48 h of incubation is shown in Fig 2 and 3. The ACPase activity of isolates ranged from 4.40±0.28 -22.77±0.32 µmol ml
-1 h
-1.
Arthrobacter sp exhibited maximum ACPase activity (22.77±0.32 µmol/ml/h) followed by
Rhodococcus sp (18.08±0.71 µmol ml
-1 h
-1),
B. altitudinis (16.31±0.71 µmol ml
-1 h
-1). The P solubilizing activity and P solubilizing rate in liquid medium observed in the isolates ranged from 7.62±0.51- 16.33±0.84 mg/l and 0.11-0.22 mgl
-1 h
-1 in 72 h of incubation respectively.
Kathiresan and Selvam (2006) reported that phosphate solubilizing efficiency of 24 isolates from Vellar estuary at Parangipettai, Tamil Nadu was in the range of 0.012-0.141 mg/l.
Vibrio sp and
Pseudomonas sp from the marine sediment reported to solubilize 0.5-0.55 mg/l (
Promod and Dhevendaran, 1987). Bacterial species such as
B. subtilis, Pseudomonas sp and
Azotobacter sp isolated from mangrove sediment of Chollangi, East Godavari, exhibited a P-solubilising ability in the range of 80-100 mg/l
(Audipudi et al., 2012). Much higher phosphate solubilising activity (400 mg/l) was also reported in the bacterial population from the arid mangrove ecosystem in Mexico
(Vaquez et al., 2000). Behera et al., (2017a) documented that
Serratia sp. isolated from mangrove soil of Mahanadi River delta showed P-solubilizing activity of 44.84 mg/ml, accompanied by a decrease in pH of the growth medium from 7.0 to 3.15.
Production of organic acids by PSB isolates from sediment samples
The organic acids produced by the selected PSB isolates (
B. altitudinis,
Arthrobacter sp and
Rhodococcus sp) were shown in Fig 4. HPLC analysis of organic acids revealed that
B. altitudinis produced 66.53 mg/kg of lactic acid, 37.8 mg/kg of acetic acid, 20.21 mg/kg of citric acid, 18.89 mg/kg of succinic acid and 15.1 mg/kg of oxalic acid;
Arthrobacter sp produced 320.62 mg/kg of acetic acid, 35.36mg/kg of oxalic acid and 14.07 mg/kg of citric acid;
Rhodococcus sp produced relatively very low quantity of citric acid (3.94 mg/kg), lactic acid (4.03 mg/kg) and oxalic acid (3.95 mg/kg).
The organic acids solubilize P by chelating metals ions that would immobilize it or forming soluble complexes with P via metal ions or both
(Neumann et al., 2000). The P-solubilization rate depends not only on the pH reduction of culture media, but also on the structure and type of the organic molecule produced and not the concentration of organic acid (
Johnston, 1952). It is reported that oxalic, 2-ketogluconic and succinic acids produced by
Bacillus sp have the capability to solubilize P (
Banik and Dey, 1983).
Park et al., (2009) documented that
Pseudomonas fluorescens produced oxalic, trans-aconitic, citric, tartaric, malic, gluconic, succinic and fumaric acids;
Pantoea sp and
Enterobacter sp produced lactic, acetic, propionic, pyruvic, malonic, maleatic, tartaratic, oxalic, succinic, fumaric, citric and trans-aconitic acids to solubilize the inorganic P respectively. The P solubilisation activity of
Alcaligenes faecalis decreased in pH levels 7.0 to 3.2 of the growth medium with the production of organic acids such as oxalic acid, citric acid, malic acid, succinic acid and acetic acid
(Behera et al., 2017b).
Microcosm experiment with PSB isolates
The P-mobilizing potential of the PSB isolates obtained in this study were compared with the commercial P biofertilizer following a microcosm study. The initial water quality parameters in the microcosm study were pH-8.2; alkalinity-160 mg/l; salinity-35ppt; ammonia-0.017mg/l; phosphorus-0.31 mg/l. And the initial sediment characteristics such as pH, EC, Available-P, Total -P are given in Table 3. Fig 5 shows the phosphate in water and ALP activity and available-P in the sediment during microcosm experiment for PSB isolates. The control tank showed stable available-P (48.10-52.31 ppm) and ACPase activity (17.86-20.80 µmol/g/h) in sediment and orthophosphate (3.07-3.17 mg/l) in water from 0-21 days. The tanks inoculated with
B. altitudinis,
Rhodococcus sp,
Arthrobacter sp and commercial product showed significant increase in ACPase activity and available-P in sediment and orthophosphate in water from 2
nd day onwards. The ACPase activity of the sediment was found to be higher in the tank inoculated with
Rhodococcus sp (12.00 µmolg
-1 h
-1) followed by commercial product (11.23 µmol g
-1 h
-1),
B. altitudinis (10.03 µmol g
-1 h
-1) and
Arthrobacter sp (8.10 µmol g
-1 h
-1). The available P of the sediment on 21
st day was in the following order: Commercial product (128.25 ppm) >
Rhodococcus sp (110.2 ppm) >
B. altitudinis (85.77 ppm) >
Arthrobacter sp (82.65 ppm). The phosphate level in the water of microcosm study on 21
st day was observed in the following order:
Rhodococcus sp and commercial product (0.49 mg/l) >
B. altitudinis (0.46 mg/l) >
Arthrobacter sp (0.417 mg/l). With regard to phosphate level in the water, there is no significant difference observed between
Rhodococcus sp and commercial product. The initial and final PSB count in water samples of treatment tanks during the study period were in the range of 2-3 CFU/ml and 26-42 CFU/ml. The initial and final PSB count in soil samples of treatment tanks during the study period were in the range of 5-7 CFU/g and 1-3x10
3 CFU/g. In the present study, water-P, ACPase activity and available-P of sediment were increased up to 0.32-0.53 times, 0.56-1.16 times and 0.58-1.45 times respectively. This study gives evidence that apart from
Bacillus sp,
Rhodococcus sp and
Arthrobacter sp could be used as biofertilizer in aquaculture.
Microcosm study with PSB isolates of
Vovk et al., (2013), was similar to the present study, where they warranted that application of
Paenibacillus polymyxa contributes to an increase in mineral phosphorus in pond water by 1.3-3.7 times compared to the control. It was carried out in a 30 L tank consisting of pond water with concentration of PSB,
Paenibacillus polymyxa KB (4.50±0.50) × 10
2 - (5.63±0.46) × 10
3 CFU/ml for 35 days. They also inferred that introducing bacterial fertilizer on the floor increased the mineral phosphorus content in the aquatic environment more rapidly (15-17 days) than introducing it on the water surface (25-30 days). Similarly, the study of
Zhou et al., (2023) revealed that application of
B. cereus mutant strain S458-M in Crucian carp culture systems improved the concentration of phosphorus in aquaculture water. Further studies are needed to evaluate the rate of application based on soil texture, organic carbon and total phosphorus.