## 4 Discussion
In nature all these factors can be limiting, depending on the physical state of the site. In situations with (species specific) non-limiting growth factors, temperature will usually set the upper limit of the growth rate. It must here be taken note of that at high biomass concentration, density dependent self-limitation may occur not only with respect to light. Translated to mass cultivation, in reactors where biomass concentrations are much higher than even peak spring bloom values, it means that it is seldom that temperature limits growth but rather irradiance and nutrients. Even if the large part of pond, tubular and plate reactors are \"thin\" constructions, at high, e.g., _Chlorella_ sp. concentrations, production rates are often described as light limited [149; 150; 151]. Several of the cultivation sessions in the reactors lasted up to 5-6 months without fouling and microbial contamination (Figure 1). It is clear that open pond systems are prone to contamination and especially fouling, but also flat plate and tubular reactors need frequent maintenance and cleaning, often after cultivation periods only lasting a few weeks [111; 152; 153; 154; 111]. The lengths of our cultivation sessions were hence clearly at the higher end of the spectrum, and the main reason for this was possibly an anti-biofilm (anti-fouling) activity of some compound in our algae [104]. Our results showed that biomass (biovolume) specific nitrate and nitrite consumption was reduced with ca. 35% when factory fume was added (Figure 9), relative to cultivation with only atmospheric air bubbled into the system (comparable flows). This indicates indirectly that NOx fume is in fact utilized by the algae. This is strengthened by the fact that both protein and lipid amount and composition were highly comparable during cultivation with and without fume (Tables 7 and 8). This represents a quite substantial reduction in the need for fertilizer. In large scale commercial situations this means reduced running costs. Microalgae contain 1-2% N. The annual emission of NOx from Finnfjord AS amounts to 700-1100 tons/year (measured as NO\({}_{2}\)), and this is equivalent to an N uptake to produce 16,500-47,000 tons of algae biomass. It is not unknown that nitrous oxides can be taken up by microalgae and utilized as an inorganic N source. [PERSON] et al. [155] cultivated _Dunaliella tertioleta_ > 14 days with continuous uptake of NO in the cells, and they concluded that NO first was absorbed in the culture water by diffusion and thereafter diffused into the algae. In addition, NO\({}_{2}\) can be oxidized to nitrite and nitrate in seawater and thereafter taken up by microalgae [156]. When factory fume is added this represents a 125 x increase in the partial pressure of CO\({}_{2}\) in the fume relative to atmosphere concentrations. Information on how diatoms react to high CO\({}_{2}\) pressure is hard to come by. Instrumental in understanding this is the functioning of Rubisco, concerning biomass specific amount [157] and if specificity (ability to discriminate between CO\({}_{2}\) and O\({}_{2}\)) increases at high CO\({}_{2}\) pressures. Hypothetically, if specificity increases, this could in fact maintain photosynthetic efficiency with reduced need for N in biosynthesis since Rubisco is a N rich enzyme [83; 158]. The lipids in the diatom biomass have earlier been found to make up approximately 8-10% of the dry weight (20% of ash free dry weight) when harvested in the exponential growth phase [159; 160], which is similar to the lipid content found in the present experiment where fume and compressed air solely were injected into the culture. In their review of the literature of lipid content of diatoms, [PERSON] and [PERSON] [161] reported that species cultivated in nutrient poor media contained 6.6-39.8% lipids (DW); however, their experimental data showed an average of 5.3-21.5% lipids (DW) when cultivated in nutrient replete media.