Advances in Microbiology, 2012, 2, 234-240 Published Online September 2012 (
Depth Integrated Microbial Community and
Physico-Chemical Properties in Mangrove Soil of
Sundarban, India
Subhajit Das1, Minati De2, Dipnarayan Ganguly1, Tushar Kanti Maiti3, Abhishek Mukherjee1,
Tapan Kumar Jana1, Tarun Kuma De1*
1Department of Marine Science, Calcutta University, Kolkata, India
2Maniktala Siksha Bhavan, Kolkata, India
3Microbiology Laboratory, Department of Botany, Burdwan University, Burdwan, India
Email: *,
Received January 18, 2012; revised March 20, 2012; accepted June 5, 2012
In the Sundarban Mangrove forest microbial activities are dominantly involved in both the mineralization and decom-
position processes that regulate nutrient profile in soil of different depth. It was found that besides changing the water
and soil quality, monsoonal cycle plays a crucial role in regulating microbial population distribution in the mangrove
soil. Statistical analyses revealed that organic carbon was the most significant factor that regulated the total microbial
population. The cellulose degrading bacteria, [mean value of CFU 14.32 × 106 (gm dry weight of soil)–1] was dominant
throughout the year. The sulfate reducing bacteria showed an increasing trend along depth with a minimum value at the
surface i.e. 6.113 × 106 (gm dry weight of soil)–1 and 12.312 × 106 (gm dry weight of soil)–1 at a depth of 60 cm. Inten-
sification of monsoonal cycle could heavily affect microbe dominated soil biogeochemistry and subsequent change in
the regional ecology of the Sundarban Mangrove Forest.
Keywords: Sundarban Mangrove; Microbial Population; Monsoonal Cycle; Nutrient Concentration
1. Introduction
Mangroves are highly productive marine ecosystem
where bacteria actively take part in biomineralization and
biotransformation of minerals [1]. The distribution of
microbial activities in estuarine systems is clearly com-
plex and variable. Much research remains to be done in
order to define the distributions of microbial activities
and the major factors involved in controlling these dis-
tributions in estuaries. Leaves and wood provided by
mangrove plants to the soil are degraded primarily by
large variety of microbes which actively participate in
the heterotrophic food chain [2-4]. Major products of
general recycling of organic matter are detritus which is
rich in enzymes and proteins and contains large micro-
bial population [5]. Bacteria are the major participants in
the carbon, sulfur, nitrogen and phosphorous cycles in
mangrove forest [6,7]. Bacterial activity is responsible
for most of the carbon recycling in mangrove soil under
both oxic and anoxic conditions. Many species of phos-
phate solubilizing rhizosphere bacteria associated with
black mangrove roots were found. The mechanism for
phosphate solubilization probably involves the produc-
tion of several organic acids [8]. Saprophytic fungi are
fundamental to many aspects of decomposition and en-
ergy flow in mangrove forests [9]. Most investigations of
anaerobic metabolism in natural ecosystem have dealt
with sulfate rich marine soils where sulfate reduction is
the dominating process or eutrophic lake soils where sul-
fate and nitrate are depleted in the hypolimnionn and in
the superficial soil layers leaving terminal carbon miner-
alization principally to methane producing bacteria
[10-12]. Sulfate reduction, methane production, denitri-
fication are the important processes for the terminal elec-
tron removal during decomposition of organic matter in
anoxic environment. The methanogens are characterized
by their ability to produce methane from hydrogen and
carbon di oxide, formate, acetate, methanol etc. [13].
