CULTURE METHOD FOR MICROALGAE THAT IMPROVES OIL CONTENT RATIO, METHOD FOR MANUFACTURING ALGAL BIOMASS, AND NOVEL MICROALGA

In a liquid surface-floating culture method in which culturing of microalgae is performed on the liquid surface, there is provided a culture method in which the oil content in microalgae is improved. In addition, there is a provided a method for decreasing the probability of recovery of bottom surface algae. Furthermore, an object of the present invention is to provide a culture method in which the proliferation rate of microalgae is improved. Culturing is performed such that a medium is suctioned from a region, in which there is a small quantity of microalgae between the liquid surface and the bottom surface, is discarded, and is replaced with a medium of which the concentration of a nitrogen compound or a phosphorus compound is lower than that of the above-described medium. In addition, a liquid is added thereto immediately before collecting microalgae on the liquid surface, and the water depth in a culture vessel is made to be deep. In addition, the microalgae are subjected to liquid surface-floating culture using a medium containing sugar. In the present invention, microalgae which can form a biofilm on the liquid surface and have at least one characteristic selected from the group consisting of the following (1) to (8) when being cultured in a medium within a culture vessel may be used. (1) The sum of the quantity of algal bodies of microalgae existing on the liquid surface and in a region from 1 cm below the liquid surface to the liquid surface, and the quantity of algal bodies of microalgae on the bottom surface of a culture vessel is greater than or equal to 10 times the quantity of algal bodies existing in the other region within the culture vessel; (2) the specific gravity of microalgae on the liquid surface is smaller than that of microalgae on the bottom surface of the culture vessel; (3) the specific gravity of microalgae on the liquid surface is greater than that of water; (4) the oil content of microalgae on the liquid surface is higher than that of microalgae on the bottom surface; (5) the size of microalgae on the liquid surface is larger than that of microalgae on the bottom surface; (6) a formed biofilm includes a film-like outer layer and an inner layer which has a plurality of bubble-like structures, and the outer layer is thicker than the inner layer; (7) a part of a formed biofilm has a pleat-like structure in a medium; and (8) in a case where microalgae obtained by collecting a biofilm which has been formed and subjecting the collec