Blue green algae – Biofertilizer (A)

a. Introduction:

The global energy crisis and dwindling mineral oil reserves have widened the gap between supply and demand of nitrogenous fertilizers. An introduction of fertilizer responsive high yielding crop varieties has further increased the demand of this important crop nutrient. This has resulted in further burden on small and marginal farmers, especially in developing countries. This has become necessary to look for alternative sources to meet atleast a part of nitrogen requirement of crop production.

In India, rice is cultivated on about 40 million hectares of area, which constitutes about 37-40% of total area under cereals. Though, rice cultivation is an age-old practice in our country, the average production is only about 1.7 t/ha. This is because more than 85% of the total area of rice is owned by small and marginal farmers. These farmers cannot afford to use various inputs needed to harvest maximum yield of rice. They do not get full returns/unit nitrogenous fertilizers in their fields because of high nitrogen losses in the ecosystem.

The past few decades have widened remarkable advancement in harnessing some of the potentially useful micro-organisms to build up the fertility of the soil to increase the crop yield. In recent years, blue-green algae, a group of soil micro-organisms have been shown to be agriculturally important, particularly in tropical rice field soils. This is because of capacity of some of the algae to synthesize organic substances and also to fix atmospheric nitrogen.

Submerged conditions of a rice field provide congenial habitat for blue green algae where they form most efficient system providing biologically fixed nitrogen to the crop. The importance of blue-green algae was first recognised by De (1936) who reported that these micro-organisms are responsible for spontaneous fertility of tropical rice field soils. Since then series of reports have been appeared emphasising their role in nitrogen cycle in general and rice field in particular. The propagation of blue green algae will not only enrich the nitrogen status of the soil by their fixation process but also provide organic matter and biologically potent substances for plant growth. These algae form a living constituent of the soil biotype and continue their activity year after year. Besides they release oxygen for the paddy roots and increase soil phosphate. Some of them prevent the loss of soil ammonia and leaching out of nitrates by converting them into organic nitrogen. Blue green algae also produce a surface humus after death and exert a solvent action on certain minerals – maintaining a reserve supply of elements in a semi-available form for higher plants either by ecretion or upon death and decomposition.

b. Distribution of blue green algae in rice field soils:

Blue green algae have been found in almost all the conceivable habitats. They are widely distributed throughout the tropical, subtropical and temperate regions. However, the frequency of their occurrence was more prominent in Southern than in Northern regions. Tropical soils harbour comparatively higher population of blue green algae.

In Southeast Asia including Japan the presence of species Tolypothrix, Nostoc, Cylindrospermum, Calothrix, Anabaena, Plectonema and Anabaenopsis was found prominent. In Senegal, the dominant species reported were Nostoc and Anabaena, whereas, Scytonema and Calothrix were respectively found in 50 and 15 percent area. In Indonesia Cylindrospermum, Anabaenopsis, Nostoc and Nodularia were found to be common. In North Australia blue green algae flora was dominated by Nostoc and Anabaena. The dominant species in Philippines weree Nostoc and Anabaena. In Russia, Nostoc and Anabaena were found most common.

In India, a general prodominance of blue green algae except in acidic soils of Kerala, Assam and parts of Tamil Nadu. Forms like Anabaena, Nostoc and Calothrix were found to be widely distributed throughout rice growing tracts of India.

Other forms like Cylindrosporum, Tolypothrix, Scytonema and Aulosira had localised distribution. The distribution of soils harbouring blue green algae in India varies from as low as 7 to as high as 80 percent in different States. Uttar Pradesh soils are rich in Aulosira and Mastigocladae is found in Gujarat. Westiella is found very dominant in Maharashtra and Cylindrospermum in Karnataka and Calothrix in Punjab soils of Vidharbha and Konkan of Maharashtra are dominated by blue green algae.

