Microalgae are potential plant biostimulants and biocontrol agents. A major hurdle towards their commercialization is the production of large volumes of biomass at the correct time of year. Secondary metabolites are unstable and the “shelf-life” of bioactive microalgal biomass needs to be investigated. The aim of the study was to investigate the effects of storage conditions on freeze-dried microalgae to determine how long the biomass retained its growth promoting and bioactive properties under various temperature and light conditions. Chlorella vulgaris biomass was stored in the dark at − 70 °C, 10 °C, and 25 °C and in the light at 25 °C. Samples were tested every 3–4 months for 15 months. Storage time significantly influenced the rate of change in the bioactivity in the C. vulgaris biomass with storage temperature also having some effect. Rooting activity decreased in the mungbean rooting assay over time up to 12 months and then increased slightly. Antimicrobial activity increased against Staphylococcus aureus and Escherichia coli for up to 12 months and then declined. Antioxidant activity measured in the DPPH assay remained relatively stable for up to 12 months and then significantly decreased with longer storage. The change in bioactivity over time was attributed to the gradual breakdown of the rigid cell wall of C. vulgaris, thereby improving extraction efficiency but exposing the secondary metabolites to oxygen, thus quickening their degradation. Biomass produced for commercial purposes requires preliminary validation as the results of the present study showed that bioactive compounds are susceptible to degradation over time.

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A plant biostimulant is any substance or microorganism which can be applied to plants to enhance seed germination and plant growth development along with its nutritional efficiency. Plant biostimulants collectively influence: plant growth development, pollen tube development, flower and fruit set, plant pigments, shoot and root development, nutritional efficiency, yield and shelf-life of crops, rhizospheric and soil microorganisms, general soil health and plant-environment interactions. Biostimulants are derived from natural origins and can help reduce the use of chemical products and also mitigate the negative impacts of harmful chemicals in the environment. This chapter focuses on their modes of application and effects on crops and horticultural plants. It also emphasizes impacts on the shelf-life and efficiencies of commercial biostimulants, as compared to synthetic chemical products and highlights the opportunities and challenges of their market expansion.