NOVELTY - A photobioreactor for cultivation of photosynthetic microorganisms, comprises at least one module (3) containing a set of transparent tubular cells (tubes) (5) aligned in parallel through which a culture medium flows in an upflow direction, for cultivation of microorganisms; a bottom collector (4) that receives the medium and distributes it through cells; a top collector (6) for collecting medium; first pump (2) for feeding cells; discharge unit; second pump for recirculating medium; escape valves; and controller.    USE - For cultivation of photosynthetic microorganisms which can be used for the treatment of effluents (claimed), for production of biomass e.g. algae and cynobacteria useful as nutritional supplements, such as long-chain polyunsatureated fatty acids; for producing nutriceuticals, such as beta-carotene and astaxanthin; in food industry, in pharmaceuticals market; in environment engineering for treating wastewater; for reducing concentration of nutrients in wastewater, such as nitrogen and phosphorus; and for purifying effluent containing pollutants like metals and/or radionuclides.    ADVANTAGE - The photobioreactor promotes growth of the biomass at the same time as various substances are produced by the photosynthetic cells. The culture medium discharged into the last module is recirculated through as many cycles as necessary in order to increase the retention time of the culture in the cells, thus increasing the production of biomass. The photobioreactor is able to overcome the problems associated with open systems such as ponds and raceways, and at the same time is more economic than the artificial enclosed photo reactors. The reactor uses solar light energy, which makes the system much more economical. The photobioreactor is easy to assemble and since it consists of a set of modules, each with a set of cells, if a cell is damaged there is no need to change the whole structure but only the cell in the module involved. Therefore the photobiorector can be repaired without stopping. The type of structure proposed also makes it possible to introduce into or withdraw from the system nutrients or products that are lacking or are in excess. The photobioreactor can have automatic control, meaning that all the modules can be computer controlled, which can save time and labor. The photobioreactor is flexible, since it is possible to switch from the production of one product to another without many modifications.    DETAILED DESCRIPTION - A photobioreactor for cultivation of photosynthetic microorganisms which can be used for treatment of effluents, comprises at least one module (3) containing a set of tubular cells (5) aligned in parallel through which a culture medium flows in an upflow direction, for cultivation of microorganisms; a bottom collector (4) that receives the medium and distributes it through cells; a top collector (6) for collecting medium; first pump (2) for feeding cells with the culture medium and nutrients necessary for development of microorganisms in the culture and the production of biomass by photosynthesis; discharge unit for removing the biomass synthesis product after the last module; second pump for recirculating medium from last to the first module; escape valves for molecular oxygen or other gases created during the process; controller for automatically controlling the variables which affect the photobioreactor; and unit for separating a portion of the culture medium removed from the photobioreactor into a liquid phase and into a solid phase which contains the biomass, and for recirculating each of the phases if necessary. The culture medium is fed to the first module and pass through the cells of all the modules that constitute the photobioreactor in order to complete a cycle, during which the culture is exposed to sunlight and photosynthesis is promoted. The culture medium is pumped through the cells at a velocity of 8-25 cm/second, in order to prevent sedimentation of the microalgae, there adherence to the walls of the tubes and consequent shadowing. An INDEPENDENT CLAIM is included for production of photosynthetic microorganisms, involving cultivation of photosynthetic microorganisms in the photobioreactor to obtain a culture medium containing photosynthetic microorganisms; removal of the culture medium from the photobioreactor when the desired concentration of biomass is reached or, if necessary, for ensuring the optimum functioning of the system; recirculation of the remaining portion of the culture medium to the beginning of the process in order to guarantee maximum exposure to sunlight and thus increasing the efficiency of the photosynthesis; separation of the culture medium into a solid phase containing photosynthetic microorganisms and into a liquid phase.    DESCRIPTION OF DRAWING(S) - The figure shows a one unit of the photobioreactor (one module) comprising a set of cells (tubes) aligned vertically.    first pump (2)    module (3)    bottom collector (4)    tubular cells (5)    top collector. (6)