# _Arthrospira platensis_ Cultivation in a Bench-Scale Helical Tubular Photobioreactor
However, they do not allow for reaching high biomass productivity, owing to the difficulty of keeping the optimum temperature; thus, they are restricted to the tropical and sub-tropical regions [28]. To overcome these problems, much widespread attention is now focused on closed systems, owing to their advantages over open cultivation systems (sterility, much greater control of cultivation conditions such as light intensity, carbon dioxide and nutrient levels, temperature, and higher biomass productivity). Particularly, a great deal of work has been done to develop suitable and efficient photobioreactors, such as flat-plate, tubular, vertical-column, and internally illuminated photobioreactors. Among them, tubular photobioreactors are often considered the most suitable ones for commercial large-scale mass cultures due to their large illumination surface area [29]. Most of these reactors are usually constructed with either glass or plastic transparent tubes, inside of which the culture is re-circulated either with mechanical pumps or preferably with airlifits. Regarding the shape, they can be horizontal/s serpentine, vertical, L-shaped, nearly horizontal, conical, inclined, multi-tubular, or helical [30; 31; 32; 33; 34; 35; 36]. Some studies have shown that the parameters that most influence the growth in a tubular reactor are the tube size, mixing, and culture circulation [37], which in turn depend on the reactor configuration. The global climate change is, today, one of the greatest environmental concerns. It is mainly due to the emission into the atmosphere of greenhouse gases, such as CO\({}_{2}\) produced by the combustion of fossil fuels [5]. An attractive alternative to reduce carbon dioxide emissions could be the use of photosynthetic microorganisms able to utilize this pollutant as a carbon source for their growth [5; 7]. From this point of view, the photoautotrophiccultivation of the cyanobacterium _A. platensis_ would allow to produce valuable biomass using the CO\({}_{2}\) from industrial processes and other human activities as a carbon source [38]. Light supply and distribution are undoubtedly the most important factors influencing the productivity of photoautotrophic cultures and represent the main source of energy for _A. platensis_. The first studies about the response of _Arthrospira_ sp. to light were carried out by [PERSON] [39], who discovered that the growth of _Arthrospira (Spirulina) maxima_ was inhibited at light intensity above 25-30 klux. Later, [PERSON] [40] demonstrated that inhibition of _A. platensis_ growth takes place at values of photosynthetic photon flux density (PPFD) exceeding 150-200 mmol m\({}^{-2}\) s\({}^{-1}\). Photoinhibition, which is a reduction of the photosynthetic activities caused by the exposition to high PPFD, was well studied and documented for algae as well [41; 42]. When the flux of absorbed photons is too high, the concentration of high-energy electrons in the cell is excessive, and they cannot be consumed in the Calvin cycle. These electrons, by reacting with water and forming hydrogen peroxide, damage the cell structure [43]. Even in densely populated _Arthrospira_ sp. outdoor cultivations, photoinhibition can be observed when light intensity is 60 to 70% of full sunlight [44]. Further studies carried out by changing light intensity in photoautotrophic cultivations suggested an optimum irradiance in the range of 30-50 W m\({}^{-2}\)[43]. Based on this background, fed-batch cultivations were performed in this work in a bench-scale vertical helical photobioreactor to check the performance of this reactor configuration under different conditions of light intensity and CO\({}_{2}\) feeding rate.