The space environment is extreme for plants growth and survival as gravity (gravitropism modification, water distribution), radiations (mutations enhancers), light spectrum regime and temperature are not optimal. Photosynthetic microorganisms are a foreseen solution for supporting plant development, growth, and stress tolerance in closed environments, like those designed for space colonisation. Indeed, photosynthetic microorganisms are known as secondary metabolites producers (exopolysaccharides, indole alkaloids, fertilisers) able to impact plant stimulation. Studying their abilities, application methodologies and best strains for space agriculture may lead to developing a sustainable and efficient approach for food production. Furthermore, as these microorganisms could also be used to produce oxygen and recycle waste materials increasing their interest in closed loop systems is undeniable. In this review we provide an overview of the current state of knowledge about existing biostimulants, their effects and applications, and the potential brought by photosynthetic microorganisms for life in closed environments.