E. Fernández Valiente

Contribution of N2 fixing cyanobacteria to rice production: availability of nitrogen from 15N-labelled cyanobacteria and ammonium sulphate to rice

This study investigate the potential contribution ofnitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N2-fixing cyanobacteria abundance and N2 fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha-1. No further increase was observed with 140 kg N ha-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from 15N fertilizer. Recovery of applied 15N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of 15N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application.

Effect of nitrogen rates on rice growth and biological nitrogen fixation

The effect of different rates (0–140 kg/ha) of nitrogen fertilizers on soil cyanobacteria and rice crop performance were studied in a rice-cropping system on an alkaline Fluvent soil at Valencia, Spain, during three consecutive crop seasons (1990–92). The results showed that the rice fields of Valencia favour the development of N 2 -fixing cyanobacteria. Nitrogen fixation varied during the cultivation cycle, reaching its highest values at the maximum tillering stage, 5–6 weeks after sowing, and showed a positive correlation with the abundance of cyanobacteria and a negative correlation with the amount of N fertilizers used. Grain yield increased with increasing amounts of N fertilizers up to 70 kg N/ha. N rates appeared to affect grain yield by causing variations in the number of panicles/m 2 . Leaf chlorophyll readings at the end of the tillering stage were positively correlated with the number of panicles/m 2 , suggesting that it could be a useful parameter for predicting productivity. There was a significant increase in the N uptake of the rice but a decrease in the apparent N recovery and Nuse efficiency of applied fertilizer N, with the application of increasing rates of N fertilizer. In all instances, except in plots fertilized with 140 kg N/ha, the amount of N removed by plants was significantly higher than that applied as N fertilizer. The differences were positively correlated with the values for N fixation, suggesting a significant contribution by N fixation to rice production. These results show that a rational use of biological N fixation, in combination with inorganic N fertilization, would permit the input of N fertilizers to be reduced by c . 50% without any significant loss of productivity and with an ecological benefit for the whole ecosystem.

Short- and long-term effects of ammonium on photodependent nitrogen fixation in wetland rice fields of Spain

Abstract Short- and long-term experiments were conducted in the rice fields of Valencia, Spain, to determine the ecological significance of ammonium on nitrogen fixation. A significant inhibition of nitrogenase activity by ammonium, at concentrations higher than 0.5mM, was observed after 8h of incubation in short-term experiments done with a bloom of the N2-fixing cyanobacterium Anabaena sp. In a second set of short-term experiments for in situ assays of nitrogenase activity in the field, a significant correlation between nitrogenase activity and the number of N2-fixing cyanobacteria in soil was found. No significant inhibition of nitrogenase activity by ammonium at concentrations up to 2mM was observed in these assays after 24h of incubation. This lack of inhibition was probably due to the rapid decrease in ammonium content in the flood water. Only 5% of the ammonium initially added remained in the water 24h later. In the long-term experiments, nitrogenase activity was assayed in plots fertilized with 0, 70 and 140kgNha–1, over the cultivation cycle, for 5 years. A partial inhibition of nitrogenase activity by deep-placed N fertilizers was observed. Differences were only significant in 2 years. Mean results from 5 years only showed significant differences between plots fertilized with 0 and 140kgNha–1. The partial inhibition of nitrogenase activity by ammonium increased over the cultivation cycle. Inhibition was only significant in September, at the end of the cultivation cycle.

Influence of pH, light intensity and oxygen on the short-term effect of ammonium on nitrogenase activity of Cyanobacteria from rice fields

Abstract The influence of pH, light intensity and anaerobiosis on the short-term effect of ammonium on nitrogenase activity was investigated in four cyanobacteria ( Nostoc punctiforme UAM 205, Anabaena variabilis UAM 202, Calothrix marchica UAM 214 and Nodularia spumigena UAM 204) isolated from rice fields in Valencia, Spain. A fast switch-off of nitrogenase activity by ammonium was observed only when cells were incubated in the light in the presence of oxygen at pH 10. A stabilizing effect of carbohydrates, which was evident at pH 7, was not effective at pH 10. Under these conditions ammonium caused a light-dependent oxygen uptake insensitive to cyanide, which could be related to inactivation of nitrogenase.

Protection by CO2 against Sodium Deficiency in a Mutant of Nostoc muscorum

The effects of Na-deficient culture were studied on a mutant of Nostoc muscorum unable to fix N(2). Na deficiency made the cells sensitive to photooxidation, thus at a light intensity of lO W m(-2) and a low concentration of CO(2) (0.03 %), Na deficiency caused chlorosis and cell lysis. At a lower light intensity, cell damages did not take place. Higher levels of CO(2) prevented photooxidation, so that only a partial inhibition of photosynthetic oxygen evolution was found under these conditions.

