TECHNOLOGY FOCUS - BIOLOGY - Preferred Process: In the process, all or part of the stream in outlet of the growth region is re-injected at the inlet of the growth region. The process comprises carrying out multiple re-injection of the entire stream out of the photosynthetic growth region towards the growth region in upstream of the outlet of the growth region. The photosynthetic growth of step (a) is carried out in at least one tubular reactor localized within a buried well provided with means for irradiation of microalgae to ensure the photosynthesis. The temperature within the tubular reactor implemented in step (a) is maintained substantially constant around a chosen value. The tubular reactor of photosynthetic growth of microalgae in step (a) is arranged vertically or substantially vertically and the stream comprising microalgae flows from top to bottom of the reactor by gravity. The photosynthetic growth of microalgae is carried out without irradiation interruption. The step (a) of photosynthetic growth of microalgae is carried out by circulating the aqueous stream in a series of several consecutive tubular reactors, preferably located more and more deeply to ensure a gravity flow of the stream containing microalgae from the first to the last of reactors. The process comprises carrying out a recirculation of the stream exiting at least one of tubular photosynthetic growth reactor more in downstream towards at least one tubular reactor of photosynthetic growth more in upstream.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The stream comprising the microalgae traverses a first region of photosynthetic growth region equipped with means allowing irradiation of microalgae of the stream to ensure the photosynthesis, and it is reinjected in the region of the main photosynthetic growth by traversing a region of auxiliary photosynthetic growth provided with means for irradiation of microalgae of the stream to ensure the photosynthesis. EXAMPLE - None given.