Biochemical composition, β-glucan and phenolic content of a marine diatom Chaetoceros muelleri cultivated in Guillard's modified medium

Abstract

The biochemical compositions of diatoms are important for agricultural and pharmaceutical applications, and they vary according to nutrients and physical factors. To study the influence of carbonate and nitrogen source supplementation in the medium on the chemical composition of the marine diatom Chaetoceros muelleri, Guillard's medium was modified for this study. Three types of Guillard medium were prepared including standard Guillard medium (T1), modified Guillard medium (0.005 g/L bicarbonate supplement, T2), and 50% nitrogen reducing medium (T3). Results showed that the maximum biomass was observed at T3 (5.75 × 10⁶ cell/mL), which was significantly different (p < 0.05) from that at T1 (3.85 ± 0.13 × 10⁶ cells/mL) but did not differ from that at T2 (4.03 ± 0.08 × 10⁶ cells/mL). T3 contained the highest level of carbohydrates (13.71 ± 1.56 ), with a statistically significant difference (p < 0.05) from T1 (5.81 ± 0.50) and T2 (6.90 ± 1.14). The total lipid content in T3 (4.34 ± 0.03) was significantly greater (p < 0.05) than that in T1 (1.1 ± 0.08) and T2 (2.21 ± 0.62). The protein content in T1 (94.84 ± 0.08 mg/g) was significantly higher (p < 0.05) compared to that in T2 (31.39 ± 0.72 mg/g) and T3 (30.91 ± 0.38 mg/g). The highest β-glucan level was measured in T3 (0.4 1 ± 0.01 g/L), and statistical analysis revealed a significant difference (p < 0.05) from that in T1 (0.41 ± 0.01 gL^-1) and T2 (0.41 ± 0.01 gL^-1). The phenolic content in T1 was 14.91 ± 0.97, while those in T2 and T3 were 4.44 ± 0.11 and 4.16 ± 0.17 μg/mg gallic acid equivalents (GAE), respectively. Antioxidation examination revealed the highest value at 94.59 ± 0.04 mg mL^-1 extract in T3, followed by T1 (89.21 ± 1.71 mg mL^-1 extract) and T2 (10.66 ± 0.38). Phenolic content showed the values for T1, T2 and T3 were 49.44 ± 4.49, 47.61 ± 5.45, and 53.23 ± 6.61 mg ascorbic acid, respectively. Statistical analysis revealed that the phenolic content in T1 significantly higher differed (p < 0.05) from that in T2 and T3. In contrast, the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) scavenging ability significantly differed (p < 0.05) among T3, T1, and T2 according to the 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging ability and reducing power. The presence of β-glucan in the diatom extracts was confirmed by the FTIR spectrum data at wavenumbers 1,065 and 1,038 cm^-1, whereas the LCMS spectrum confirmed the presence of gluconic acid at m/z 198, 196, and 194. Our results demonstrate that the modified Guillard T3 medium is optimal for cultivating C. muelleri to enhance the production of carbohydrates, lipids, proteins, and β-glucan. These findings are critical for advancing the large-scale production of diatom-derived biochemical components, particularly for pharmaceutical applications.

Keywords: Antioxidation; Biochemical compositions; Biomass; Chaetoceros muelleri; Guillard medium; Phenolic content; β-glucan.

Figure 1. FTIR spectrum of standard β-glucan derived from Saccharomyces cerevisiae—(CAS 9012-72-0—Calbiochem).

Figure 2. FTIR spectrum of crude diatom (T3).

Figure 3. LC-MS/MS spectrum of crude extract from cultured diatom.