Occurrence and dynamics of phototrophic purple nonsulphur bacteria compared with other asymbiotic nitrogen ®xers in rice®elds of Egypt M. Elbadry*, A. El-Bassel and Kh. Elbanna Department of Agricultural Microbiology, Faculty of Agriculture, Fayoum Branch, Cairo University, 63514 Fayoum, Egypt *Author for correspondence: Fax: 002084334964, E-mail: makramas@main-scc.cairo.eun.eg Received 20 February 1999; accepted 1 March 1999 Keywords: Phototrophic purple nonsulfur bacteria, rice®elds Abstract The occurrence and the dynamics of phototrophic purple nonsulphur bacteria (PPNSB) as well as Azospirillum, Azotobacter, Clostridium, and cyanobacteria at dierent rice growth stages were studied in two rice®elds, at Kafr-El- Shiekh and Al-Fayoum in Egypt. The PPNSB existed in the both rice ®elds examined, but their numbers varied according to ®eld conditions, habitat and rice growth stage. After transplanting, the number of PPNSB increased gradually, reached its maximum at maximum tillering stage, and thereafter declined toward harvest time. Numbers of PPNSB were generally comparable with that of the heterotrophic N 2 -®xers namely Azospirillum, Azotobacter, Clostridium and cyano- bacteria, while that of phototrophic purple and green sulphur bacteria were relatively lower. The highest PPNSB numbers were generally found in rhizosphere (10 3 ±10 6 per g 1 dw soil) followed by soil (10 3 ± 10 5 per g 1 dw soil) and ¯oodwater (10±10 2 per ml). Rice plants showed a positive rhizosphere eect on PPNSB, clostridia, Azotobacter and Azospirillum, negative rhizosphere eect on cyanobacteria and green sulphur bacteria, and no eect on purple sulphur bacteria. Introduction Nearly all studies so far available on the occurrence of anoxygenic phototrophic bacteria (APTB) in rice®elds have been carried out in Japan. The ®rst published in- formation showing the occurrence of APTB in rice®elds was that of Okuda et al. (1957). They found that in every case, APTB were scarcely observed in forest soil and ordinary in crop soil samples, while in rice®elds they occurred frequently. Studies on the occurrence, distribution and seasonal changes of APTB and heterotrophic nitrogen ®xers have been carried out in paddy soils of Southeast Asia (Ko- bayashi et al. 1967), in a rice®eld in Japan (Haque et al. 1969); in paddy ®elds of Thailand (Matsuguschi & Tangcham 1974) and in dierent aquatic environments (Hiraishi & Kitamura 1984). However, only little in- formation on their microbial dynamics and abundance relative to other N 2 -®xers in these ecosystems were studied. In Egypt, neither the occurrence of PPNSB nor their dynamics in rice®elds has been investigated. Therefore, the objectives of this study are to follow the following seven N 2 -®xing bacterial populations: phototrophic purple nonsulphur bacteria (PPNSB), phototrophic purple sulphur bacteria (PPSB), phototrophic green sulphur bacteria (PGSB); N 2 -®xing cyanobacteria, Azotobacter, Azospirillum, and Clostridium and total indigenous bacterial populations in soil, rhizosphere and ¯oodwater samples at about one-month intervals rep- resenting ®ve growth stages in rice®elds of two gov- ernerates, Kafr-El-Sheikh and Al-Fayoum in Egypt, where large areas are cultivated with rice. Materials and Methods Samples from three habitats; soil, rhizosphere and ¯oodwater were collected over a rice-growing season (rice cultivar, Indica/Japonica Giza 176) at two dierent rice®elds in Egypt (Kafr-El-Shiekh season 1993, and Al-Fayoum season 1994) at ®ve growth stages of rice: transplanting, initial tillering, maximum tillering, panicle initiation and maturity. Soil samples were taken from areas between widely spaced plants. Floodwater samples were collected after removal of the ¯oating materials. Since there is no ab- solute de®nition of rhizosphere soil, we considered it roots plus adhering soil after careful removal from soil. To provide a basis for count, rhizosphere soil was quanti®ed as follows: in the 10 1 and 10 2 dilutions, soil was recovered, dried at 105  C and weighed. Bacterial World Journal of Microbiology & Biotechnology 15: 359±362, 1999. 359 Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands.