Introduction
There is a rising demand for alternative and sustainable food sources such as seaweed. In order to secure tomorrow’s food the European Union is concerned how we can obtain more biomass from the ocean without depriving future generations of their benefits. Seaweed (or macroalgae) are low-trophic marine organisms and meet the criteria when it comes to low environmental impact when cultivated and for their variety of health benefits for humans and animals. Due to the high level of fibres, proteins, minerals (Ca, K, Fe, …), vitamins (B1, B12, …) and antioxidants seaweed would be a good addition to our current diet. According to the Food and Agriculture Organization (FAO), protein required by Europe in 2054 will be around 56 million metric tonnes. Seaweed-derived protein may offer a viable solution. For seaweed to grow, it does not compete with conventional agricultural land and it does not require fresh water and pesticides. In comparison with at-sea cultivation, where seasonality has a huge impact on the quality and quantity of the production, on land cultivation can deliver year-round production of high-value products that are suitable for food applications.
Materials and Method
The red algae Palmaria palmata and Porphyra sp. were collected from the wild and kept in closed cultivation systems. A series of experiments on lab scale (1 L bottles) and small scale (40 L buckets) were conducted to obtain optimal cultivation conditions (light intensity, light quality, day length, water movement, stocking density and nutrient concentration) in order to maximize the specific growth rate (SGR) of the seaweeds.
Based on the findings from the lab and small-scale experiments a recirculating aquaculture system (RAS) was developed for the cultivation of P. palmata. The system consisted of three interconnected tanks with a volume of 1 m³ each, a first step towards commercial production. The seawater was recycled continuously with a turnover rate of 2400 % day-1. Water exchange was practically zero, except for a small amount of water that was added weekly to compensate for evaporation. UV was used to supress the outgrowth of micro-organisms. On a weekly basis the seaweeds were checked for quality, weighted and growth media was added to the system. In between two measuring days, the system operated autonomously and was followed up remotely by continuous measurements of salinity, temperature, dissolved oxygen and pH using submerged probes. Macro-nutrients in the water, such as nitrate, phosphate and ammonium are essential in the field of aquaculture, but unfortunately they remain hard to measure analytically or require expensive apparatus. In order to be able to follow up the nutrient uptake kinetics of the seaweeds and better understand their nutrient requirements an inexpensive autonomous in situ wet chemical phosphate detector was developed and tested.
Results
On a lab scale maximum growth rates of 8 % day-1 and 10 % day-1 for P. palmata and Porphyra sp. were reached, respectively. Under small-scale conditions different strains of P. palmata showed similar growth rates. However, the natural cycle of the seaweed (alteration between growth and forming new proliferations) introduced a lot of variation in the data ranging from 2 to 8 % day-1. In the RAS P. palmata gave average yields of 600 ± 17.1 g week-1 m-3 or 3 ± 0.1 % day-1.
Discussion and conclusion
The high growth rates of 8 % day-1 that were obtained in the lab and small-scale experiments with P. palmata could not be repeated in the RAS. By further optimizing tank dimensions, water circulation, nutrient requirements and light conditions we hope to reach higher yields in the future. Since we now better understand the natural cycle of the seaweed we can exploit this knowledge and develop a cultivation scheme that is adapted to this. Furthermore, we are working on developing new hatchery techniques in order to close the life cycle of both Palmaria palmata and Porphyra sp. to become self-sustaining and no longer dependent and wild populations. And finally, the idea is to further explore the possibilities for automatization of the RAS system, so labour and energy costs can be reduced significantly.
References
Corey, P., Kim, J. K., Duston, J., Garbary, D. J., & Prithiviraj, B. (2013). Bioremediation potential of Palmaria palmata and Chondrus crispus (Basin Head): effect of nitrate and ammonium ratio as nitrogen source on nutrient removal. Journal of applied phycology, 25(5), 1349-1358.
European Commission, Directorate-General for Research and Innovation, Group of Chief Scientific Advisors, Food from the oceans : how can more food and biomass be obtained from the oceans in a way that does not deprive future generations of their benefits?, Publications Office, 2017, https://data.europa.eu/doi/10.2777/66235.
Grote, B. (2019). Recent developments in aquaculture of Palmaria palmata (Linnaeus)(Weber & Mohr 1805): cultivation and uses. Reviews in Aquaculture, 11(1), 25-41.
Markets and markets. (2020). Seaweed Cultivation Market by Type (Red, Brown, Green), Method of Harvesting (Aquaculture, Wild Harvesting), Form (Liquid, Powder, Flakes, Sheets), Application (Food, Feed, Agriculture, Pharmaceuticals), and Region - Global Forecast to 2025.
Schmedes, P. S., & Nielsen, M. M. (2020). Productivity and growth rate in Palmaria palmata affected by salinity, irradiance, and nutrient availability—the use of nutrient pulses and interventional cultivation. Journal of Applied Phycology, 32(6), 4099-4111.