Vol.1, No.2, 35-40 (2011)
http://dx.doi.org/10.4236/oje.2011.12004
Open Journal of Ecology
Salt tolerant culturable microbes accessible in the soil
of the Sundarban Mangrove forest, India
Subhajit Das1*, Minati De2, Raghab Ray1, Dipnarayan Ganguly1, Tapan Kumar Jana1, Tarun
Kumar De1
1Department of Marine Science, University of Calcutta, Calcutta, India; *Corr esponding Author: [e-mail]
2Maniktala Siksha Bhavan, Calcutta.
Received 20 May 2011; revised 20 June 2011; accepted 30 June 2011.
ABSTRACT
Sundarban Mangrove forest is highly productive
marine ecosystem where halophilic microbes
actively participate in bio-mineralization and
biotransformation of minerals. The population
of aerobic halophilic microbes was studied to
determine their distribution with the availability
of different physicochemical parameters with
increasing depth of this forest sediment. The
present study revealed that microbes present in
the top soil region w ere less tolerant to fluctua-
tion in salinity than the middle and bottom
segment. Microbes isolated from bottom seg-
ment showed higher growth rate in anaerobic
condition. A decreasing trend of total microbial
population and organic carbon content of soil
were found with increase in depth. In contrary a
reverse profile was found for salinity. A signifi-
cant stratification was found to exist among
microbial population and the salty nature of the
soil of Sundarban Mangrove forest.
Keywords: Sundarban Mangrove Forest;
Ecosystem; Halophilic Microbes; Aerobic Condition;
Anaerobic Cond i ti o n
1. INTRODUCTION
Some studies investigated the impacts of soil saliniza-
tion on the microbial community and found that in-
creasing salt levels had a significant negative impact on
microbial populations. Yuan et al. [1] found that there
was a significant negative exponential relationship be-
tween soil salinity and soil microbial biomass and basal
soil respiration.
The distribution of microbial activities in estuarine
systems is clearly complex 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 distributions in mangrove dominated
tropical estuaries. Salinity and sodicity properties of
coastal soil determine the degrees of inhibition of mi-
crobial activity and biochemical processes that are fun-
damental in maintaining ecological quality and produc-
tivity in soils of coastal regions [2]. Major products of
general recycling of organic matter are detritus which is
rich in enzymes and proteins and contains large micro-
bial population [3]. Microbial compositions are the ma-
jor participants in the carbon, sulphur, nitrogen and
phosphorous cycles in mangrove forest [4-6]. Microbial
activity is responsible for most of the carbon recycling in
mangrove sediment under both in oxic and anoxic condi-
tion. Many species of phosphat e sol ubilizi ng
rhizosphere
bacteria associated with black mangrove roots were
found from the prev ious research works. The mechanism
for phosphate solubilization probably involves the pro-
duction of several organic acids [5]. Effects of NaCl,
salinity (EC 5, 10, 15 dSm–1) were studied on the popu-
lations of ammonium oxidizers, nitrite oxidizers and
Azotobacter in rice rhizosphere in a pot-culture experi-
ment. Increasing salinity reduced the population of both
the groups of nitrifying bacteria. The growth rate of
4 ox idizers was found to be more susceptib le to salt
stress than 4 oxidizers [7]. Halophilic and halotol-
erant microorganisms are able to thrive and grow in sa-
line and hypersaline environments. These microorgan-
isms are being the object of basic studies in relation to
the origin of life in our planet and the molecular mecha-
nisms of adaptation to saline and hypersaline conditions
[8]. Most investigation of anaerobic metabolism in natu-
ral ecosystem have dealt with sulfate rich marine sedi-
ments where sulfate reduction is the dominating process
or eutrophic lake sediments where sulfate and nitrate is
depleted in the hypolimnionn and in the superficial
sediment layers leaving terminal carbon mineralization
principally to methan e producing bacteria [9-11]. Sulfate
+
NH +
NH

S. Das et al. / Open Journal of Ecology 1 (2011) 35-40 36
reduction, methane production, de-nitrification were the
important processes for the terminal electron removal
during decomposition of organic matter in anoxic envi-
ronment.
