**Figure 2.**

*Diagram of floating food production system on the example of Jellyfish Barge.*

farms to work as effectively as possible. At the moment, it's an extremely ambitious concept. Yet, it raises a significant point: we could feed ourselves with low ongoing costs if we simply used endless and predictable resources such as the sun and the ocean.

Due to the high expense of desalination systems to produce irrigation water, and to the low salt tolerance of crops, alternative technologies have gradually emerged. Japanese start-up N-ARK has combined salt-tolerant technology with floating architecture to tackle the issues of sea-level rise and salt damage. In partnership with CULTIVERA agri-tech company, they aim to build a prototype of a floating marine farm "green ocean," conceived to float on the coast along urban areas. The facility makes use of a seawater agriculture technique based on moisculture, a humidity-controlled cultivation technology that reproduces the natural soil surface layer of about 15 cm using special fibers of 5 mm in diameter. Saline agriculture fertilizer is produced, thanks to a special circular process that absorbs water and nutrients in the air and mixes and neutralizes alkaline seawater and acidic rainwater. Moisculture requires only one tenth of the amount of water used in conventional irrigation farming.

The current challenge toward more resource-efficient cities is to shift cities metabolism from linear to circular, so that discarded material can become a resource for another process. Nutrients and carbon dioxide are two of the most common waste products generated by cities, and both are rarely reused or recycled before being discharged into the environment. A possible way to recycle nutrients and carbon dioxide is to use them as input for algae cultivation. Because of their ability to fix carbon via photosynthesis at up to 50 times the rate of terrestrial animals, algae are among the greatest organisms for CO2 sequestration.

Cities are suitable locations for local recycling of waste due to their high concentration of nutrients and carbon dioxide. Unfortunately, dense urban areas often lack the space to implement large-scale algae cultivation. One alternative is to cultivate algae on the water, resulting in floating systems for biofuel and food production [34].

Another widespread practice is the integration of aquaculture within wider farming systems, contributing to the development of synergies between farming operators. Such systems are known as integrated agri-aquaculture systems (IAAS) and can help to improve water-nutrient balance through chemical or natural fertilization [35]. Agri-aquaculture systems generally comprise three major subsystems: aquaculture, agriculture, and household. Common positive interactions of agri-aquaculture systems include the use of animal manure as pond fertilizer, the use of crop by-products as supplementary feed for fish, the use of pond sediments as terrestrial crop fertilizers, and the use of aquaculture wastewater for crop irrigation.

Overall, producing, processing, and packaging food inside the city can significantly shorten the supply chain, add a certain social value and level of food security, contribute to new forms of urban circularity, and promote an efficient use of scarce space due to the lack of it. As a result, combining production facilities with the urban environment is expected to boost economic feasibility while also providing climateproof expansion for a growing urban population.
