Algal biofertilizers as plant growth stimulants for sustainable agriculture

Population of the Earth has been projected to reach ~9.7 billion by 2050 and Asia and Africa would be the major contributors to this enhanced population. This increased population is directly related with higher demand of food security in very near future. Modern agricultural management is heavily dependent on the fertilizers for increased crop production, but increasing use of chemical-based and inorganic fertilizers may be a serious threat to the environment and human health. Microalgae have biostimulant and biofertilizer properties which are attracting the interest of farmers and agrochemical industries. Application of microbes such as cyanobacteria, microalgae, endo/ecto-mycorrhizal fungi, rhizobacteria, and others serves as an ecofriendly approach for sustainable agriculture practices. Biostimulants are the products derived from organic matter that are applied in small quantities and are able to stimulate the development and growth of several crops under stressed as well as optimal environmental conditions. Plant biostimulants encompass diverse organic and inorganic substances (humic acids and protein hydrolysates) as well as prokaryotes (plant growth promoting bacteria) and eukaryotes such as mycorrhiza and macroalgae (seaweed). Biofertilizers are products containing living microorganisms or natural substances that are able to improve chemical and biological soil properties, stimulating plant growth, and restoring soil fertility. Microalgae, which comprise algae and cyanobacteria (blue-green algae), are attracting interest from scientists, extension specialists, private industry and plant growers because of their versatile nature, simple unicellular structure, high photosynthetic efficiency, ability for heterotrophic growth, adaptability to domestic and industrial wastewater, amenability to metabolic engineering, and possibility to yield valuable co-products. This review article focuses on the research achievements on microalgae and cyanobacteria based plant biofertilizers and biostimulants in the agricultural applications. The challenges to commercializing these kinds of biofertilizers are also discussed.