The methanogens are characterized by their ability to
produce methane from hydrogen and carbondioxide,
formate, acetate, methanol etc [12]. 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 [13]. Nitrogen
fixing bacteria are the other group of bacteria that are
involved in formation of ammonia or organic nitrogen
from atmospheric nitrogen. It has been studied that N2
fixation by heterotrophic bacteria are generally regulated
by specific environmental factors like oxygen, combined
Nitrogen and the availab ility of carbon source fo r energy
requirement [14]. Aerobic, autotrophic nitrifiers oxidize
ammonia to nitrite and nitrate, with molecular ox ygen as
electron acceptor. Nitrite and nitrate are reduced to
di-nitrogen gas by heterotrophic denitrifying bacteria
that use NOx instead of oxygen as electron acceptor [15].
The coastal wetland forests con sists of intertidal zon es
of estuaries, brackish waters, deltas, creeks, lagoons
marshes and mudflats of tropical and subtropical lati-
tudes are called as Mangroves. Mangrove forests are
usually considered to be high productive areas that sup-
port highly developed detritus-based food webs. The
high primary productivity of mangroves implies a high
demand for nutrients essential to plant growth and this
demand appears to be met by a highly efficient system of
nutrient trapping, uptake and recycling. The organisms
within mangrove ecosystems, including microorganisms,
plants and animals, show complex interactions. Micro-
organisms are intimately involved in biogeochemical
cycling and in many instances are the only biological
agents capable of regenerating forms of the elements
used by other organisms, particularly plants. Therefore,
Mangrove provides a unique ecological niche to differ-
ent microbes, which play various roles in nutrient recy-
cling as well as different environmental activities. The
decomposition involves in this forest at various trophic
groups of microorganisms acting in a multi-step process.
The first step is an enzymatic hydrolysis of polymeric
material to soluble monomeric and oligomeric com-
pounds. Under oxic conditions, the soluble compounds
are directly mineralized to carbon dioxide and water
where as under anoxic conditions various physiological
groups are involved in degradation after the initial de-
polymerisation. Fermentative bacteria convert the prod-
ucts of hydrolysis to a variety of products, mainly short
chain fatty acids, carbon dioxide and hydrogen. Further
conversion through the action of secondary fermenters,
sulphate-reducers, acetogens and methanogens produce
the end products as CO2, CH4 and H2S, which may es-
cape into the atmosphere. First two gases among them
are important greenhouse gases. The organisms within
mangrove ecosystems, including microorganisms, plants
and animals, show complex interactions. Microorgan-
isms are intimately involved in biogeochemical cycling
and in many instances are the only biological agents
capable of regenerating forms of the elements used by
other organisms, particularly plants. Distribution of bac-
teria depends on changes in water temperature, salinity
and other physico-chemical parameters [16]. Due to high
salinity, halophilic bacteria are believed to be predomi-
nant in this ecosystem. It serves as important source of
food for a variety of marine organisms and maintains
pristine nature of the environment. It also acts as a bio-
logical mediator through their involvement in the
bio-geochemical process [17]. In the present study an
attempt has been taken to explore the vertical distribu-
tion of microbial population along with different phys-
icochemical parameters of the soil and their response to
fluctuation in salinity an d availability of O2.
2. MATERIALS AND METHODS
2.1. Study Area
Sundarban Mangrove forest that is located geographi-
cally in between 21˚31´N and 22˚30´N and longitude
88˚10´E and 89˚51´E along the North East coast of Bay
of Bengal, India. Sampling zone of present study is rep-
resented in Figure 1.
This mangrove forest is a part of the estuarine system
of the River Ganges, NE coast of Bay of Bengal, which
covers 9630 km2, out of which 4264 km2 of inter-tidal
Figure 1. The map presenting the zone of the present study.

