Aquaculture & Applied Limnology Research Unit, Department of Zoology,
University of Kalyani, Kalyani
– 741235, West Bengal, INDIA
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Phosphorus,
though required in small quantities, has often been implicated as the most
limiting element controlling biological productivity in natural waters. Aquaculture ponds with high input of
fish biomass demand for frequent application of phosphate fertilizer for
increasing phytoplankton production.
It is estimated that about 10% of the fertilizer applied caused increase
in soluble phosphate in the water
phase, which is absorbed by the phytoplankton within few minutes of fertilizer
application, whereas the rest is rapidly precipitated and settled at the bottom and converted into insoluble compounds. Thus, the
pond bottom acts as a sink of phosphorus in fertilized ponds, whereas a source
of P in unfertilized ponds.
In the
biogeochemical cycle of phosphorus, a mixed population of microbes is essential
to promote enzymatic degradation of naturally occurring organic phosphorus
compounds. Extracellular products
of the microbial community such as enzymes and chelating agents (organic acids)
have substantial effect respectively on phosphorus mobilization from organic P
esters and inorganic salts. Phosphatases are stated to promote the degradation of complex
phosphorus compounds into orthophosphate, which is readily utilized by phytoplankton. Alkaline phosphatase can catalyze the
liberation of orthophosphate from organic P compounds and inorganic
pyrophosphate and tripolyphosphate. Synthesis of external alkaline phosphatases
is often repressed by high concentrations of phosphate and depressed at low
phosphate concentration. As a consequence, phosphatase activity can be used as
the indicator of the degree of nutrient regeneration in surface sediments.
Increasingly high cost of chemical phosphate
fertilizers has led to search for alternative
cheap, effective and dependable source of phosphorus from natural sources for
pond fertilization. Igneous rock
phosphate is trade name of mineral
phosphates, which denotes the product obtained from mining and subsequent
metallurgical processing of
phosphorus containing ores. India has a vast reserve of 126.90 million tones of
rock phosphate. Though the available form of phosphorus obtainable from rock
phosphate is very little, it
contains essential nutrients like calcium, magnesium, zinc, molybdenum, silica,
organic carbon and potash which
are useful in biological
production. Despite rock phosphate is potentially rich in phosphate contents, a
major problem encountered in the direct application of rock phosphate in fish ponds is that it is sparingly
soluble in water; the association of tricalcium phosphate and calcium fluoride
forming a mineral fluorapatite had made it more resistant to weathering.
A large
number of phosphate solubilizing
microorganisms such as bacteria, fungi and cyanobacteria occurring in pond ecosystem are capable of assimilating insoluble
inorganic phosphate like hydroxyapatite, tricalcium phosphate and rock
phosphate and make a large portion
soluble by the production of organic and inorganic acids. Based on this principle,
attempts were made to enhance the fertilizer value of rock phosphate for
aquaculture ponds. . Bacteria induced solubilization of rock phosphate is
of considerable economic
importance in the managed fish
ponds. It is suggested that
solubilization of rock phosphate during the process of its composting with
organic substances is accelerated by the liberation of organic acids in the
first step and proliferation of phosphate solubilizing microorganisms at a
later stage. It is reported that amongst the microbes, Bacillus is reported to be the
most effective one in respect of phosphate solubilization and phosphates
production. Experimental evidences suggest that exogenous introduction of
phosphate solubilizing bacteria charged
with compost increased the
concentrations of different
species of phosphate in water and sediments, which was responsible for
increased phytoplankton production.
Invertebrates can contribute to the
mineralization of dissolved and particulate compounds in the sediment and their
burrowing activity can affect several exchange processes by increasing the
mixing of the sediment surface. Several studies indicate that a number of bottom grazing fishes are also able to increase the
fertilizer value of rock phosphate through their bioturbation effects in bottom
sediments. This effect has been clearly demonstrated in a number cultivable
fishes of India. Different trials using
rock phosphate as P fertilizer in fish ponds have conclusively shown that
rock phosphate can be effectively used as phosphate fertilizer in fish ponds.
The purpose of the present paper was to examine the state-of-the-art of various
manipulation approaches for using rock phosphate as direct source of phosphate
fertilizer and the role of phosphate solubilizing bacteria in the enhancement
of fertilizer value of rock phosphate in aquaculture ponds.