## 3 Results
### L. india Growth and pH Evolution under Magnetic Field  
The growth of _L. india_ PCC 8005 is shown in Figure 4, where the curves represent the daily biomass concentration. All assays started with 0.1 g L\({}^{-1}\) and the growth showed a small lag phase until the second day. After this, the growth phase was observed and it continued until the 16 th day, when the stationary phase began for all conditions. Despite the same behavior shown in the growth phases starting in the same period, under the MF treatment, there was a significant increase (\(p\leq 0.05\)) in the growth rate from the sixth day and it remained until the end with the MF treatment for 1 h per day and until the 15 th day with the MF treatment for 24 h per day. According to other studies that applied an MF in _Spirulina_ assays [4; 26; 27], the action during a short period (1 h per day) was enough and it worked better if the main objective was biomass production. The X\({}_{\text{max}}\) obtained on the 13 th day with an MF applied for 1 h per day (2.12 \(\pm\) 0.16 g L\({}^{-1}\)) was 39.5% higher than in the control assay (1.52 \(\pm\) 0.01 g L\({}^{-1}\)), while with an MF for 24 h per day (2.02 \(\pm\) 0.28 g L\({}^{-1}\)), the increase was 32.9%. As the growth was observed until the end of the cultivation for the assay with an MF applied for 1 h per day, this biomass concentration even reached 2.57 g L\({}^{-1}\). [PERSON] et al. [4] also obtained an enhancement in the biomass concentration when the cyanobacteria _Spirulina_ was cultured under 60 mT, achieving 37% more biomass than the control group. An MF of 11 mT was demonstrated to be more efficient than high intensities (60 mT, as studied by [26]) since the highest biomass concentration of _L. indica_ was 2.57 g L\({}^{-1}\) under 11 mT. In the study of [PERSON] et al. [33], the high intensities tested (30 and 40 mT) did not stimulate the growth of _Nannochloropsis aculata_, while the lowest intensity, 20 mT, was enough to increase the growth rate. As demonstrated in some papers, there is no linearity regarding the intensity used and the effect obtained, which led us to conclude that these modifications depend on the strain, MF intensity, frequency and functional time. According to [34; 35], there is potential damage in the algal cell at high MF intensities applied for a long period due to the increase in the free radicals. [PERSON] et al. [36] stated that the increase in microalgae growth rates may be associated  
Figure 4: Growth curves of _L. india_ PCC8005 cultivated under an MF with different application times and the control culture. with the higher nutrient consumption since the permeability of the membrane can increase under some MF treatments with an electromagnetic field. [PERSON] et al. [37] reported that high MF intensities can change the enzyme conformation, which impacts the intracellular biochemical reactions. By measuring the O\({}_{2}\) production rate, [PERSON] et al. [38] showed that an MF can have a positive effect by enhancing the photosynthesis rate and carbon fixation, with an optimal field intensity of around 100 gauss (equivalent to 10 mT). Furthermore, it is difficult to make an effective comparison since, in those studies, different MF application modes were tested, such as magnets, which do not present a homogeneous field, as in the present study. At this step, it is thus difficult to provide a hypothesis for the mechanism by which MF could influence biomass growth rates and concentrations as it may imply several targets. Further research remains necessary to go further into the elucidation of this mechanism. Although the assay with MF applied for 24 h per day produced less biomass than the one for 1 h per day, the mean biomass productivity obtained for both conditions were similar (0.14 g L\({}^{-1}\) day\({}^{-1}\)) but significantly greater (\(p\leq 0.05\)) than that obtained in the control group (0.11 g L\({}^{-1}\) day\({}^{-1}\)).