S. Das et al. / Open Journal of Ecology 1 (2011) 35-40 37
area, covered with thick mangroves, is subdivided as
forest sub-ecosystem and 1781 km2 of water area as
aquatic sub-ecosystem. The tide in this estuarine com-
plex is semidiurnal in nature with spring tide range be-
tween 4.27 and 4.75 m and neap tide range between 1.83
and 2.83 m. It is a unique bioclimatic zone in land o cean
boundaries of Bay of Bengal and the largest delta on the
globe. Several numbers of discrete islands constitute
Sundarbans. One of these Islands, Lothian Island cover-
ing an area of 38 km2 has been notified as a sanctuary
and is situated at the confluence of Saptamukhi River
and Bay of Bengal. In the southern part of the island, the
ground level is high while in the northern areas the land
is low and gets inundated during highest high tide.
Avicennia alba, Avicennia marina and Avicennia offici-
nalis are the dominant mangrove species, Excoecaria
agallocha and Heritiera fomes are thinly distributed and
Ceriops decandra is found scattered all over the island.
The deltaic soil of Sundarban Biosphere Reserve com-
prises mainly with saline alluvial soil consisting of clay,
silt, fine sand and coarse sand p articles. It is described as
very deep, poorly drained, fine soils occurring on level
to nearly level lower delta with loamy surface, severe
flooding and very strong salinity (extensive extent) as-
sociated with very deep, very poorly drained, fine loamy
soil.
2.2. Soil Sample Collection
Triplicate soil samples were collected aseptically from
three different depths ranging from 0 - 10, 10 - 20 and
20 - 30 cm as top, middle and bottom segments respec-
tively using a hand-held soil corer. The samples were
collected in sterilized polythene containers and trans-
ported to the laboratory in iced co ndition without delay.
2.3. Quantification of Bacteria
Separately, from each replicate, 10 g·m of aliquot
sample from different soil segment was homogenized
with sterilized phosphate buffer solution (PBS). Serial
dilutions upto 1 0–4 were made and inoculation was done
with 0.1ml. Quantification of bacteria from mangrove
sediments was carried out by spread plate method in
Marine Agar 2216 Medium [18] under incubation for 24
hours at 32˚C temperature.
2.4. Sediment Quality Measurement
From aliquot soil sample 30 g of subsample was
added in 75 ml of 2 mol·L–1 potassium chloride (KCl).
The mixture was shaken un til well mixed and allowed to
stand overnight [15]. After 24 h, 4 ml of the supernatant
was collected for the estimation of Nitrate-Nitrogen and
Phosphate-Phosphorous of the soil sample using stan-
dard spectrophotometric methods [19].
For estimation of Sulfate-Sulfer concentration in the
soil sample, 20 gm of it was dissolved in 100 ml distilled
water. After vigorous shaking for 1 hr the solution were
filtered through Millipore filter paper (0.45 m). The
filtrate was used to determine sulphate concentration
turbidometrically [20]. Soil was dissolved in distilled
water and chlorinity (Cl) of the water were determined
by Mohr-Knudsen titration method and standard sea-
water of chlorinity 19.374 procured from the National
Institute of Oceanography, Goa, was used for the stan-
dardization. From the knowledge of chlorinity, salinity
(S) was calculated using the Knudsen relation: S (× 10 –3)
= 1.80655 × Cl (× 10–3). The soil pH was determined
following a water paste and determined by using micro
pH meter (Systronics, model No, 362) [21].The organic
matter was determined by the modified Wakly-Black
method (oxidation with potassium dichromate in sul-
phuric acid solution) to obtain organic carbon [22].
2.5. Enumeration of Viable Count of
Microbes in Different Salinity
After extraction of soil from 3 distinct zones with PBS,
inoculations with 0.1 ml were done into Marine Agar
2216 medium with different salinity. After same incuba-
tion period CFU we re counted sepa rately for each distinct
zone.
