Cellulose from the green macroalgae Ulva lactuca: isolation, characterization, optotracing, and production of cellulose nanofibrils

Abstract

We report (1) successful extraction and characterization of cellulose from northern hemisphere green macroalgae Ulva lactuca (Ulva fenestrata) collected along the Swedish west coast and cultivated indoors under controlled conditions, followed by (2) its utilization in the production of lignin-free cellulose nanofibrils (CNF). Cellulose was extracted by sequential treatment with ethanol, hydrogen peroxide, sodium hydroxide, and hydrochloric acid, yielding a celluloserich insoluble fraction. The extracted cellulose was disintegrated into CNF using a mechanical homogenization process without any further enzymatic pretreatments. In addition, regenerated cellulose was prepared. XRD characterization of the CNF showed characteristic peaks for the cellulose I allomorph and confirmed that the nanofibrils were semicrystalline with a crystallinity index of 48%. Regenerated cellulose was mostly amorphous with an XRD pattern indicating the presence of the cellulose II allomorph. The cellulose fractions were essentially free from inorganic substances and thermally stable up to around 260°C. Structural mapping with CP-MAS 13 C-NMR sustains the cellulose content of CNF and regenerated cellulose, respectively, yet ion chromatography identified the presence of 10-15% xylose in the fractions. Optotracing was used as a novel and non-disruptive tool to selectively assess the polysaccharide composition of the cellulose fractions and produced CNF aiming to shed light on this hitherto non-resolved origin of xylose in Ulva cell wall matter. Fluorescence excitation and emission spectra of a panel of 4 oligothiophenes identified and verified the presence of cellulose and sustain the conclusion that the isolated fractions consist of cellulose intertwined with a small amount of a xylose-containing glucan copolymer.

Ulvan, a Polysaccharide from Macroalga Ulva sp.: A Review of Chemistry, Biological Activities and Potential for Food and Biomedical Applications

“…Before advanced ulvan characterization, thermogravimetric analysis (TGA) is used to determine the percent of product loss due to extraction [57]. TGA can also reveal the effect of the extraction temperature on the quality of ulvan obtained.…”

“…Specific structural analyses employ chromatographic methods, such as high-performance liquid chromatography, coupled with conductivity or Fourier transform infrared (FTIR) spectroscopy [58,59]. FTIR spectra of dry cellulose fractions show peaks in the range of 400-600 cm −1 at room temperature [53,57]. There are absorption bands at 3412, 1645, 1251, 1080, and 852 cm −1 caused by hydroxyl groups, uronic acid, SO stretching (sulfate) vibrations, CO hand COC glycosidic linkages, and COS sulfate flexural vibrations in the axial and/or equatorial position (in C-2 or C-3 rhamnose), respectively [58,60].…”

“…HPAEC determines the carbohydrate composition and purity of glucose. Most of the carbohydrate content of ulvan is dominated by glucose and xylose [57]. HPAEC can selectively measure those monosaccharides and determine the quantity of acid and sugars present in ulvan without further derivatization [63].…”

“…GC has been used to measure volatile compounds contained in ulvan, such as alditol [41]. Bond structures, link positions, and substitution patterns may be analyzed with a two-dimensional nuclear magnetic resonance (2D NMR) instrument [41,57,64]. Signals at 65 and 63 ppm correspond to the carbon C-6 in crystals and amorphous regions in cellulose.…”

“…Signals at 65 and 63 ppm correspond to the carbon C-6 in crystals and amorphous regions in cellulose. Strong signals at 76 and 73 ppm correspond to the carbons C-2, C-3, and C-5 [57]. In addition to the uronic acid component, 2D NMR can analyze hydrolyzed saccharide fragments.…”