Cultivation of microalgae on ocean provides a promising way to produce massive biomass without utilizing limited land space, and using seawater as culture medium can avoid consumption of valuable fresh water. Bicarbonate is proved as a better approach for carbon supply in microalgae cultivation, but Ca2+ and Mg2+ in seawater is subjected to precipitate with carbonate derived from it. In this study, cultivation with this medium for a marine Chlorella sp. resulted in productivity of 0.470 g L-1 day-1, despite of continual precipitation caused by increased pH due to bicarbonate consumption. Actually, this precipitation is favorable, since it can work as a flocculation harvesting method for microalgae. The highest flocculation efficiency of 98.9 ± 0.0% was observed in cultures with 7.0 g L-1 NaHCO3, which was higher than that of cultures without bicarbonate (44.1 ± 0.2%). Additionally, the spent medium after flocculation supported better growth (1.60 ± 0.0 g L-1) than the fresh medium (1.26 ± 0.0 g L-1). Outdoor cultivation with floating photobioreactor on ocean resulted in the productivity of 0.190 g L-1 day-1, which was higher than that in land-based culture systems. The floating system also benefited from better temperature control with range from 20.6 to 37.2 °C, due to solar heating and surrounding water cooling. These results showed feasibility of efficient microalgae biomass production with fully utilizing of ocean resources, including culture medium preparation and temperature control with seawater, as well as wave energy for mixing, holding great potential to produce massive biomass to support sustainable development of human society.
Keywords: Bicarbonate supplement; Flocculation; Microalgae; Ocean farm.
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