Biological studies on algae in rice-field soil from the Iraqi marshes
The algae in one soil sample from a rice-field in the southern marshes of Iraq were studied in detail. 11 taxa were found by direct microscopic inspection, with Microcoleus chthonoplastes and Nostoc muscorum the dominants. A total of 49 taxa were noted after various enrichment culture techniques. The influence of temperature, light, salinity, reducing conditions, nitrogen sources and phosphorus (phosphate) were tested on mixed populations. Blue-green algae dominated all cultures incubated at temperatures between 25-45 C, while eukaryotic algae dominated cultures at lower temperatures (10-20 C). Six taxa showed good growth in a medium enriched with 0.5 M NaC1and three also grew at 1 M NaC1, though growth was very slow. Most of heterocystous blue-green algae were sensitive to reducing conditions (0.1 mM Na(_2)S). As a consequence, in mixed populations there was an increase in the relative abundance of non-heterocystous forms. Fifteen strains were brought into clonal culture, nine of them also being axenics Anabaena cylindrica, three Calothrix spp., Cylindrospermum muscicola, Gloeotrichia sp., Hapalosiphon welwitschii, Microcoleus chthonoplastes, Nostoc muscorum. Detailed morphological and physiological studies were made for 14 strains under different nitrogen and phosphorus conditions. All asenic cultures showed marked alkaline phosphatase activity under phosphorus-deficient conditions. They were capable of utilizing a wide range of organic phosphorus compounds as a sole source of phosphorus. Axenic cultures of heterocystous strains were capable of fixing nitrogen, judged by positive results obtained with acetylene reduction assays. Nitrogen fixation and heterocyst differentiation of these strains was almost completely suppressed when the strains were grown with high concentrations of combined nitrogen (initial = 140 mg 1(^-1)NO(_3)-N or 50 mg l (^-1) NH(_4)-N).A discussion is included of how field research on these algae might indicate ways of modifying cultivation practices to increase the phosphorus and nitrogen status of the soil and also how algal morphology might be used to help indicate the nutrient status of the soil.