Accumulation of Proteins in Giant Cells, Induced by High Boron Concentrations in Chlorella pyrenoidosa

Summary Cell division was altered when the cells of Chlorella pyrenoidosa were exposed to boron concentrations ranging between 50 and 100 mgl −1 . Nuclear division was normally achieved, as electron micrographs and DNA contents showed, but cytokinesis was delayed for 72 h. During this time photosynthesis was only 50 % inhibited. In this manner the formation of daughter cells was more severely inhibited than the synthesis of biomass, leading to the formation of giant cells. These giant cells attained a high protein content which resulted from an increase in the nitrate uptake. As expected by the increase in protein content, total amino acids were increased in the giants cells. This enhancement affected not only protein amino acids but also non-protein amino acids principally ornithine and citrulline. On the other hand, chloroplasts showed a drastic alteration, due to an accumulation of a dense material, probably proteins, which appeared surrounded by thylakoids.

Effect of Sodium Deficiency on the Ultrastructure of the Cyanobacterium Anabaena PCC 7119

Summary The effects of sodium deficiency on the ultrastructure of Anabaena PCC 7119 are different depending upon the culture conditions. In cells grown under N 2 -fixing conditions and gassed with air supplemented with CO 2 , sodium deficiency did not induce any structural change. In cells grown in air, however, sodium deficiency gave rise to a progressive accumulation of cyanophycin and to the degradation of thylakoids. No change was observed in the ultrastructure of heterocysts. In ammonium-grown cells, sodium deficiency also induced accumulation of cyanophycin although in this case no degradation of thylakoids was observed. The results are discussed in terms of their relationship with the metabolic alterations induced by sodium deficiency in this strain.

Effects of High Boron Concentrations on Nitrate Utilization and Photosynthesis in Blue-green Algae Anabaena PCC 7119 and Anacystis nidulans

Summary The effect of excess boron on growth, cell composition, photosynthesis, and nitrogen metabolism, in two species of blue green algae (cyanobacteria), Anabaena PCC 7119 and Anacystis nidulans , was examined. High concentrations of boron in culture media inhibited growth and decreased the contents of phycobiliproteins and chlorophyll. In addition, an accumulation of carbohydrates and a decrease in lipids were observed in both species. Parallel to the fall in photosynthetic pigments, an inhibition of photosynthetic oxygen evolution was observed, after 72 h of culture, in Anabaena PCC 7119. However, initial exposure of the cultures to high concentrations of boron did not affect photosynthesis. In contrast, nitrate uptake decreased in both species with initial exposure to excess concentrations of boron. This inhibition was followed by a decrease in the nitrate reductase activity.

Contribution of biological nitrogen fixation to rice production in a Mediterranean rice field

In this report, we present data of a study conducted to investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). The amounts of N fertilizers usually added by farmers in these rice fields have led to the progressive pollution of nearby water. The utilization of nitrogen fixation as a complementary source of N for rice production would allow reduction of the use of N fertilizers, in this way decreasing the level of water pollution. Field experiments were conducted in three consecutive crop seasons (1990–1992) in plots (100 m2) fertilized with different amounts of N fertilizer (0–140 kg N/ha). Nitrogen fixation varied along the cultivation cycle, reaching its maximum value 4–5 weeks after sowing. Diurnal nitrogen fixation showed a positive correlation with the abundance of cyanobacteria and a negative correlation with the amount of N fertilizer. Grain yield increased with increasing amounts of N fertilizer up to 70 kg N/ha. No further increase was observed with 140 kg N/ha. Grain yields in unfertilized plots (5838–6048 kg/ha) were only 20% lower than in plots with the maximum level of fertilization. In all cases, except in plots fertilized with 140 kg N/ha, the amount of N incorporated by plants was significantly higher than that applied as N fertilizer. The differences were positively correlated with the values of nitrogen fixation, indicating a significant contribution of nitrogen fixation to rice production in plots fertilized with ≤70 kg N/ha. These results indicate that a rational utilization of biological nitrogen fixation in combination with inorganic N fertilization would allow the reduction by about 50% of N fertilizers, without significant loss of productivity and with ecological benefits to the ecosystem.

Effect of Sodium Deficiency on Mixotrophic Cultures of Anabaena variabilis

Summary The effects of sodium deficiency on fructose uptake and utilization, photosynthesis, nitrogenase activity and phosphate uptake were studied in the facultatively heterotrophic, dinitrogen-fixing cyanobacterium Anabaena variabilis ATCC 29413. Fructose uptake was not altered by the absence of sodium. However, oxidation of exogenous fructose was enhanced by sodium deficiency. Most of the CO 2 produced was reutilized in photosynthesis. This reutilization of CO 2 resulted in a partial recovery of the rate of photosynthetic oxygen evolution, which was markedly inhibited in sodium-deficient cells grown under photoautotrophic conditions. Exogenous fructose increased nitrogenase activity of sodium-supplemented cultures. Fructose, however, did not enhance nitrogenase activity of sodium-deficient cells, which was depleted under these conditions. Phosphate uptake was inhibited in sodium-deficient cells cultured both under photoautotrophic or mixotrophic conditions. A model for the effects of sodium deficiency on cyanobacteria is proposed.