Southeast Asia has been known as one of the biodiversity hotspots in the world. Repeated glacial cycles during Pleistocene were believed to cause isolation of marine taxa in refugia, resulting in diversification among lineages. Recently, ocean current was also found to be another factor affecting gene flow by restricting larval dispersal in animals. Macroalgae are unique in having mode of reproduction that differs from that of animals. Our study on the phylogeographical pattern of the brown macroalga Sargassum polycystum using nuclear Internal Transcribed Spacer 2 (ITS2), plastidal RuBisCO spacer (Rub spacer) and mitochondrial cytochrome oxidase subunit-III (Cox3) as molecular markers revealed genetic homogeneity across 27 sites in Southeast Asia and western Pacific, in sharp contrast to that revealed from most animal studies. Our data suggested that S. polycystum persisted in single refugium during Pleistocene in a panmixia pattern. Expansion occurred more recently after the Last Glacial Maximum and recolonization of the newly flooded Sunda Shelf could have involved asexual propagation of the species. High dispersal ability through floating fronds carrying developing germlings may also contribute to the low genetic diversity of the species.
Agar polysaccharides extracted from two Thai species of Gracilaria (G. fisheri and G. edulis) and one Japanese species (Gracilaria sp.) were investigated by physical and chemical analysis, and 1 H, 13 C NMR and FT-IR spectroscopy. Agar with partial 6-O-methylated on 3-linked β-D-galactopyranosyl, 2-O-methylated on 4-linked 3,6-anhydro-α-L-galactopyranosyl and 4-O-methyl-α-L-galactopyranosyl units attached to the C6 of 3-linked D-galactopyranosyl units were isolated from G. fisheri. The large parts of 6-O-methylated on 3-linked β-D-galactopyranosyl units and partial methylation on C2 of 4-linked 3,6-anhydro-α-Lgalactopyranosyl units were observed in the agar extracted from G. edulis which corresponded with higher gelling temperature (»60 C°). In contrast, the Japanese agar extracted from Gracilaria sp. showed a typical pattern of agarobiose with partial methylation at C6 of the D-galactopyranosyl units. All agars extracted exhibited sulfate substitution at different positions in the polymers. High sulfate contents were obtained in native agar of G. fisheri (4.56%) and G. edulis (7.54%) that mainly branched at C-4 of the D-galactopyranosyl unit. The presence of this unit was responsible for poor gelling ability of the agar polymers. Alkali treatment was effective both in removing alkali-labile sulfate and increasing the gel strength in Gracilaria sp. (334.5 ± 14.1 g/cm 2) whereas only a slight effect was noted on G. fisheri (228.27 ± 48.18 g/cm 2) and G. edulis (239.95 ± 28.35 g/cm 2). Further investigation may need to determine the constituent sugars and an alternative utilization of the Thai gracilaroids.
The chemical composition of agars from Gracilariopsis lemaneiformis, newly reported from Japan, was investigated. Native agars were isolated by a sequential extraction of plants in water at 22 °C and 100 °C, and in boiling 20, 40 and 60% ethanol. Agars in each extract were analyzed by chemical methods, H, 3 C NMR; and IR spectroscopy. The highest yield of agar (total carbohydrate) was obtained from the 40% ethanol extract (55%). Highest sulfate content was attained in non-alkali treated agars extracted with hot water (4.81 %, DS 0.2). The 3,6-anhydrogalactose content was highest in the 40% ethanol extract (36.1% in non-alkali treatment, 40.3% in alkali treatment). The highest methoxyl content (6.51%, DS 0.66) was obtained in the 60% ethanol extract. The G. lemaneiformis agar is composed of the biological precursor to agarobiose repeating units and agarobiose containing 6-0-methyl agarobiose and a small amount of 2-0-methyl-a-L-galactopyranose residues. Alkali treatment improved the chemical quality of the agar fractions, which was comparable with Japanese commercial agar and agarose.
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