During recent years, quantitative studies showed consistent presence of N2 fixing blue green algae at high densities in soils under rice cultivation in countries like India, Malaysia, Philippines, Portugal, Nostoc sp. Was dominant followed by Anabaena and Calothrix. Blue green algae occurred at densities from 1.0 x 10^-2 to 8.0 x 10^-6CFU/cm^-2 and their abundance was correlated to pH and available `P’ content of soil.

c. Strain variation:

Biological N₂fixation in nature or agricultural ecosystem is rarely limited by a lack of N₂, fixing micro-organisms. Nevertheless, very little nitrogen fixed in nature. Apart from the ecological stresses the efficiency of strains themselves may play an important role. Therefore great differences in the amount of nitrogen fixed by various genera and sometimes by the same species from different localities. The need for wide collection, culturing and testing for the relative efficiency of different strains is, therefore, obvious. The possible reasons for the reported differences in their efficiency in fixing nitrogen may be due to variations in the cultural conditions like light, temperature, nutrient deficiences, etc. It is also known that the secrete of increasing nitrogen fixing capacity lies in the adequate supply of trace elements like molybdenum.

Likewise, the influence of genetic constitution might also play a vital part in determining the capacity to fix nitrogen. Apart from the natural inherent variations, combined nitrogen and various agrochemicals play a vital role in determining the nitrogen contribution by the added blue green algae in field. This offers an opportunity to select strains for specific ecosystem.

d. Strain competition:

The successful induction of a micro-organism in an ecosystem depends upon its ability to adopt and compete with the indegenous biotypes. For the positive introduction of an effective blue green algae strain in an area depends upon its ability to survive and compete with the native flora for establishment, growth and effective nitrogen fixation.

Prior to seeding, the paddy field water may be sprinkled with lime powder, which will be effective in suppressing the growth of other algae and at the same time will lower the acidity of water to a favourable level. It is possible that antagonistic effects of other organisms may affect the successful survival of particular algae strain. Similarly, possibilities for the presence of algophages cannot be ruled out. There is a great deal of indirect evidence to show that some algae can liberate antibiotic substances. There are reports that there was no marked difference in the total amount of nitrogen fixed and bacteria under water logged conditions. Thus nitrogen fixation is essentially, an algae process and the part played by bacteria is relatively unimportant. The decomposition of blue green algae is enhanced by the presence of bacteria in an adequate quantity. Besides fertilizing action of nitrogen fixing blue green algae in the field is considerably improved by bacterial flora.

e. Strain selection:

The Production of strains, likely to be superior, in nature to those which already exist, is daunting challenge.

First it is essential to select N₂fixing strains capable of rapid growth. Well studied Cynobacteria such as Anabaena cylindrica are relatively slow growing with a generation time of 16 – 24 hours. Anacystis nidulans, which was doubling time of 2 hours does not fix nitrogen. However, with improved culture media and methods of obtaining axenic cultures, the selection of rapidly growing N₂ fixing strains has a reality.

Secondly, Cyanobacteria should be selected which can fix N₂ equally well under aerobic, micro aerobic and anaerobic conditions, so that they can tolerate the very wide range of oxygen tension found in rice fields. The heterocystous and certain unicellular forms satisfy these conditions. However, N₂fixing unicellular forms which can tolerate Oxygen extremes and high light intensities and grow rapidly are not yet available. Heterocystous forms are currently better alternatives.

Third, it is important to choose Cyanobacteria that fix N₂ under Photoautotrophic, Phoheterotrophic and chemoheterotrophic conditions. These Cyanobacteria include species of Anabaena, Anabaenopsis, Nostic and Tolypothrix.

Fourth, strain that show little or no H₂ evolution should be selected. The extent of H₂ production varies in different N₂ fixing Cynabacteria and it is important to select strains that little show such production and ATP wastage.

Fifth, the selection of strains that possess non-repressible nitrogenase could possibly be important.
In Cyanobacteria nitrogenase is inhibited by high level of NHX4 –N and it is thus important to obtain strain in which this does not occur.

Sixth, strains should be selected that not only liberate extracellular Nitrogen but liberate it in substantial amounts, exceeding the requirement of Cyanobacteria for optimal growth and is released it in a form that can be readily assimilated.

Seventh, the way in which glutamine synthetase regulated is of importance in Nitrogen fixing Cyanobacteria.