Methanotrophs are a subset of a physiological group of
bacteria known as methylotrophs. They are unique in
their ability to utilize methane as a source of carbon and
energy [14]. Nitrogen fixing bacteria are the other group
of bacteria that are involved in formation of ammonia or
organic nitrogen from atmospheric nitrogen. They may
be free-living or symbiotic in Nature. It has been studied
*Corresponding author.
opyright © 2012 SciRes. AiM
S. DAS ET AL. 235
that N2 fixation by heterotrophic bacteria are generally
regulated by specific environmental factors like Oxygen,
combined Nitrogen and the availability of Carbon source
for energy requirement [15]. Aerobic, autotrophic nitrifi-
ers oxidize ammonia to nitrite and nitrate, with molecular
oxygen as electron acceptor. Nitrite and nitrate are re-
duced to dinitrogen gas by heterotrophic denitrifying
bacteria that use NOx instead of oxygen as electron ac-
ceptor [16]. The purpose of the present study was to ex-
amine seasonal and spatial variations in microbial popu-
lation (bacteria and fungi) in mangrove soil and to find
out the correlation between different microbes with nu-
2. Materials and Methods
2.1. Study Area
The Sundarban Mangrove Forest is located between
21˚31N and 22˚30N and longitude 88˚10E and 89˚51E
along the North East coast of Bay of Bengal, India. This
mangrove forest is a part of the estuarine system of the
River Ganges, NE coast of Bay of Bengal (Figure 1).
The climate in the region is characterized by the south-
west monsoon (June-September), northeast monsoon or
post-monsoon (October-January), and pre-monsoon (Fe-
bruary-May); 70% - 80% of annual rainfall occurs dur-
ing the summer monsoon (southwest monsoon), The tide
in this estuarine complex is semidiurnal in nature with
spring tide ranging between 4.27 and 4.75 m and neap
tide range between 1.83 and 2.83 m. It is a unique bio-
climatic zone in between the land and ocean boundaries
of the Bay of Bengal and the largest delta on the globe.
The deltaic soil of Sundarban Biosphere Reserve com-
prises mainly saline alluvial soil consisting of clay, silt,
fine sand and coarse sand particles.
2.2. Sample Collections and Analyses
Soil samples were collected aseptically using a hand-
held stainless steel core sampler (3.2 cm diameter, 100
cm long) from six different depth i.e. 1) 0 - 10 cm; 2) 10
- 20 cm; 3) 20 - 30 cm; 4) 30 - 40 cm; 5) 40 - 50 cm and 6)
50 - 60 cm at five different sites in dense mangrove area
(Deep forest) covering different seasons. Samples were
collected into sterilized containers and immediately trans-
ported to the laboratory for analyses. Three replicates
Figure 1. Map showing the study area.
Copyright © 2012 SciRes. AiM
from each site were analyzed for five sites at different
depths. The result represents the average value at each
2.3. Quantification of Bacteria and Fungi
Soil samples were stored at 4˚C immediately after
collection and transported to the laboratory, for analysis
with adequate care. 10 gm of sample from different depth
of different regions were homogenized with sterilized
phosphate buffer solution. Serial dilutions up to 10–4
were made and inoculation was done with 0.1 ml. Quan-
tification of bacteria and fungi from mangrove soils was
carried out by spread plate method for different types
of bacteria such as Phosphorous Solubilizing Bacteria
(PSB), Cellulose Degrading Bacteria (CDB), nitrifying
bacteria, free living nitrogen fixing bacteria and fungi
and they were incubated at different conditions [17].
Sulfate reducing bacteria was cultured in Starkey’s me-
dium in anaerobic condition [18].
2.4. Soil Quality Measurement
Concentration of Nitrate-Nitrogen, Nitrite-Nitrogen, Pho-
sphate-Phosphorous and Silicate in the soil sample was
measured at 10 cm interval (from 0 to 60 cm depth). 30 g
of soil subsample was collected from the different depth
and was immediately extracted in 75 mL of 2 mol·L1
potassium chloride (KCl). The mixture was shaken until
well mixed and allowed to stand overnight [16]. After 24
h, 4 mL of the supernatant was collected for the estima-
tion of different nutrients using standard spectropho-
tometric methods [19]. Concentration of Sulfate-Sulphur
of the soil was measured using standard protocol [20].
