Distribution pattern and role of phosphate solubilizing bacteria in the enhancement of fertilizer value of rock phosphate in aquaculture ponds: state-of-the-art

Abstract

Phosphorus, though required in small quantities, has often been implicated as the most limiting element controlling biological productivity in natural waters. As a result, aquaculture ponds demand for frequent application of phosphate fertilizer for enhanced fish 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. Increasingly high cost of chemical phosphate fertilizers has been the main stimulus for searching alternative cheap, effective and dependable source of phosphorus from natural sources for pond fertilization. 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. It has proved to be an important phosphate fertilizer for agriculture soils under acidic conditions. 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. 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 can be readily utilized by phytoplankton. Alkaline phosphatase can catalize 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. This enables to use phosphatase activity as a good indicator of the degree of nutrient regeneration in surface sediments. Solubilization of insoluble inorganic phosphate by bacteria is of considerable importance in the anthropogenically-managed system. A large number of phosphate solubilizing microorganisms such as bacteria, fungi, and cyanobacteria occurring in water and sediments of fish ponds 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. 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 the most effective one in respect of phosphate solubilization and phosphates 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. 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 has been clearly demonstrated in experimental studies. Exogenous introduction of phosphate solubilizing bacteria is of considerable interest in solubilization of rock phosphate in fish culture ponds because of extremely low natural microbial solubilization of rock phosphate in fish ponds. Exogenous introduction of phosphate solubilizing bacteria with compost resulted in the highest concentrations of different species of phosphate in water or sediments among all treatments. This was attributable to the combined effects of the phosphate solubilizing bacteria population of both exogenous and compost origin with short generation time. The present paper reviews the state-of-the-art of various approaches for using rock phosphate as direct source of phosphate fertilizer and the dynamics of phosphate solubilizing bacteria and bacteria induced solubilization of rock phosphate in the fish culture ponds of India.