2.6. Measurement of Growth Rate of
Microbes Found from Three Distinct Soil
Segments in Aerobic and Anaerobic
Condition
After extraction of soil from 3 distinct zones with PBS,
inoculations with 0.1 ml were done into Marine Agar
2216 medium and they were allowed to grow separately
in aerobic and anaerobic condition. After each 12 hour
interval CFU of microbes were counted.
3. RESULT & DISCUSSION
In recent work it was found to show a decreasing
trend of total organic carbon content of soil with in-
crease in depth (1.08 ± 0.209% in top soil segment, 1.01
± 0.186% in bottom soil segment and 0.9 ± 0.115% in
bottom soil segment). Similar type of profile was found
for phosphate-phosphorous ( 0.438 ± 0.167 µg·gm–1 dry
weight of soil in the top soil segment, 0.389 ± 0.142
µg·gm–1 dry weight of soil in the middle soil segment
and 0.359 ± 0.116 µg·gm–1 dry weight of soil in the bot-
tom soil segment) and sulfate-sulfur concentration (1.39
± 0.329 mg·gm–1 dry weight of soil in the top soil seg-
ment, 1.22 ± 0.257 mg·gm–1 dry weight of soil in the
middle soil segment and 1.14 ± 0.191 mg·gm–1 dry

S. Das et al. / Open Journal of Ecology 1 (2011) 35-40 38
weight of soil in the bottom soil segment). From top to
middle soil segment a decreasing trend was found for ni-
trate-nitrogen concentration (0.194 ± 0.014 µg·gm–1 dry
weight of soil in the top soil segment and 0.178 ± 0.01
µg·gm–1 dry weight of soil in the middle soil segment).
Bottom soil segment (0.185 ± 0.026 µg·gm–1 dry weight
of soil) was found to show a little increase in ni-
trate-nitrogen concentration. Soil temperature (18.58 ±
4.813˚C, 18.56 ± 4.926˚C and 18.44 ± 4.827˚C in top
soil segment, middle soil segment and bottom soil seg-
ment respectively) and population of culturable microbes
(12.437 ± 0.821 × 106 CFU gm–1 dry weight of top soil
segment, 10.966 ± 0.725 × 106 CFU gm–1 dry weight of
middle soil segment and 9.647 ± 0.788 × 106 CFU gm–1
dry weight of bottom soil segment) were found to de-
crease from top to bottom soil segment. Huge population
of halophilic microbes found in present study from the
soil of Sundarban mangrove forest may be supported by
Kathiresam, K in 2001, [23] for predicting microbial
(Halophilic aerobic bacterial) load as it gives too nu-
merous to count (TNTC) colonies even at 10–8 dilution.
Maximum salinity of soil was found for bottom segment
(16.37 ± 0.546 psu) following middle segment (15.23 ±
0.403 psu) ant top soil segment (11.6 ± 1.41 psu). On
contrary middle soil segment showed maximum of soil
pH value (8.28 ± 0.086) following bottom segment (8.24
± 0.058) and top soil segment (8.19 ± 0.197) (Table 1.).
The organisms from the three different segments were
allowed to grow on salinity of 0.5, 1, 1.5 upto 4 (%)
modified marine agar medium. Among the three seg-
ments studied, the bottom segment showed higher halo-
philic microbial load with medium of increasing salinity
and middle segment showed insignificant variation. Sur-
prisingly, microbes isolated from top segment showed a
decreasing trend of their population with increasing sa-
linity (Figure 2).
The above mentioned result evoked the urihaline na-
ture [24] of microbes present in the top soil segment.
Microbes isolated from top segment was found to show
maximum growth rate when they were allowed to grow
in aerobic condition. It may be attributed that aerobic
microbes were dominant in 0 - 10 cm top soil segment
[3]. In contrary a reverse profile was found for microbes
isolated from bottom segment (Figure 3).
The bottom segment was found to show maximum
growth rate compare to middle and top soil segment
when the microbes isolated from three segments were
allowed to grow in anaerobic condition (Figure 4).
