## 4 Discussion
The addition of factory fue gas did not affect the lipid content of the applied diatom in the present study, but it has previously been found that the addition of CO\({}_{2}\) can increase the lipid content [162]. The reason for these discrepancies between our results and, e.g., [PERSON] and [PERSON] [161], we believe is simply because different species with different physiological traits were applied. What concerns \"internal\" differences may result from the fact that in [45] pure CO\({}_{2}\) was added while in the present study uncleaned factory fume with CO\({}_{2}\) was added. The fatty acid composition was highly comparable between the two treatments. The slightly higher omega-3 content, caused by the elevated 18:4n-3 content in samples with factory flue gas, was the main difference. The EPA content seems to be stable at approximately 30% no matter the cultivation conditions [163; 164; 165; 166; 159]. Other diatoms usually contain 5-20% EPA; however, species such as _Skeletonema costatum_ and _Phaocdactylum tricornutum_ have been reported to contain 30% of this fatty acid [165]. Proteins are important ingredients in all animal feeds as they are essential nutrients for growth. Both the amount of protein and the protein quality are important. Microalgae are generally rich in proteins, and the protein content of microalgae varies between 20 and 60% of dry matter [166]. The quality of protein resources is usually described by different metrics taking into account factors as digestion, absorption and assimilation capacity. [PERSON] et al. [167] describe such common metrics in detail including content and percentage of EAA, chemical score (CS) and assessment of limiting amino acids. Microalgal proteins are excellent sources of essential amino acids (EAA) and the content of EAA (tryptophan, methionine, phenylalanine, leucine, isoleucine, lysine, histidine, valine and threonine) are comparable to protein sources of aquaculture feed ingredients such as soybean and pelagic fishes. For many fish, arginine is also essential, owing to lack of a functional urea cycle. In general, in terms of protein quality, digestibility and absorption, the requirements for feed ingredients are met [168]. Amino acid analysis of our diatom revealed that the total amino acid content was higher (339 mg g\({}^{-1}\) DW) when cultivated with fume compared to without (300 mg g\({}^{-1}\) DW). Accordingly, protein content was also higher (29% vs. 25% with and without fume, respectively) (Table 8). The EAA contributed to about 36-37% of the total amino acids by both fermentation procedures. Similar protein contents and amino acid distribution patterns were also observed in samples collected throughout a cultivation season (2018-2019) when random samples for quality controls were analyzed (protein content 28.7% \(\pm\) 4.6%, n = 11) [169]. The ratio of each amino acid was stable throughout the year. The largest variation was observed in the total protein content (\(\pm\)). A stable amino acid profile is necessary for a reliable feed ingredient. The chemical score equals the ratio between each EAA in the food protein and the corresponding EAA in a reference protein proposed by FAO/WHO. Proteins of animal source normally have a chemical score of 1.0, while cereal proteins normally range from 0.4 to 0.6. Legumes, beans and nuts normally range in between these. Our diatom has a high protein quality with contents of all essential AA above the reference-scoring pattern (CS) for adults [170], which indicates that the protein quality is superior to most terrestrial plants. Tryptophan was denatured during the pre-treatment of the samples, acid hydrolysis, thus it was not possible to assess whether it is a limiting amino acid. [PERSON] et al. [167] reviewed mechanisms and indicators for assessing microalgal suitability as a feed ingredient based on toxicological analysis. Microalgae are capable of sequestering potentially harmful heavy metals from the environment. Thus, the heavy metal content of the biomass is dependent on the growth environment.