The pH value was measured in a 1:5 (w/w) soil water
suspension using electric digital pH meter [21] and soil
organic carbon was measured by standard methods [22].
Salinity of a soil saturation extract (ECe) was determined
by measuring the electrical conductance of soil water
saturation extract with the help of a conductivity meter
[23]. Soil redox potentials (Eh) at each sampling site
were measured with brightened platinum electrodes
which were allowed to equilibrate in situ for 1 hr prior to
measurement. Each electrode was checked before use
with quinhydrone in pH 4 and 7 buffers (mV reading for
quinhydrone is 218 and 40.8, respectively, at 25˚C). The
potential of a calomel reference electrode (+244 mV)
was added to each value to calculate Eh value for the soil
samples [24].
3. Result and Discussion
Mangrove soil at Indian Sundarban showed seasonal
variation with respect to both major nutrient concen-
trations and microbial population. Beside monsoonal
influx of nutrients to the system, mangrove litters also
played a significant role in regulating the nutrient status
that in turn controlled the microbial population. Among
several physical factors tidal inundation, wave action,
presence of mangrove roots and bioturbation are the im-
portant ones considered for determining microbial abun-
dance in the mangrove soil from surface to a depth up to
60 cm. The physico-chemical parameters of the study are
shown in Table 1. Temperature and Eh value of soil
sample showed a decreasing trend from surface to 60
cm of depth during all season. A reverse profile was
Table 1. Seasonal variations of phys ico-chemical parameters of soil at d ifferent d epth in Sundarban mangrove environment.
Season Parameters (soil) 0 m 10 m 20 m 30 m 40 m 50 m 60 m
Eh (mV) –95 –100 –105 –112 –125 –135 –145
pH 7.95 8.25 8.27 8.23 8.25 8.21 8.19
Temp (˚C) 17.85 17.84 17.83 17.82 17.82 17.78 17.78
Salinity (PSU) 16.80 16.96 17.05 17.20 17.35 17.40 17.60
Eh (mV) –100 –102 –110 –118 –145 –170 –175
pH 8.22 8.20 8.15 8.14 8.13 8.12 8.12
Temp (˚C) 24.71 24.69 24.67 24.65 24.60 24.59 24.59
Salinity (PSU) 14.99 15.01 15.05 15.15 15.20 15.35 15.41
Eh (mV) –121 –125 –128 –130 –139 –165 –187
pH 8.42 8.35 8.32 8.30 8.25 8.22 8.19
Temp (˚C) 12.90 12.94 12.98 12.99 13.05 13.12 13.12
Salinity (PSU) 15.35 15.39 15.45 15.50 15.60 15.65 15.69
Copyright © 2012 SciRes. AiM
S. DAS ET AL. 237
found for pH and salinity. During monsoon, the salinity
was found to be 14.99 psu in surface soil and it was
15.41 psu at 60 cm depth. Less soil salinity in monsoon
with respect to pre monsoon and post monsoon may be
due to maximum dilution by river run off during mon-
soon period [25]. Eh value showed a decreasing trend
from surface soil (–95 mV) to 60 cm of depth (–145 mV)
which represented more anoxicity of bottom soil than
that of surface during premonsoon (Table 1). Soil redox
potential value (Eh) from surface to 60 cm of depth re-
gion in three distinct seasons suggested that the soil of
deep forest region of Sundarban Mangrove is relatively
anoxic or it can be referred to as oxygen-starved soil.
During pre-monsoon nutrient concentration showed very
weak stratification from surface to 30 cm of depth with
almost uniform distribution. Intense bioturbation up to 30
cm depth by several benthic organisms could cause uni-
form mixing of soil nutrients.
No significant variation of silicate concentration was
found throughout the entire depth. Gradual decrease in
organic carbon and Phosphate-Phosphorous concentr-
ation was observed from depths of 30 to 60 cm.
During transportation of organic matter from surface
to bottom, it is decomposed by microbes. As a result,
organic content of soil decreased with increasing depth.
