Blue green algae (Part B)

Blue green algae, biofertilizer has been proved to be most efficient source of organic nitrogen in low level Paddy.

N₂ from blue green algae

Nitrogen constitutes in general 1-2 per cent of total dry weight of plants and in unfertilized soils this often limits crop production. Use of chemical fertilizers during 1960-1970 was preferred by farmers due to its cheapness and easiness of application. Later on these fertilizers become most expensive and hence farmers were unable to use these fertilizers as required by crops. The production of N-fertilizer is the energy intensive process and this energy is provided from fossil fuels to convert Nº NÕ NH₃. However, with the energy crisis of the late 1970 the cost of chemical fertilizer was escalated. Hence it was necessary to search for alternative source to maintain the production level of grains to feed the increasing population.

The most inexhaustible energy source is the solar radiation thus there is particular interest in those organisms which can use this energy within their protoplasm to produce ammonia from nitrogen. There are two groups of microorganisms viz. Cyanobacteria (blue-green algae) and photosynthetic bacteria.

Blue green algae (BGA) are photosynthetic procaryotic micro-organisms. Their main photosynthetic pigments are chlorophyll-a carotenes. Xanthophylls, together with phycobiliproteins, c-phycocyanin (blue) and e-phycoerythrin (red). Due to the presence of these latter pigments and mucilage, the colour of BGA in nature ranges from dirty yellow, through various shades of blue-green to brown or black.

Some blue green algae can fix atmospheric nitrogen because they contain an O₂ sensitive enzyme nitrogenase. The term algal biofertilizer was coined in early sixties to embody such blue green algae which have the capacity to metabolize the molecular nitrogen and bring about an addition to the nitrogen content of soil. The conversion of elemental nitrogen to ammonia was the monopoly of heterocystous blue green algae till the findings of Whyatt and Silvey in 1961 who reported nitrogen fixation by a unicellular algae Gloeocapsa. Since then more than a dozen non-heterocystous genera of blue green algae have been found to fix air nitrogen.

Heterocysts

Some blue green algae have empty looking thick walled structures in their trichomes, known as heterocysts. These specialised cells lack pigment system-II and as such there is no endogenous evolution of oxygen. The oxygen from air cannot diffuse through their thick walls. The anaerobic conditions are so created inside the heterocysts keep nitrogenase active in them. Thus, under aerobic conditions only the heterocysts blue green algae can fix nitrogen.

The nitrogenase in the vegetative cells also can become active if the entire trichome is transferred to microgerobic or anaerobic conditions. The non-heterocysts blue green algae find such an environment in the subsoil region and soil water interphase in a rice field where they add substantial amount of nitrogen through nitrogen fixation.

Growth promoting effects of blue green algae

In addition to contributing about 30kg N/ha /season, blue green algae help in maintaining the soil fertility by way of liberating growth promoting substances like auxin, vitamins. They add organic matter because of their photolithotrophic nature. They also solubilize insoluble phosphate and improve physical and chemical properties of soil.

Blue green algae have been found to synthesize and liberate biologically potent substances into the medium. The liberation of auxins, vitamin B₁₂ and amino acids has been found to be maximum during the stationary phase of the growth. The substances benefit the crop growth and enable plants to utilize more of the applied nitrogen.

Photosynthesis by blue-green algae

Blue green algae possess permanent property of metabolizing both elemental nitrogen and carbondioxide from the atmosphere simultaneously. The process of photosynthesis in these organisms meets the entire energy requirement including the power needed for reducing nitrogen to ammonia. As such these algae form a completely independent system and they don’t dwell upon soil organic matter for energy supply. As a consequence of algal growth, organic matter is added to the soil. But because of high nitrogen content and high rate of decomposition in the water logged conditions no appreciable addition to the organic matter content in soil is observed. The presence of organic matter in the soil has been found to favour the growth of the blue green algae. This is attributed to the increased availability of carbon dioxide for the process of photosynthesis.

The polysaccharidic sheath present around the trichomes of these algae binds the soil particles and increases the particle size. This improvement in soil aggregates formation increases aeration and water holding capacity. Some saprophytic algae like Calothrix, Tolypothrix and Scytonema grow on moist soil surface forming a velvety growth and protect the soil from erosion.

Iron toxicity

Algae have been found to grow in the subsoil zone upto a depth of about 20cm. Being photosynthetic in nature they liberate oxygen in this zone which helps in bringing down oxidizable matter content of soil. This has an important implication in areas where 2 to 3 crops of rice are taken in one year. In these areas continuos water-logging conditions create reducing conditions which results iron toxicity. The oxygen liberated by the blue green algae in the microaerobic or anaerobic zones of a rice field converts Fe⁺⁺ to Fe⁺⁺⁺. The later being insoluble gets participitated and iron content of the water is reduced. Iron, if present beyond 5 ppm, is known to adversely affect the cell permeability.

Phosphate solubilization

Many algae have been found to solubilize the insoluble phosphate to the extent of 2.27mg P₂O₅/ml/15 days. This attains importance in view of the fact that most of the phosphatic fertilizer, when applied to soil, is immediately converted into insoluble calcium phosphate and becomes unavailable to the plants.

Response of BGA to various external stresses

Algae when introduced in the field are subjected to physical, chemical and biotic stresses. The physical stress is exerted by soil texture, temperature and moisture. Chemical properties of the soil, pH, fertilizers and various agricultural chemicals constitute the chemical stresses. Variety of micro-organisms present in soil exert antagonistic and synergistic effects on the introduced algae.

Physical stresses

Heavy soils rich in organic matter and with higher water holding capacity support good algal growth. Saline alkali soils with higher water table and poor drainage harbour rich flora of blue green algae. Conversely, sandy soils have a poor algal growth. Blue green algae can grow at temperature range of 30 to 45⁰C. Blue green algae essentially true hydrophytes, although many of them exist in sub-aerial and terrestrial habitats. Although water-logged conditions favour the growth of BGA, quite a few of them grow as true saprophytes. In such forms no significant reduction in growth and nitrogen fixation was observed even when water was present upto 50% of the total water holding capacity of soil. Increased humidity coupled with high temperature and shade favour luxurious growth of algae in rice fields.

Chemical stresses

Soil pH plays an important role in distribution and predominance of algae. Acidic soils show a higher incidence of green algae while neutral to slightly alkaline soils support a rich blue green algae flora. The ideal pH range for luxuriant growth of blue green algae is 6.5 to 8.5.

Use of chemical fertilizers and pesticides has become an integral part of the present day agriculture. In the presence of fertilizers nitrogen blue green algae are expected to "shut off" the process of nitrogen fixation. This effect is more pronounced in presence of NH₄⁺ than NO₃^- nitrogen. However, upto 40 ppm ammonium nitrogen, no significant reduction in nitrogen fixation by blue green algae occurs.

The blue green algae have been found to accumulate pesticides within their cells, in concentrations several folds higher than that of surroundings. At the recommended field application doses, most of the pesticides do not have any adverse affects on the activity of algae.

Biotic stresses

Variety of micro-organisms inhabit the soil. In an undisturbed soil ecosystem, there always exists an equilibrium. A disturbance in this is likely to be met with resistance. The capacity of introder to withstand, overcome and adjust to the new environment qualifies the organisms to be used as inoculum. Fungi like Alternaria and Cephalosporium have synergistic effect on algae. Antibiotics producing organisms are expected to have a regulatory effect on algae. Protozoa, mosquito larvae and snails are common grazers of algae in rice fields.