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Applied and Environmental Microbiology, September 2004, p. 5391-5397, Vol. 70, No. 9
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.9.5391-5397.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Epiphytic Cyanobacteria on Chara vulgaris Are the Main Contributors to N₂ Fixation in Rice Fields

Yoanna Ariosa,¹ Antonio Quesada,¹ Juan Aburto,¹ David Carrasco,¹ Ramón Carreres,² Francisco Leganés,¹ and Eduardo Fernández Valiente^1*

Departamento de Biología, Universidad Autónoma de Madrid, Madrid,¹ Departamento del Arroz, Instituto Valenciano de Investigaciones Agrarias, Sueca, Valencia, Spain²

Received 16 December 2003/ Accepted 10 May 2004

The distribution of nitrogenase activity in the rice-soil system and the possible contribution of epiphytic cyanobacteria on rice plants and other macrophytes to this activity were studied in two locations in the rice fields of Valencia, Spain, in two consecutive crop seasons. The largest proportion of photodependent N₂ fixation was associated with the macrophyte Chara vulgaris in both years and at both locations. The nitrogen fixation rate associated with Chara always represented more than 45% of the global nitrogenase activity measured in the rice field. The estimated average N₂ fixation rate associated with Chara was 27.53 kg of N ha^–1 crop^–1. The mean estimated N₂ fixation rates for the other parts of the system for all sampling periods were as follows: soil, 4.07 kg of N ha^–1 crop^–1; submerged parts of rice plants, 3.93 kg of N ha^–1 crop^–1; and roots, 0.28 kg of N ha^–1 crop^–1. Micrographic studies revealed the presence of epiphytic cyanobacteria on the surface of Chara. Three-dimensional reconstructions by confocal scanning laser microscopy revealed no cyanobacterial cells inside the Chara structures. Quantification of epiphytic cyanobacteria by image analysis revealed that cyanobacteria were more abundant in nodes than in internodes (on average, cyanobacteria covered 8.4% ± 4.4% and 6.2% ± 5.0% of the surface area in the nodes and internodes, respectively). Epiphytic cyanobacteria were also quantified by using a fluorometer. This made it possible to discriminate which algal groups were the source of chlorophyll a. Chlorophyll a measurements confirmed that cyanobacteria were more abundant in nodes than in internodes (on average, the chlorophyll a concentrations were 17.2 ± 28.0 and 4.0 ± 3.8 µg mg [dry weight] of Chara^–1 in the nodes and internodes, respectively). These results indicate that this macrophyte, which is usually considered a weed in the context of rice cultivation, may help maintain soil N fertility in the rice field ecosystem.

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* Corresponding author. Mailing address: Departamento de Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. Phone: 34-914978186. Fax: 34-914978344. E-mail: eduardo.fernandez{at}uam.es.

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Applied and Environmental Microbiology, September 2004, p. 5391-5397, Vol. 70, No. 9
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.9.5391-5397.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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