It could be attributed to mangrove litter fall with an an-
nual rate of 1603 g·m–2·year1 [26]. Organic carbon was
found maximum during postmonsoon followed by pre
monsoon and monsoon (Figure 2(a)). Nitrate-Nitrogen
concentration was increased from surface to 40 cm of
depth but decreased from 40 cm to 60 cm. Vertical
movement of materials, nutrient cycling and reuse driven
by various burrowing organisms could have an effect on
this Nitrate-Nitrogen distribution along the depth profile
at up to 40 cm. Less abundance of bioturbation below 40
cm could enhance the anoxic condition which in turn
initiate denitrification causing sudden depletion of Nitrate-
Nitrogen. The Nitrite-Nitrogen concentration showed no
significant variation throughout depth but slight increased
below 50 cm of depth which may be an indication of deni-
trification. Population of SRB was found to increase with
increase in depth in all seasons. Thus, more anoxic con-
dition preferred the more population of SRB in the bot-
tom soil than that of surface soil. Fungal population
showed decreasing trend with increasing depth. Free liv-
ing nitrogen fixing bacterial population showed de-
creasing trend from surface to 30 cm depth and increased
again from 30 cm to 50 cm of depth. After the death of
plant, the woods are carried away by tidal action or con-
sumed by herbivorous animal but the root attached to the
bottom soil below the 50 cm depth seldom may act as the
source of Carbon to fungus and cellulose degrading
bacteria (Figure 2(a)).
During monsoon Nitrate-Nitrogen, organic carbon con-
tent of soil showed decreasing pattern along with de-
crease in population of nitrifying bacteria with increase
in depth. Silicate concentration showed little variation
with increasing depth. Population of PSB was found to
be decreased with increase in depth and at the same time,
Phosphate-Phosphorou s concentration was also found to
decrease with increase in depth. Population of CDB
decreased with increase in depth as the organic carbon
content of the soil was also found to decrease with
increase in depth. Population of SRB showed increasing
trend from surface to 60 cm of depth (Figure 2(b)).
During postmonsoon Nitrate-Nitrogen organic carbon
content of soil showed decreasing pattern along with
decrease in population of nitrifying bacteria with increase
in depth. Silicate concentration showed little variation
with increasing depth. Population of PSB was found to
be decreasing with increasing depth and at the same time,
Phosphate-Phosphorou s concentration was also decrea-
sing with increase in depth. Population of CDB de-
creased with increase in depth as the organic carbon
content of the soil was also decreasing with increase in
depth. Sulfate concentration did not show distinct strati-
fication though population of SRB showed increase in
trend from surface to bottom (Figure 2(c)). Free living
nitrogen fixing bacteria showed decrease in population up
to 30 cm of depth but below 30 cm to the next 30 cm of
depth their population was recorded to increase. Popu-
lation of PSB and free living nitrogen fixing bacteria was
more in proportion than population of nitrifying bacteria.
Organic carbon content of the soil was found to be most
significant factor on the growth rate of cellulose decom-
posing bacteria (Pearson correlation of OrgC (%) and
C.D.B. (CFUs × 106) = 0.500, P-Value = 0.000). The
populations of cellulose decomposing bacteria were
found to be more in post monsoon period than that of
premonsoon and monsoon. Again the zone with more
population of phosphate solubilizing bacteria showed
more concentration of available phosphate. Presence of
phosphatase enzyme within such type of bacteria might
be responsible for these findings [27]. It might be for
availability of more organic carbon source. Sulfate re-
ducing bacteria was found to be correlated with sulfate
concentration of soil sample (Pearson correlation of Sul-
fate-Sulphur mg·gm–1 dry wt of soil and S.R.B. (CFUs ×
106) = 0.595, P-Value < 0.001).
