Carbohydrates in microalgae: Comparative determination by TLC, LC-MS without derivatization, and the photometric thymol-sulfuric acid method

Algal carbohydrates are investigated for many potential applications, for example renewable energies or human health products, revealing the need for different analytical methods suitable for the respective use. In this study, 46 microalgae species were screened for their carbohydrate content and composition. A TLC method with Na2HPO4-impregnated silica gel plates and an LC-MS method were developed and validated for qualitative and quantitative analysis of monosaccharides. Algal biomass was hydrolyzed with HCl, purified with SPE anion/cation exchange and analyzed on silica gel plates with different solvent systems. For comparison, the same samples were separated without derivatization on a HILIC column. For total carbohydrate content, a photometric thymol-sulfuric acid method was used and values between 16.5% (Mychonastes sp.) and 71.6% (Porphyridium purpureum) showed the high variability of the sugar content in the different algae. With the TLC method, fast and simple separation and qualitative estimation of the eight main monosaccharides in algal hydrolysates was possible. The results indicated that glucose was present in every screened alga, the presence of galactose, mannose, rhamnose, arabinose, xylose, ribose and fucose depended on the analyzed species. In comparison, the LC-MS method revealed the presence of the same monosaccharides. Glucose was the main monosaccharide with a species-depending content between 5.6% and 66.8%. Mannose, galactose, rhamnose and xylose were detected in most algae species in higher amounts (up to 15%, 7%, 3%, and 17%, respectively). Arabinose, ribose and fucose were only present in lower concentrations (<5%).In summary, a comparison of the different methods revealed that the TLC method is a simple screening method for detecting the main components of the algal carbohydrates. For quantification of the individual sugars the LC-MS method and for determination of the total carbohydrates the colorimetric thymol-sulfuric acid method are more suitable.