## Results and Discussion
### Metabolite Network Analysis in Diatom C. calcitrans  
In the present study, 11 amino acids, including glutamate, glutamine, proline, alanine, valine, leucine, isoleucine, lysine, methionine, glycine and choline were detected. These amino acids were manufactured from the cellular carbon (C) and nitrogen (N) metabolisms through glycolysis, gluconeogenesis, respiration and the citric acid cycle [35]. However, based on the VIP values, only isoleucine, leucine, glycine and proline were selected for relative quantification. In diatoms, isoleucine is derived from the reduction of threonine. This amino acid production starts with synthesis of aspartate, which contained oxaloacetic acid as the backbone. Isoleucine was significantly more abundant (\(P\leq 0.001\)) in all the solvent extracts, except for the methanol extract. Leucine was significantly (\(P\leq 0.001\)) higher in hexane, methanol and 70% ethanol extracts compared to the acetone and chloroform extracts. Its metabolism in diatoms occurs through the synthesis of pyruvate. Glutamate has also been identified as a precursor for proline biosynthesis in diatoms [35]. Diatoms utilize nitrogen-rich amino acids for growth and cell-defense. Proline was found to be significantly higher (\(P\leq 0.001\)) in acetone and chloroform extracts compared to other solvent extracts of \(C\). _calcitrans_. Many diatomic microalgae produce abundant carbohydrate and sugar alcohols. As shown in Figure 6, these metabolites stem from pathways such as glycolysis, gluconeogenesis and sugar-like metabolism. Acetone and chloroform extracts of \(C\). _calcitrans_ accumulated significantly higher (\(P\leq 0.001\)) amounts of the end products of carbohydrate biosynthesis pathways, such as sucrose and glucose. These sugars are found in medium polar solvents as they might be present in the form of glycosides, which are bonded to the other compounds. A number of sugar-alcohol metabolites, in particular the end products of sugar breakdown, were also detected in marine microalgae. Cyclitols, particularly myo-inositol was found to be significantly (\(P\leq 0.001\)) more abundant in acetone and chloroform extracts. This storage compound plays an important role in preserving the bio-membrane of diatom cells [36]. Figure 6: Metabolic map of different biosynthetic pathways (amino acids, carbohydrates, fatty acids, cholesterol, photosynthetic pigments) for various functions in the diatom _Chactoxersclaims_. Abbreviations: THF, tetrahydrofolate; SAM, \(S\)-adenosyl methionine; SAH, \(S\)-adenosyl homocysteine; RuBP, Ribulose 1,5-bis phosphate; DMAPP, dimethylallyl diphosphate; GGPP, geranyl digraph phosphate; GTP, geranyl diphosphate; ITP, isopentenyl diphosphate; MEF, 2,2-C-methyl-ly-erythitol 4-phosphate; MVA, mevalonate. Fatty acids are frequently studied among other low-molecular weight metabolites from diatoms. Several metabolites have been suggested to be derived from degradation of fatty acids in which acetyl-CoA acts as intermediate for their biosynthesis [1]. In the present study, significant amounts of essential PUFAs were identified and quantified (\(P\leq 0.001\)) to be highly concentrated in the hexane extract compared to the other solvent extracts. The C20, C18, C16 chains including arachidonic acid [C20:0] and \(\alpha\)-linolenic acid [C18:3] were found to be the prominent fatty acid constituent in the hexane extract, whereas stearic acid [C18:0] and palmitic acid [C16:0] were higher in the acetone and chloroform extracts of _C. calcitrans_. In this regard, the amount of fatty acids produced by the diatom cells may be varied due to differences in growth phase and culture conditions [37]. The abundance of these microalgal lipids were also reported in the earlier study by [PERSON] et al. [38] in the same species and attracted much interest among microalgal biotechnologists to use as a feedstock for safer biodiesel production. Cholesterol was also detected in the extract of _C. calcitrans_. Generated from squalene biosynthesis, this metabolite is also an important constituent of the biomembrane.