Phosphate solubilizing bacteria was also found to be
correlated with phosphate concentration of the Sundarban
mangrove soil (Pearson correlation of Phosphate-Phos-
phorous µg·gm–1 dry wt of soil and P.S.B. (CFUs × 106)
= 0.766, P-Value = 0.000). No such correlation was
found for nitrogen fixing bacteria with nitrate and nitrite
concentration. Organic carbon from the leaves, wood
from forest and other organic dead or waste products
from other living creatures are easily degraded by cellu-
lose degrading bacteria in the mangrove soil because
they are the most dominating group of microbes prior to
Copyright © 2012 SciRes. AiM
D epth pro f ile of nutrient status and Org
deep forest region during premonsoon
Conc. of Nutrients and Org.Carbon
Depth (cm)
. C in
µg/gm dry wt of
µg/gm dry wt o
Org.C (%)
Phosphorous µg/
gm dry wt of soi
mg/gm dry wt o
Silicate µg/gm
dry wt of soil
Verti cal Di stribut ion of Micro bes D uring Premonso on
C.F.U X 10
dry wt of soil
Depth (cm)
Ntrfyng Bacteria
(1) (2)
D epth profi l e of nutri ent st atus and O r
deep forest region during monsoon
00.5 11.52
Conc. of Nutrients and Org.Carbon
Depth (cm)
g. C i n
Nitrogen µg/g
dry wt of soil
µg/gm dry wt
of soil
Org.C (%)
µg/ gm dry wt
of soil
Sulfate- Sulp hu
mg/gm dry wt
of soil
Silicate µg/gm
dry wt of soil
Vert ical Di stribu tion of Microbes Du rin g Monsoon
C.F.U X 10
d ry w t of soil
Depth (cm)
Ntrfyng Bacteria
(1) (2)
Vertical D i stribution of Microbes During
C.F.U X 10
dry wt of soil
Depth (cm)
D epth pr ofil e of nutri ent status and Or g
deep forest r egi on during postmonso
Conc. of Nutrients and Org .C arbo n
Depth (cm)
. C i n
o n
itrogen µg/g
dry wt of soil
µg/gm dry wt
of soil
Org.C (%)
µg/ gm dry wt
of soil
Su lfa te -Sulphur
mg/g m dry wt
of soil
Silicate µg/gm
dry wt of soil
(1) (2)
Figure 2. (a) Depth profile of organic carbon, nutrient concentrations (1) and microbial populations (CFUs) (2) during
pre-monsoon at different depth; (b) Depth profile of organic carbon, nutrient concentrations (1) and microbial populations
(CFUs) (2) during monsoon at different depth; (c) Depth profile of organic carbon, nutrient concentrations (1) and microbial
opulations (CFUs) (2) during post-monsoon at different depth. p
Copyright © 2012 SciRes. AiM
S. DAS ET AL. 239
fungi. Other groups of microbes have also shown sig-
nificant population which is a good sign for a mangrove
forest with respect to mineralization of organic debris
and as a result mangrove plants can easily get nutrients in
their simplest forms. From seasonal perspective, the
monsoon period was significantly different (Student’s t
test, P < 0.01) than other periods of the year in terms of
total bacterial abundance and other key parameters. Sea
level rise due to global warming may hamper the stable
ecological zone of Sundarban mangrove forest which
may ultimately reflect to net flux of several biologically
active trace gases between soil and atmosphere. Intro-
duction of huge amount of nutrients during monsoon
have a positive feedback on the bacterial population of
mangrove sediment. Beside the changes in several phys-
icochemical parameters, transport of huge amount of
aquatic microbes could lead to the significant increase in
the microbial population in the soil of this mangrove eco-
system. This may contribute to the aquatic biogeochem-
istry of this tropical wetland.
4. Acknowledgements
The financial assistance from DOEn, Govt. of West
Bengal and U.G.C., New Delhi are gratefully acknowl-
edged. The authors are also grateful to the Forest De-
partment, Govt. of West Bengal for assisting the research
team in collecting data and providing all infrastructural
facilities to reach the remote island.
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