It may be attributed that anoxicity increases with
depth and anaerobic microbial population was dominant
in the bottom soil segment [25]. Recent study revealed
that anaerobic microbes present in the bottom segment
are more tolerant to fluctuation of salinity in the sur-
Table 1. Physico-chemical parameters & microbial load in
different depth of mangrove soil.
Top soil Middle soil Bottom soil
Physicochemical
Parameters and
microbial load Avg± stdv Avg ± stdv Avg± stdv
Salinity (PSU) 11.61.41 15.23 0.403 16.370.546
pH 8.190.197 8.28 0.086 8.240.058
Temp (ºC) 18.584.813 18.56 4.926 18.444.827
Org.C (%) 1.080.209 1.01 0.186 0.900.115
N-NO3- µg·gm–1 dry wt o
f
sediment 0.194 0.014 0.178 0.01 0.1850.026
S-SO4–2 mg·gm–1 dry wt
of sediment 1.390.329 1.22 0.257 1.140.191
P-PO4–3µg·gm–1 dry wt of
sediment 0.438 0.167 0.389 0.142 0.3590.116
Microbial C.F.U (× 106)
gm–1 dry wt of sediment12.437 0.821 10.966 0.725 9.6470.788
Effec t of salinit
y
o n mic ro bial populatio n
0
1
2
3
4
5
6
7
8
012345
Salinity of the medium
Micro bial CFU
TopSoil
Middle Soil
B ottomSoil
Figure 2 . Variation of microbial CFU (× 106 gm–1 dry weight of
soil) of three soil segment with medium having different
salinity.
Micro bial growt h of 3 so il segmen t in a erobic condition
0
1
2
3
4
5
0 20406080
Incubation Period ( hour)
CFU X10
6
gm
-1
dry weight of soi
l
TopSoil
Mid dleSoil
BottomSoil
Figure 3. Growth rate of microbes from 3 soil segments in
aerobic condition.

S. Das et al. / Open Journal of Ecology 1 (2011) 35-40 39
Mic r obial growth of 3 so il segment in anaerobic c on dit ion
0
1
2
3
4
5
0 20406080
I ncubation Period (hour)
CFU X10
6
gm
-1
dry w e ight of soil TopSoil
Midd leSoil
B ottomSoil
Figure 4. Growth rate of microbes from 3 soil segments in
anaerobic condition.
rounding environment than the aerobic microbes present
in the top soil segment. This observation can be ex-
plained from the study by Lowe et al. [26]. According to
their report it can be predicted that anaerobic bacteria
can grow at environmental extremes of temperature, pH,
salinity, substrate toxicity, or available free energy and
anaerobes, unlike aerobes, appear to have evolved more
energy-conserving mechanisms for physiological adapta-
tion to environmental stresses such as novel enzyme ac-
tivities and stabilities and novel membrane lipid compo-
sitions and functions. Sea level rising due to global
warming may cause fluctuation of water as well as soil
salinity which may ultimately hamper the activity of
aerobic bacteria a little more than that of anaerobic bac-
teria. Soil salinity is a stress factor relating to microbial
selection process and can reduce bacterial diversity and
control microbial abundance, composition and functions
[27]. Thus oxidation of the reduced trace gas like meth-
ane by aerobic bacteria like methanotrops could be hin-
dered more than that of in present. In such condition
mangrove sediment may emit more methane to the at-
mosphere.
4. CONCLUSIONS
This bacterial growth profile study reveals that a per-
fect stratification exists between the depths of soil in the
mangrove ecosystem and salt tolerance nature of the
bacteria. This stratification may be responsible for a
perfect nutritive management of the mangrove forests.
Thus they provide unique ecological niche to variety of
microorganisms. More anoxic and salty nature of the
Sundarban Mangrove Forest may play a crucial role to
reflect on the microbial activity regarding biogeochemi-
cal cycles.
5. ACKNOWLEDGEMENTS
We would like to acknowledge UGC, New Delhi for providing fel-
lowship to Sudhajit Das and department of forest, Govt. of West Ben-
gal for permitting us to do this study in Sundarban mangrove forest.
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