## 2 Materials and Methods
### Biomass Proxies, Growth Rates, Light Utilization and CO\({}_{2}\) Uptake  
The water content of the harvested algae-paste varied with drum filter and centrifuge systems used, as well as centrifugation time. We therefore applied computed biovolume and in vivo Chla fluorescence (FL) [55] as biomass proxies during the test production runs. Dry weight (60 \({}^{\circ}\)C 36 h) was tested occasionally and varied between 11% and 38% with an approximated mean of ca. 22%. Occasionally in vitro Chla was also measured. Cell counting was performed according to the Utermohl [88] method with Leica and Zeiss inverted microscopes (400\(\times\) magnification) on cells fixed with [PERSON]'s iodine solution [89] using Nunc 4 well 1.9 mL cultivation chambers. Settling time was minimum 2 h. Cell sizes (diameter, height) were normally measured on ca. 10 cells once each week. Biovolume was thereafter computed from measurements of cell size and number according to the method in Menden-Deuer and Lessard [64]. In vitro Chla was measured according to the Holm-Hansen and Riemann [90] method with ethanol as extractant. From this cell number/biovolume/FL/Chla ratios could be established. Growth rates were registered as doublings day\({}^{-1}\) (\(\mu\)) during two-to-three-day periods:  
\[\mu\ =\ \frac{Log2(N_{t}\ )-Log2(N_{0})}{t} \tag{3}\]  
where \(\mu\) = doublings day\({}^{-1}\), \(N_{t}\) and \(N_{0}\) are cell numbers (or biomass) at time \(t\) and zero and \(t\) is time in days. The collected data were screened manually, and obvious outliers (excessively high biomass concentrations and doubling rates) were deleted. This amounted to 4-5% of the data. To estimate photosynthetic efficiency, it is necessary to include the energy content of the microalgae. [PERSON] and [PERSON] [91] reported caloric contents of 2.151 to 3.529 calories mg\({}^{-1}\) DW in microalgae (diatoms) in a field spring bloom situation. [PERSON] et al. [92]reported 19-27 MJ Kg\({}^{-1}\) dry biomass, i.e., 4.5-6.45 calories mg\({}^{-1}\) with the highest values in blue-green and green microalgae and the lowest in a diatom. We applied 2.1 calories mg\({}^{-1}\) (8.7 J) as a conservative estimate of the energy in our mass cultivated diatoms, this taking the measured protein and lipid content into consideration. Example computation with 1/3 of the large reactor full is a mean production value of 0.28 g L\({}^{-1}\) day\({}^{-1}\) DW (see Results chapter), which will therefore amount to ca. 280 \(\times\) 8.7 \(\times\) 100,000/86,400 = 2.85 kW (1 W = J s\({}^{-1}\)) \"produced\" in 24 h. During production peaks total irradiation in the reactor was 50-55,000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), i.e., equivalent to 14.39 kW using the conversion from [PERSON] and [PERSON] [93]. Light utilization was then 2.85/14.39% = 19.8% relative to total light energy delivered to the reactor. The final efficiency was calculated with different volumes in the reactor. Estimation of potential maximum amount produced of algae biomass from CO\({}_{2}\) in future was calculated from several cultivation sessions from measured gas flow in pipes, CO\({}_{2}\) content in the fume (Table 2) and an algae DW: CO\({}_{2}\) conversion factor of 2, i.e., time \(\times\) flow \(\times\) pipe area \(\times\) CO\({}_{2}\) fraction in fume. Example is CO\({}_{2}\) uptake during 24 h (86,400 s \(\times\) 105 cm s\({}^{-1}\)\(\times\) 44.16 cm\({}^{2}\)\(\times\) 0.05 = 20,030 L). 1 L of CO\({}_{2}\) at atmospheric pressure has a weight of 1.84 g, then the added CO\({}_{2}\) can maximum be converted to 36.5 kg biomass. Harvested biomass (DW) was 12 kg. From this the logged uptake efficiency was ca. 33%. Applying measured CO\({}_{2}\) pressure in the culture and 2 m above culture surface (but well below rim of reactor) in similar situations yielded 709 ppmv in culture and 494 ppmv above, i.e., d ppmv was 215 ppmv. This method, since the measured fluxes were stabilized over time (>24 h), yielded a CO\({}_{2}\) uptake efficiency of 30.03%.