Soil salinity is a major problem that makes
soil unfit for Agriculture. Historical records
of the past 6000 years of civilization evidenced
that, humans have never been able to continue
a progressive civilization in one locality for
more than 200 to 800 years. The major reason
for the decline of any civilization in any area
seems to have been the destruction of the resources
base of that area. In Mesopotamia, major salinity
damage occurred from 2400 BC to 1700 BC and
the slow increase in salinity
caused a decline in agriculture productivity
to as approximately as 65% over a 700-year period.
At present salinity is one of the most serious
environmental problems influencing crop growth
around the world. In India, 7 m ha are affected
by salinity and alkalinity and marginal decrease
of productivity is expected from these lands.
In Tamilnadu coast, salt causes stress and damage
on the plant during the vegetation period from
germination – emergence through growth
– development and harvesting time.
It is generally accepted that three major hazards
are associated with saline habitats. These may
be described as follows:
(a) Water stress arising from the more negative
water potential (elevated osmotic pressure)
of the rooting medium.
(b) Specific with toxicity usually associated
with either excessive chloride or sodium intake
, and
(c) Nutrient ion imbalance when the excess of
sodium or chloride leads to a diminished uptake
of potassium, nitrate, or phosphate or due to
impaired internal distribution of one or another
of these ions.
More than that, inoculation of crops with any
useful microorganisms would not yield desired
success. Excess salts in soil adversely affect
the survival, growth and nutrient supply to
the plants.
During late 20th century, research
has been started to find out the saline tolerant
Azotobacter and Phosphobacteria
from marine aquatic sediments but focus on the
preparation of marine biofertilizer for coastal
agriculture has not been made. Recently, identification
of saline tolerant biofertilizers for possible
utility to use for agricultural crop cultivation
has been recognized. Besides that microbial
biofertilizers have also been identified to
improve the growth of mangrove plants. Azotobacter,
Azospirillum, Phosphobacter and
Phosphate producing bacteria and Blue Green
algae were isolated and identified from saline
sediments. Even the presence of high phenolic
compounds and prevalent anaerobic condition
in the mangrove habitat and their biofertilizer
effects have been proved with the rice and balckgram
crop seedlings. Compared with the existing biofertilizers
the morphological and biochemical characteristics
are similar except the saline induced effects
on growth and physiology.
Genus Azotobacter
Azotobacters are aerobic, free-living,
thermotrophic bacteria with unique ability of
fixing atmospheric nitrogen. The bacteria are
gram negative, often motile by peritrichous
flagella or non-motile. The Azotobacters
produce copious amount of capsular slime. They
do not or endospores but some species may form
cysts. Three species of Azotobacters
such as Azotobacter chroococcum,
A.berijerinkii and A. vivelandii
were identified from mangrove rhizospheresediments.
All the three species are able to tolerate high
saline concentrations (up to 35 g 1-1
and 30 g1-1). These species of Azotobacter
enhanced the germination and growth of rice
and black gram seedling even at high saline
conditions by fixing atmospheric nitrogen and
producing phytohormones. Among them, A.chroococcum
was highly recommended than the other bacterial
species.
Genus Azospirillum
Azospirillum species are free living
bacteria know to fix atmospheric nitrogen. They
occur as free living organisms in the soil and
are associated with mangrove and associated
plant species. Azospirillum are plumpy,
slightly curved and straight rods gram negative
to gram variable. They are motile in liquid
media by a single plar flagellum.
Four species of Azospirillum such
as Azospirillum lipoferum, A.brasilense,
A.halopreferns and A.irakense
were identified from the marine sediments. Of
them, Azospirillum lipoferum was found
to be the dominant species. All these species
were found to have tolerance ability to various
salinity levels (0- 35 g.1-1) and
grown better in 30g. 1-1 NaC1. However,
the level of phytohormoneProduction (IAA) and
the rate of nitrogen fixation was better at
10 g .1-1 NaC1 and reduced activity
could be observed at higher salinity levels.
Moreover all these species could be used as
marine biofertilizers. Of them, Azospirillum
brasilense are preferable than the other
species.
Group Phosphate
Solubilising Bacteria (PSB)
Phosphorous is an important limiting nutrients.
The phosphate form of phosphorous is one of
the least soluble mineral nutrients in soil.
The phosphorous content of soils may range up
to 19 g k-1 but usually less that
5 % of this is available to the plants and microorganisms
in soluble form and the rest 95 % is unavailable
being in the form of insoluble inorganic phosphate
and organic phosphorous complexes. These forms
of phosphorous being held in the sediments far
a long time remain excluded from cycling. Microbes
play a significant role in the transformation
of phosphorous and referred to as phosphobacteria.
Eight species of saline tolerant inorganic phosphate
solubilizing bacteria such as Bacillus subtilis,
B.cereus, B.megaterium, Arthrobacter illicis,
Escherichia coli, Pseudomonas aeruginosa, Enterobacter
aerogenes and Micrococcus luteus
were identified. Of them, Bacillus subtilis
was predominantly found in mangrove sediments.
All the nine species could able to grow better
at 4 g.1-1 NaCl concentrations. However
the phosphatase activity was good at 2 g1-1
NaCl salinity levels. Moreover except Pseudomonas
aeruginosa and Micrococcus luteus,
all the other species of bacteria could be used
as a biofertilizer to enhance the growth of
rice seedlings. Of them Bacillus Megaterium
could be used as a marine biofertilizer for
saline soil cultivation.
Group Phosphatase
Producing Bacteria (PPB)
Organic phosphorous in the marine environment
is macromolecular and not readily available
for incorporation into the marine organisms.
So the organic phosphorous compounds are to
be preconditioned by extra cellular bacterial
enzymes called “phosphatases” for
making them available to the nutrient cycles.
Three groups of bacteria viz., Pseudomonas,
Vibrio and Bacillus were identified
from mangrove sediments. Of them, Bacillus
cereus was dominant form and the phosphatase
activity was also higher. All the three groups
of PPB could enhance the growth of rice seedlings
at 25 g. 1-1 NaCl level of soil salinity
at which the phosphatase activity was significantly
high.
Recommended biofertilizers
for the saline soil crop cultivation on priority
basis.
Biofertilizer |
Recommended
crop species |
| Azospirillum |
Black gram, Rice |
| Azotobacter |
Black gram, Rice |
Inorganic phosphate
solubilizing bacteria |
Black gram, Rice |
Phosphatase producing
bacteria |
Rice |
Species abundance of saline tolerant
biofertilizers in the mangrove sediments
-Phosphate
solubilizing bacteria (45%)
-Azospirillum
(25%)
-Azotobacter
(15%)
-Phosphate
producing bacteria (15%)
