Optimization of microalgal-fish symbiotic culture was tried to produce edible biomass at low cost. Chlorella sp., Spirogyra sp., and Ulothrix sp. obtained from natural habitats were used as microalgae. Loach (Misgurnus sp.) purchased from local fish stores were used as a representative of fish. Erlenmyer flasks of 3-liter and mess cylinders of 2-liter capacity were uses to grow single-celled Chlorella together with loaches. The filamentous Spirogyra and Ulothrix were grown in nylon cages of 50cm × 25 cm × 15 cm, pertially submerged to 10 cm depth in the water of glass-walled water baths of 60cm × 30cm × 50cm containing 70-liter of water. Loaches were kept outside of the cage except some for agitation in the cage. The cultures were illuminated with fluorescent lamps of 40-watt. The utilization of the loach respiratory carbon dioxide as carbon source for microalgal photosynthesis and the loach excreted ammonia as nitrogen source for the microalgal assimilation were confirmed. The utiliztion of the oxygen liberated from the microalgal photosynthesis for the loach respiration was also confirmed. The optimal loach density for Chlorella sp. and Ulothrix sp. was found to be 50g loaches per liter of water. It was difficult to obtain exact yield of microalgal biomass and fish biomass from the small-scale experiments, but we can safely conclude that the physical separation of the filamentous microalgae from the fish in the same body of water can be used for the furture symbiotic culture to increase biomass yield from a land as well as to save the microalgal separation cost.
