Fucose-containing sulfated polysaccharides from brown seaweed: Extraction technolgy and bioactivity assessment

Abstract

Marine seaweed that is washed up on the coastline is a nuisance as its degradation produces a foul a smell and generates waste problems. Exploitation of coastline-polluting seaweeds such as Sargassum sp., Ulva sp., and other beach-cast seaweed species for various commercial applications will generate new valuable products that may help lessen coastal pollution by seaweeds and create new seaweed-based resources. Thus, utilization of these natural resources is of great importance. The objectives of this PhD study were to develop a technology to extract bioactive compounds from nuisance brown seaweeds, and investigate their bioactivity. To this effect, designed optimized extraction of fucose-containing sulfated polysaccharides (FCSPs) and/or crude fucoidan from brown seaweed were performed, and the bioactivity of the isolated FCSPs was investigated. Moreover, to assess the potential of seaweed to assimilate nitrogen-based nutrients, a technology for accurate monitoring of differential seaweed growth responses to nutrient assimilation was also developed. Fucoidan is a term used to describe a class of sulfated polysaccharides extracted from brown seaweed, which contains substantial amounts of fucose; varying amounts of galactose, xylose, and glucuronic acid; and differing glycosidic linkages, and are variously substituted with sulfate and acetyl groups and side branches containing fucose or other glycosyl units. These FCSPs principally consist of a backbone of (1→3)- and/or (1→4)-linked α-L-fucopyranose residues that may be substituted with sulfate (SO3-) on C-2, C-3, or C-4 and acetyl groups at C-4 on the main chain or may have short fucoside side chains that are usually linked from the O-4 of one or several of the fucopyranose backbone residues. FCSPs are known to exhibit crucial biological activities including anti-tumor activity. Although differently extracted, purified, fucose-rich, modified fucoidans have been reported to exert bioactive properties such as anti-coagulant and enhance immune response activity, few studies have investigated the bioactivity of unfractionated FCSPs, notably FCSPs extracted using milder and fewer processing steps. Crude fucoidan from Sargassum sp. and Fucus vesiculosus were examined for their bioactivity against lung and skin cancer cell lines in both in vitro and in vivo studies. This study showed that unfractionated FCSPs hinder the in vitro proliferation of Lewis lung carcinoma and melanoma B16 cell lines by induction of apoptosis. Moreover, the anti-tumor activity of crude fucoidan seems to be associated with an enhanced immune response as depicted by an increase in natural killer cell activity in mice. The classical extraction of FCSPs involving long, repetitive, multi-step acid and alkaline treatments is detrimental to its structural properties, yield, and compositional attributes. In this study, statistically designed, optimized extraction of a single-step extraction of FCSPs from Sargassum sp. was carried out. The effects of the different extraction parameters on the natural chemical composition of the isolated sulfated polysaccharides were also investigated. The data showed that classical multi-step extraction using ≥0.2 M HCl at elevated temperature and extended time had a detrimental effect on the FCSPs yield, as this treatment apparently disrupted the structural integrity of the polymer and evidently degraded carbohydrate chains of fucose residues during extraction. The results also revealed a maximal FCSPs yield of approximately 7% dry weight with Sargassum sp. using 0.03 M HCl at 90°C and 4-h extraction conditions. Accurate monitoring of the differential growth response of seaweed to different nutrient assimilation is crucial to explore various applications of seaweed resources, such as biomass for bioenergy production and source of functional healthy components and bioactive compounds. A major prerequisite for the successful exploitation of cultivated seaweed like Ulva lactuca for commercial purposes is that the growth rate and yields should be optimized. In this study, the growth response of U.lactuca to ammonium and nitrate assimilation was investigated using a photoscanning technique to monitor the growth kinetics in U.lactuca. Photoscanning images revealed differential increases in the surface area of U.lactuca discs over time in response to different nitrogen-based nutrient sources. The results also showed a favorable growth response to ammonium as a nitrogen source, and the presence of ammonium discriminated the nitrate uptake by U.lactuca upon exposure to ammonium nitrate. This study exhibits the applicability of a photoscanning approach for acquiring precise quantitative growth data forU.lactuca. In conclusion, we demonstrated that nuisance seaweed can be a potential source of biomass and bioactive compound notably FCSPs. This study proved the hypotheses that different extraction conditions have crucial influenced to the chemical nature of FCSPs. The study also demonstrated that unfractionated FCSPs are able to exert bioactive actions such as anti-tumor and immune-modulating properties in both in vitro and in vivo studies. This study illustrates the importance of a precise monitoring technique of the growth of U.lactuca in order to successfully exploit it for commercial application.