Nitrogen and Sulfur Metabolism in Microalgae and Plants: 50 Years of Research

Microalgae and higher plants are primary producers which convert solar energy into chemical energy associated with biomass, which is used by the other organisms as life support. The knowledge and improvement of mineral nutrition of microalgae and plants are very important in order to obtain a good productivity, even in low fertile soils. This review describes, in chronological order, the research topics in which the author has been involved in the past 50 years, which are mostly connected with the nitrogen and sulfur metabolisms in bacteria, fungi, microalgae, and higher plants, with emphasis on the microalgal model Chlamydomonas reinhardtii and the plant models Arabidopsis thaliana and Lotus japonicus. It is described the outstanding contribution of the author to each topic described, and the state of art actualized. The data from our studies on the nitrate and sulfate assimilation pathways aid in clarifying plant nutrition mechanisms, indicating that there are certain key substrates which also trigger the regulatory signals, commonly participating in the photosynthetic organisms. In addition, nitrate and sulfate participate in the metabolic pathways required for plant adaptation mechanisms to abiotic stresses encountered commonly in agriculture, such as salt and metal toxicity, drought, temperature, and herbicides. There are many species of microalgae in the world but only a few of them have been studied in terms of biomass producers as source of foods, biofuels and/or high added value products for aquaculture, functional foods, pharmaceutical, and cosmetic industries. Actually we are involved in the use of extremophilic microalga, such as Coccomyxa onubensis, as a natural source of C40 carotenes, such as lutein, and the halophilic bacteria Haloferax mediterranei as source of C50 carotenoids, such as bacterioruberin.