Commercial mushrooms are produced on lignocellulose such as straw, saw dust, and wood chips. As such, mushroom-forming fungi convert low-quality waste streams into high-quality food. Spent mushroom substrate (SMS) is usually considered a waste product. This review discusses the applications of SMS to promote the transition to a circular economy. SMS can be used as compost, as a substrate for other mushroom-forming fungi, as animal feed, to promote health of animals, and to produce packaging and construction materials, biofuels, and enzymes. This range of applications can make agricultural production more sustainable and efficient, especially if the CO2 emission and heat from mushroom cultivation can be used to promote plant growth in greenhouses.

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Despite being a comparatively new branch of agriculture, algae production is often considered to be a solution to many food security-related problems, such as land scarcity, climate change, inefficient and unsustainable fertilizer usage, as well as associated nutrient leakage and water pollution. Algae can be cultivated independent of arable land and, especially in the case of many microalgae, produce oil- and/or protein-rich biomass with spatial efficiency which far exceeds that of terrestrial plants. Nevertheless, algae and algae-derived products are almost exclusively produced for high-value, low-volume markets and are far from being able to compete with cheap commodities such as plant-based proteins or fossil fuel. High investment and production costs are considered the main reason for this, but a lack of economic incentives for sustainable production and CO2 mitigation should not be overlooked. The development of new production technologies; the monetization of ecosystem services, such as water treatment, CO2 sequestration, and nutrient recycling; as well as the simultaneous production and marketing of “high-value, low-volume” and “low-value, high-volume” products from the same algal biomass are the most promising ways forward. A sustainable “algae industry” could be an integral part of the future bioeconomy, enabling more resource-efficient food and fuel production and creating new products, companies, and jobs.

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