## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reactor Type**} & **Production** & **Mean** & **Mean** & **Mean Volume** \\  & **Range** & **Production** & **Growth Rate** & **Reactor** \\  & **g L\({}^{-1}\) day\({}^{-1}\)** & & **Doublings day\({}^{-1}\)** & **L** \\ \hline Plate & 0.2–3.8 & 2.4 & 1.2 & 6500 \\ Tubular & 0.05–2.7 & 1.2 & 0.6 & 2800 \\ Column & 0.06–0.4 & 0.2 & 0.2 & 550 \\ Pond & 0.1–0.35 & 0.6 & 0.6 & 50,000 \\ Other & 0.05–0.5 & 0.08 & 0.08 & 100 \\ Finnfjord \({}^{*}\) & 0.18–0.42 & 0.22 & 0.47 & 300,000 \\ \hline \hline \end{tabular} \({}^{*}\) Values from 300 m\({}^{3}\) reactor during selected cultivation periods in 2019–2021.  
\end{table} 
Table 12: Production, growth rates and volumes of selected photoboreactor types. Numbers in table are rounded and growth rates are in most of the instances computed from reported increases in biomass in these publications [117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176 creates currents shears that may be harmful to the cells. In a typical cultivation session, we measured vertical mixing rates of 0.01 to 0.05 m s\({}^{-1}\) (Table 3). This is within the range of a normal well performing vertical airlift reactor [139]. The highest (mean) values were in the surface of the reactor (0.1 m depth). The horizontal values were high at 1.2 m depth, and this we believe was due to the rotating gas injection arm at the bottom. The overall CO\({}_{2}\) transfer rate from injector to fluid and algae was measured/calculated to an approximate mean of ca. 26%. This is not considered to be a high transfer rate, and this is a well-known problem [76]. Substantial improvements must take place here in the development of larger reactors. In our analysis of the data, we also observed that cell doubling rates did not decrease proportionally to culture volume when, e.g., the amount of water was doubled, and the same amount of total light was available to the cells. This points towards some energy utilization advantage by giving cells small pauses in the exposure to light. This is because illumination was above the reactor surface (LEDs and daylight) and submerged LEDs at one layer (0.7-1.4 m) below the surface. This may be connected to the fact that photosynthesis functions in pulses [140]. Furthermore, the efficiency may vary on longer timescales [141] and the explanation to this discrepancy may also be that cell specific utilization of the light is better at lower mean overall light intensities. The large part of the cited cultivations in Table 12 took place at temperatures between 10 and 20 \({}^{\circ}\)C. The classic [PERSON] paper [142] predicts (maximum) doubling rates of 1.5 to 2.6 doublings day\({}^{-1}\) at these temperatures, leading to the conclusion that growth in mass cultivation reactors is normally not temperature limited. The mean annual temperature in our 300 m\({}^{3}\) reactor was 6.64 \({}^{\circ}\)C (Table 5) and the total range was from below zero (\(-\)2.0 \({}^{\circ}\)C) to 16 \({}^{\circ}\)C. Our species has a wide temperature range, with maximum growth at 12-14 \({}^{\circ}\)C and in fact positive growth even at sub-zero seawater temperatures [46; 143]. However, growth was modified by stress imposed upon the cultures in various experiments (light limitation, e.g., decreased production early in 2021, see Figure 12). The main factors that regulate microalgae growth are irradiance, temperature, pH, CO\({}_{2}\), inorganic nutrients (N, P, Si compounds), and trace nutrients [142; 144; 145]. Other intrinsic or external compounds may also affect growth, e.g., phytohormones [146] and the combination of photoperiod and light intensity may have effects on growth and chemical composition [147; 148]. If a surplus of the necessary resources is present, the growth rate will be constant.