NOVELTY - Preparing lipid by microalgae, comprises: (a) growing photosynthetic microalgae; (b) culturing microalgae in stress conditions inducing an overproduction of fat by microalgae; and (c) extracting lipid produced by the microalgae followed by storing or recovering lipid.    USE - The lipid is useful as biofuel.    ADVANTAGE - The process is cost effective for preparing the lipid with high yield.    DETAILED DESCRIPTION - Preparing lipid by microalgae, comprises: (a) growing photosynthetic microalgae; (b) culturing microalgae in stress conditions inducing an overproduction of fat by microalgae; (c) extracting lipid produced by the microalgae followed by storing or recovering lipid, where the process comprises circulating an aqueous stream comprising, by mixing, microalgae, carbon dioxide and nutrients, in at least one photosynthetic growth region (3) provided with means allowing irradiation of microalgae of the stream to ensure the photosynthesis, between an inlet (25) of the growth region and outlet (85) of the growth region, and all or part of the stream in outlet of the growth region is reinjected in the photosynthetic growth region in upstream of the outlet, by which all or part of a stream traverses through at least a part of the region of the photosynthetic growth, in step (a) of photosynthetic growth of microalgae. An INDEPENDENT CLAIM is included for installation for the implementation of the process, particularly a pre-existing industrial site, comprising: means to provide an aqueous stream comprising, by mixing, micro-algae, carbon dioxide and nutrients; a region (3) of photosynthetic growth of microalgae of the aqueous stream, provided with means to convey the aqueous stream from inlet (25) of the region of photosynthetic growth region towards an outlet (85) of the photosynthetic growth region and means for irradiating microalgae stream to ensure photosynthesis; at the outlet of the region of photosynthetic growth, means of reinjection of all or part of the outflow from the region of photosynthetic growth towards the region of photosynthetic growth in upstream of the outlet, preferably at inlet of the growth region; a reactor (9) for algal lipid overproduction, comprising means for culturing microalgae obtained from the photosynthetic growth zone, in stress conditions inducing the production of lipid by microalgae; and means (10) for extracting lipid produced by microalgae in algal lipid synthesis zone, generally associated with means of storing and/or recovering the lipid.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of an installation for the implementation of the process.    Photosynthetic growth region (3)    Reactor for algal lipid overproduction (9)    Means for extracting lipid (10)    Inlet (25)    Outlet. (85)