Epiphytic Cyanobacteria onChara vulgarisAre the Main Contributors to N2Fixation in Rice Fields

Abstract

ABSTRACTThe 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 N2fixation was associated with the macrophyteChara vulgarisin both years and at both locations. The nitrogen fixation rate associated withCharaalways represented more than 45% of the global nitrogenase activity measured in the rice field. The estimated average N2fixation rate associated withCharawas 27.53 kg of N ha−1crop−1. The mean estimated N2fixation rates for the other parts of the system for all sampling periods were as follows: soil, 4.07 kg of N ha−1crop−1; submerged parts of rice plants, 3.93 kg of N ha−1crop−1; and roots, 0.28 kg of N ha−1crop−1. Micrographic studies revealed the presence of epiphytic cyanobacteria on the surface ofChara. Three-dimensional reconstructions by confocal scanning laser microscopy revealed no cyanobacterial cells inside theCharastructures. 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 chlorophylla. Chlorophyllameasurements confirmed that cyanobacteria were more abundant in nodes than in internodes (on average, the chlorophyllaconcentrations were 17.2 ± 28.0 and 4.0 ± 3.8 μg mg [dry weight] ofChara−1in 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.