## 5 Conclusions  
Based on the results, the following conclusions can be drawn: The hybrid reactor has the undeniable advantage of having an online monitoring system. At the same time, the tested basic device needs to be supplemented in order to be suitable for laboratory optimization experiments and to be economically feasible during scaling up. The necessary additions are well exemplified by the fact that we had to solve the mixing at the very beginning of the experiments due to the unexpected large-scale sedimentation and adhesion of the cells. As a temporary solution, we placed a table-top magnetic stirrer under the fermenter tank, but in our opinion, incorporating a stirrer motor should be considered, as in the case of a conventional fermenter. In addition, even on a laboratory scale, optimization experiments could benefit from more finely adjustable mixing and circulation of the culture than it is available with peristaltic pump performance to improve control and avoid unexpected adhesion phenomena. In accordance with the literature, continuous operation of a helical tubular hybrid reactor would be much more beneficial, since the batch mode allows for significantly less efficient biomass production for the tested algal species than conventional flask cultivation (as an air bubbled column or tank reactor) in the studied volume. Automatic setting of light intensity in parallel with culture density was not found to be useful because this design does not allow the optimization of light intensity either in the case of young cultures or in the case of different algae species requiring lower or higher light intensity than the intensity provided by the default setting. This study highlights that reactor geometry and complexity plays a crucial role in the obtained results. Some default designs which have certainly been developed considering scalability requirements, are not optimal (or sometimes not suitable) for optimization experiments in certain laboratory volumes. On the other hand, the results also highlight that conventional laboratory methods (such as Erlenmeyer flasks considered as air bubbled column or tank reactors) are very useful for the physiological optimization of algae cultivation for a certain purpose, but may be not appropriate from the point of view of scalability. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12042196/sl](https://www.mdpi.com/article/10.3390/app12042196/sl)]([https://www.mdpi.com/article/10.3390/app12042196/sl](https://www.mdpi.com/article/10.3390/app12042196/sl)), Figure S1: _Mononphidium pusillum_ cells (a) in inoculum, without staining; (b) in inoculum, with Nile red fluorescent staining; further photographs were taken on the 3 rd and the 7 th day with Nile red fluorescent staining (c,d) in control cultures; (e,f) in 75% nitrate containing cultures; (g,h) in 50% nitrate containing cultures and (j,k) in 25% nitrate containing cultures. Golden yellow spots represent neutral lipid drops; Figure S2: Preliminary analysis of the lipid content of _Mononphidium pusillum_ control (C) and nitrate-depleted (75-25% N) cultures from the hybrid reactor by thin layer chromatography. (a) the TLC plate after the run; (b) the TLC plate after development; (c) the negative image of the developed TLC plate for pattern analysis; (d) the peaks found by using CpAtlas 2.0 software. 1-8: the separated bands (peaks); TOG: triroleil glycerol and TPG: triroleil glycerol standards; Figure S3: Preliminary analysis of the lipid content of _Monorapidium pusillum_ control (C) and ni-rate-depleted (75-25% N) cultures from the tank reactor by thin layer chromatography. (a) the TLC plate after the run; (b) the TLC plate after development; (c) the negative image of the developed TLC plate for pattern analysis; (d) the peaks found by using CpAtlas 2.0 software. 1-8: the separated bands (peaks); TOG: triroleil glycerol and TPG: tripalmitoyl glycerol standards. Conceptualization, I.B. and F.T. ; methodology, I.B. and S.G.; formal analysis, F.T., K.M., I.B.