The bioprocess was taken on 400 mL of photobioreactors with a volume operation of 300 mL, and the strain was maintained under a 12:12 light:dark photoperiod with a light intensity of 73 µmol/m^-2s^-1 provided by cool white fluorescent tubes (Trademark MAGG) and a constant air flow of 1.5 vvm. No axenic samples from Fischerella sp. were taken.

There were 105 samples prepared, where 30 were combined with BG-11 medium (Rippka, Dervelles, Waterbury, Herdman & Stanier, 1979; Stanier, Kunisawa, Mandel & Cohen-Bazire, 1971) and distilled water, 60 with living biomass prepared with BG-11 medium, and 15 with inactivate biomass (DBM) or dead biomass prepared with BG-11 medium too. Table 1 specifies the making of each sample analyzed. DBM was obtained from a 250 Ml cyanobacteria culture that was sterilized under the conditions of 15 psi, 15 min, at 121 °C. The supernatant was retired, and dead biomass was washed with distilled water and resuspended in fresh medium BG-11.

Table 1

#	Fresh medium		Distilled water		Living biomass		Inactive biomass
	FM [%]	V[mL]	DW [%]	V[mL]	BM [%]	V[mL]	DBM [%]	V[ml]
3	0%	0	100 %	2.8	X	X	X	X
3	75%	0.7	75%	2.1	X	X	X	X
3	50%	1.4	50%	1.4	X	X	X	X
3	75%	2.1	25%	0.7	X	X	X	X
3	100%	2.8	0%	0	X	X	X	X
5	0%	0	X	X	100%	2.8	X	X
5	75%	0.7	X	X	75%	2.1	X	X
5	50%	1.4	X	X	50%	1.4	X	X
5	75%	2.1	X	X	25%	0.7	X	X
5	100%	2.8	X	X	0%	0	X	X
3	0%	0	X	X	X	X	100%	2.8
3	75%	0.7	X	X	X	X	75%	2.1
3	50%	1.4	X	X	X	X	50%	1.4
3	75%	2.1	X	X	X	X	25%	0.7
3	100%	2.8	X	X	X	X	0%	0

Source: Author's own elaboration.

The characterization of the system was achieved using graphite electrodes with a length of 3 cm and a diameter of 0.07 cm. The electrodes were placed on a plastic cover which has an area of 0.81 cm². The cover was used to seal the plastic cuvette containing the sample to evaluate. The cells volume was 2.8 mL and a signal generator (Stanford Research Systems, Mod. DS340) was used to supply a sine signal with an amplitude of 5 Vpp to the electric circuit varying the frequency of the sine signal in 10 Hz, 50 Hz, 100 Hz, 500 Hz, 1000 Hz, 5000 Hz, 10 000 Hz, 50 000 Hz, 100 000 Hz, 250 000 Hz, 500 000 Hz, 750 000 Hz and 1 000 000 Hz. In Figure 1, the used experimental setup is shown.

Figure 1 Source: Author's own elaboration.

All samples were run in triplicate, both at the electric system and at its dry weight determination. A comparative analysis of each sweep was performed with different concentrations of biomass; this was done in order to observe the response of the system in each frequency sent by the function generator. The data obtained were processed through the software Origin version 8.0.

Dry weight measurements were given through drying the filter with a humidity balance PMB (Adam Equipment, Mod.17250) at 140 °C for 5 min; next, it was weighed in an analytical balance (RADWAG Wagi Elektroniczne, Mod. AS 220.R2). The biomass was filtered with the equipment KG 47 GLASS SUPPORT (Advantec MFS, Inc., Mod. 311400) of 300 mL and a Kitasato flask (KIMAX, Mod. 27060) of 1000 mL, as well as with a vacuum pump (WACO, Mod. 73021-00). The filter with living biomass was dried using the humidity balance PMB at 140 °C for 5 min, then it was weighed in the analytical balance. Figure 2 shows the flow of the drying process of the biomass evaluated by electrical admittance.

Figure 2 Source: Author's own elaboration.

All samples were evaluated with different frequencies included in the range from 1 × 10^-1 Hz to 1 × 10^-6 Hz. All the sweeps reflect the resistive, capacitive and inductive regions. Figure 3 shows the electrical behavior of the solutions without Fischerella sp. biomass (only fresh medium and distilled water, with different concentrations), and where the electrical response was relatively very low. On the other hand, Figure 4 shows the response of the electrical system for living and inactive biomass solutions prepared with fresh medium at different concentrations. In the comparative analysis between non-active biomass and living biomass, Figure 4 shows a higher variability with non-activated biomass with respect to living or activated biomass; that is, the electrical impedance values were higher when the dead cyanobacteria Fischerella sp. was analyzed. However, the only focus of this research is the quantification of the living biomass. For this, in Figure 5 only the electrical response of the living biomass for the different frequencies evaluated is shown.

Figure 3 Source: Author's own elaboration.

Figure 4 Source: Author's own elaboration.

Figure 5 Source: Author's own elaboration.

Our results are supported by studies (Deurenberg, 1992) which postulate that when the frequency results to be lower or equal to 1 kHz (<10 kHz), the current does not pass through the cell membrane, and therefore the resistance obtained is only related to the extracellular mass. Otherwise, with 50 kHz-100 kHz the current crosses the membrane and allows determining the total cell mass (intra and extracellular components).

Therefore, for the calibration curves preparation, it was necessary to select a frequency under which the sweeps did not show high variability, whereby the frequency of 50 kHz was chosen. In Figure 5, the electrical behavior of the activated biomass prepared with fresh medium is shown. In comparison with the response of the system in the samples of distilled water and fresh medium, a higher electrical response was presented for the living biomass.

Considering the admittance values obtained from the response of the electric system at 50 kHz, we adjusted to a linear system to determine concentrations of biomass in fresh culture medium. The linear behavior reflects an R² = 0.95. In Figure 6, the constants for the slope and the intercept for the calibration system are verified.

Figure 6 Source: Author's own elaboration.