The Algae as a Sustainable Feed for Aquaculture
Based on the principles of circular economy, SIMTAP project has developed a prototype (“algae unit”) that exploits the hydroponic greenhouse crops byproducts to grow algae and feed the aquaculture more sustainably.
Even though aquaculture is considered the most sustainable animal farming sector, it still releases a certain amount of nitrogen, phosphorus, and organic substances. In addition, aquaculture still depends on fishmeal and fish oil production, which are among aquafeeds’ main ingredients. Since they are largely obtained by fish catch (“forage fish”), their production negatively affects the consistency of the natural fish wild stocks and, therefore, is no longer sustainable. Moreover, the production and transportation of other ingredients, such as soybean, and corn, have a massive impact on land use (frequently resulting in deforestation) and CO2 emission (due to the combustion of fossil fuels). Likewise, greenhouse crops also need large quantities of water and fertilizers, such as nitrogen and phosphorus, which are emitted into natural water bodies, thus resulting in environmental pollution. Based on that, aquafarmers, policymakers, and scientists are looking for a possible solution that might be found in the circular economy principle.
The aim of the SIMTAP project* is to develop an innovative integrated farming system for producing marine fish and plants (halophytes). The idea that underlies the SIMTAP concept is to mimic what naturally happens in marine ecosystems, where a very well-combined (naturally selected) pool of organisms live together in a perfect balance according to the ecosystem’s carrying capacity. This means the organisms share a common environment without overexploiting the environmental resources and/or accumulating waste and polluting substances.
Essentially powered by solar light (energy), the SIMTAP concept consists of a primary production of unicellular algae-fed mineral elements) in a specific section of the SIMTAP prototype (“algae unit”). In addition to freshwater, run-off from the greenhouse can be used for algae growth according to the circularity concept. Basically, SIMTAP is a multi-trophic aquaponic system in which several organisms can be cultivated in a mutualistic way. Moreover, SIMTAP is designed to be implemented with brackish to marine water.
The produced algae biomass represents the basic diet for the detritivore and filter-feeder organisms (DFFO) hosted in the second section of the SIMTAP prototype (“DFFO unit). The DFFO are polychaetes (e.g., Nereis diversicolor), mussels (Mytilus galloprovincialis), clams, sea cucumbers (Holothuroidea), shrimps (Crustacean), sea snails (marine gastropods), etc. These organisms must be carefully chosen to occupy the greatest possible number of ecological niches and maximize the biomass production of the DFFO unit. In turn, this biomass can be used for feeding the marine fish reared in the third section of the SIMTAP prototype, the “fish unit.” While producing fish for the market, in the fish unit, waste material, such as uneaten feed and feces (the solid waste in suspension), and ammonia nitrogen, are also produced and used by the other units. The suspended organic waste, still rich in nutrients, is feed for DFFO. Ammonium is the end-product of fish metabolism and is highly toxic at low concentrations. It can be converted into less harmful nitrate in the “bioreactor unit,” where the nitrification (i.e., the oxidation of ammonium to nitrate) takes place thanks to the aerobic nitrifying microorganisms; then, nitrate and other dissolved nutrients can be assimilated by plants. The SIMTAP concept is illustrated in Figure 1.
Fig. 1. The concept of SIMTAP (Self-sufficient integrated multi-trophic aquaponic system.
Currently, in the SIMTAP system set up at the University of Pisa, macroalgae such as Chaetomorpha and Ulva rigida are cultivated. Among the halophytic plants, Salicornia Europea is cultivated together with Beta vulgaris var. cicla. Gilthead sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax) are used as fish species considering their high market relevance and adaptability to different salinities. As DFFO species, Nereis diversicolor has been successfully introduced into the system. At the same time, a Chlorella-like microalga has been cultivated in the algae unit. Experiments on using low-salinity water are ongoing to widen the variety of fish and plant species that can be produced and boost the production efficiency of the SIMTAP prototype.
SIMTAP systems based on the same principles have been implemented in France, Turkey, and Malta. Also, Life Cycle Assessment (LCA) studies are also carried out for all these systems to evaluate their sustainability level.
In conclusion, the SIMTAP system is meant mainly to be a saltwater aquaponic system coupled with hydroponic greenhouse crops that provide nutrient-rich effluents for algae production.
The whole SIMTAP cycle results in fish and plant products for the market and the remediation of greenhouse wastewater. In addition to these advantages, reducing the use of fishmeal, fish oils, and, more generally, feed, whose raw materials are now produced and imported from overseas, should also be considered.
*The international project SIMTAP (Self-sufficient integrated multi-trophic aquaponic systems for improving food production sustainability; www.simtap.eu), funded by the PRIMA (Partnership for Research & Innovation in the Mediterranean Area) program and coordinated by the University of Pisa, involves the University of Bologna, the University of Milan, the INRAE and the Lycée de la Mer et du Littoral (France), the Mediterranean Fisheries Research Institute (MEDFRI, Turkey), the Ministry for the Environment, Sustainable Development & Climate Change (MESDC) of Malta and the German company Korolev.