Successive recycling of culture medium in the outdoor cultivation of Nannochloropsis oceanica in pilot-scale photobioreactors

Medium recycling has been investigated to improve resource efficiency and reduce production costs, and is essential for the environmental sustainability and economic viability of the algal industry. However, it is not straightforward in monoculture algal cultivation, as it requires monitoring dissolved organic matter and inorganic elements, as well as the appearance/accumulation of bacteria and other biological contaminants, all of which can affect algal growth. In this work, the marine microalga Nannochloropsis oceanica was cultivated outdoors in pilot-scale tubular photobioreactors (2.6 m³) in an artificial seawater medium that was successively recycled. Following biomass harvest and centrifugation, the supernatant was recycled without further pretreatment for 43 days across seven cultivation cycles. Medium recycling did not adversely affect the biomass biochemical profile or productivity. The biochemical alterations in biomass throughout the experiment were not due to medium recycling, but rather to metabolic acclimation to environmental conditions. Productivity was initially maintained and enhanced in the last cultivation cycles compared to the control reactor using freshly prepared medium. Medium recycling resulted in a 90% decrease in water and marine salt consumption, and a 77% decrease in magnesium supplement use. Recycling the medium once reduced costs by 50% compared with N. oceanica grown in fresh medium. After seven cycles of medium recycling, production costs decreased by 83%. Overall, directly recycling the medium after dewatering the harvested culture proved effective in the short term, significantly reduced cultivation costs, and can be readily integrated into existing large-scale algal farming facilities.