## 4 Discussion  
In many ways the present project was based on the conviction that a large part of the future's fish feed, nutritious biomass and oil in general will and should be harvested or produced from the lowest trophic level in the marine food web, i.e., from fast growing photoautotrophic organisms. Based on the experience from numerous other microalgae projects, it was clear that this was indeed a high-risk journey to begin. The upside is that there is still room for optimization of methods to use and organisms to apply, similar to the optimization that the cultivation of terrestrial crops continuously experiences. Both growth rates and biomass concentrations in our cultivation sessions increased steadily during the five-year period (2015-2021) (Figures 10 and 11). The main, but not only mechanism behind these increases was the gradual implementation of an improved illumination concept. This was based on a combination of atmospheric radiation and artificial light on and below the surface of the bioreactor. We used daylight white LED lamps with variable intensity, and total illumination on/in the reactor was gradually increased with time. A maximum was reached early 2021 with ca. 50,000 umol quanta s\({}^{-1}\). In our reactors we observed a mean conversion rate of light utilization efficiency of ca. 20% (Table 6). This number is not exact since the precise chemical composition of the diatoms are not accounted for in terms of energy. In our opinion, the applied computation method is acceptable, and our logged efficiencies must be considered high relative to comparable initiatives where maximum varied between 5% and 15% [110; 111; 112; 113]. It must be mentioned though that we use the term \"light utilization efficiency\" since the way photon efficiency is measured influences the result. The way the reactor \"captures\" and \"re-uses and keeps\" the light will be important (all light one can \"see\" is wasted energy). It has also been shown in raceway reactors that cells can adapt to short (s\({}^{-1}\)) on-off light cycles [114]. In addition, our large cells concept with low self-shading, allowing for long light depths, also played a major role here (Table 4). The annual mean atmospheric irradiance incident upon the reactor was ca. 20 W m\({}^{-2}\) (Figure 6). The approximate mean annual radiation in the Finnsnes area is much higher, i.e., 80 W m\({}^{-2}\)[115]. This discrepancy was due to the fact that our reactors for practical reasons were situated in a shadow zone close to the factory's energy retrieval system buildings. Further upscaling of reactor size and production is therefore bound to take place in open areas without shadowing from buildings and terrain. Tight monitoring of inorganic nutrient content and addition was also important for the growth optimization. Sufficient levels and consumption of N, P and Si are mandatory if nutrient limited growth shall be avoided, and we tried as much as possible to keep concentrations > 2 \(\times\) of the respective nutrient's half saturation constants. At the beginning, nutrients were measured ca. weekly, and the results from the autanalyzer were only available weeks after sampling. This is unsustainable if fast responses to unfavorable culture environments shall be implemented. From 2018, nutrients were therefore measured every second day and results were available the same day. This allowed for frequent adjustments to maintain appropriate nutrient levels in the cultures, as well as to obtain reproducible chemical content (fatty acids and amino acids) of the cells. It can be observed that production decreased after early spring 2021 (Figures 12 and 13). This was mainly due to application of fewer light units (Figure 7). Initial simple linear correlation analysis between cultivation parameters vs. biomass and production revealed few significant correlations between many (16) parameters. We therefore performed (data reduction) PCA analysis on the data sets (Table 5). Here Factor 1 and 2 explained ca. 35% of the data (Figure 14).