Nitrogen-fixing cyanobacteria make a major contribution to the fertility of paddy fields (Vaishampayan et al., 2001). In many Eastern countries, peasant farmers do not fertilize their fields; the nitrogen is fixed by cyanobacteria, thus permitting a moderate harvest when in their absence there would be a poor one. There are about 100 million km2 of paddy fields, of which some are in southern Europe and the United States, but about 95% are in India and the Far East. Usually rice is grown on land that is submerged under 10 cm or so of water for 60 to 90 days during the growing season, and then allowed to dry to facilitate harvest. The warm conditions demanded by the rice, the availability of nutrients, the reducing conditions in the soil, and the cyanobacterial ability to withstand desiccation all favor growth of cyanobacteria. Over 70% of algal species in Indian paddy soils are cyanobacteria (Pandey, 1965). In paddy soils in India there is a succession of cyanobacteria over the growing season of the rice (Singh, 1961). Early in the rainy season (starting at the end of June), the soil becomes covered with a thick patchy growth composed of a variety of algae. In July, the fields are flooded, and the top 20 cm of soil and algae are mixed to form a muddy suspension. The rice seedlings are transplanted, and after about a fortnight the soil and algae have settled. By the middle of September, there is an extensive brownish-yellow gelatinous growth on the soil, composed primarily of Aulosira fertilissima

(Fig. 2.46(a)), the most important nitrogen-fixing species in the fields. This species is dominant for about 3 months and forms a papery growth on the soil surface when the fields are dry.

From the above discussion, it is plain that cyanobacteria occur in most environments on earth. There is, however, one important exception: cyanobacteria are usually absent in waters or soils with a pH below 5, and they are uncommon between pH 5 and 6 (Brock, 1973).

Adaption to silting and salinity

In salt marshes and mud flats, felts of cyanobacteria, particularly Microcoleus chthonoplastes (Fig. 2.42(e)) are important in stabilizing mud surfaces. Filaments of M. chthonoplastes are phototactic and chemotactic, resulting in migration to the surface of the mud at a rate of about 7 mm per 24 h. The movement of the filaments is an adaption that allows the cyanobacterium to survive during silting (Whale and Walsby, 1984). M. chthonoplastes is euryhaline, it is able to survive in a wide range of salinities in the estuarine environment by producing glycosylglycerol (Fig. 2.47) as an osmolyte to counteract the osmolarity of the surrounding environment. It also synthesizes trehalose to stabilize the phospholipid membrane bilayers of the cell (Karsten, 1996).

In hypersaline environments, such as the Great Salt Lake in Utah, halotolerant and halophilic cyanobacteria can adapt to the high salinity in three ways (Fulda et al., 1999):