Research progress in agricultural application of nitrogen-fixing cyanobacteria

As autotrophic photosynthetic micro-organisms with the ability to fix nitrogen and carbon into the soil, nitrogen-fixing cyanobacteria belong to an important microbial resource. The first report on their application in agriculture came from India in 1939. Although it subsequently had global attention, large-scale popularization and application of nitrogen-fixing cyanobacteria were not implemented in the end. The reasons were related to the wholesome use of chemical fertilizers in the world and the immature application technology of nitrogen-fixing cyanobacteria with high complexity, high cost and low reliability. For the former, although chemical fertilizers promoted crop production dramatically, its negative effects on agriculture and the eco-environment were already a growing concern in the public thinking. For the latter, with the development of new technologies in large-scale production and inoculation of nitrogen-fixing cyanobacteria, these defects were gradually eliminated. In the big background of current national policy advocating for "reduction" and "low carbon" development in agriculture, the application of nitrogen-fixing cyanobacteria (as green bio-fertilizer) has ushered in a new development opportunity for future application. In addition to bio-fertilizers in agriculture, studies in recent years have shown that nitrogen-fixing cyanobacteria were used in crop growth regulation, bio-pesticide development, agricultural pollution control and agro-ecological protection. This paper reviewed the progress in studies on agricultural application of nitrogen-fixing cyanobacteria in the past five years (2013-2017) at national and international scales. Moreover, the existing problems and future development directions were discussed. It was hoped that the review provided a summary reference for the development of this field in China. In recent years, there have been new ideas and considerable progresses in the application of nitrogen-fixing cyanobacteria in crop growth regulation, bio-fertilizer production and application, pest control, agricultural pollution treatment and agro-ecological protection. However, there were still problems in this field. Before large-scale application, these problems needed to be solved through further re-search. This included research on algal hormone metabolism and regulation mechanism, metabolic interaction of micro-organisms in biofilm, field evaluation of the degree of crop protection, protection mechanism, pathways and key enzymes of pesticides degradation, field-applied technologies, and effects and mechanism of drought and salt resistance.