Agarophyte

Abstract: Structural investigations of the agar from Gelidium amansii , a principal agarophyte in Japan, have shown that the agar is a mixture of two different polysaccharides, one a neutral linear molecule composed of alternating residues of 1,3-linked β-D-galactopyranose and 1,4-linked 3,6-anhydro-α-L-galactopyranose, and the other an acidic polymer containing both sulphate and organic acid residues in addition to the above two sugar residues. The present paper begins with a brief review of the above investigations and then refers of additional work on the structures of agars obtained from several species of agarophytes. The agar samples examined have been shown to be mixtures of agarose and agaropectin in varying proportion depending upon their sources. The agarose samples prepared therefrom are free from sulphate, and contain 6- O -methyl-D-galactose in the amounts ranging from 1 to 20%, in addition to the two sugars described above. Analytical data, coupled with evidence that O -6- O -methyl-β-D-galactopyranosyl-(1 → 4)-3,6-anhydro—L-galactose dimethylacetal (6′- O -methylagarobiose dimethylacetal) was isolated by the partial methanolysis of a sample of agar, has pointed to the conclusion that the structure previously suggested for the agarose of G. amansii is a pattern common to agaroses of several sources with the modification that D-galactose residues are partially 6- O -methylated to a varying degree depending upon the species of agarophyte. In connection with the work outlined above, synthetic proof is also presented as to the structure of the constitutional disaccharides of agar and of x -carrageenan.

Abstract: Agar, a gelatinous polysaccharide in the cell wall of many red algal species, is widely used as a gelling, thickening and stabilizing agent. The commercial value of seaweed is judged by their agar content and gel quality. Seaweed materials with higher agar yield and better gelling properties are desired due to the growing demand for agar in the global market. Agar biosynthesis in seaweeds is affected by genetic variations, developmental stages and environmental conditions, while different agar extraction techniques can also affect the yield and quality of agar. In this paper, the effects of different physiological states of seaweed, abiotic and biotic factors, seaweed storage and agar extraction techniques on the agar yield and gelling characteristics, are reviewed. This information is important as a guide for marine aquaculture of potential agarophytes and the possible effects of climate change on the stock of this natural resource.

Abstract: The agarophyte Gracilaria cornea, collected over 2.5 y in the Florida Keys, shows adaptations to oceanic salinities and subtropical to tropical water temperatures in its photosynthetic and respiratory responses as measured with a respirometer. No seasonal pattern in responses to irradiance, temperature, and salinity were evident between five collections over a 20-month period, indicating the tropical nature of the populations from Bahia Honda and Pigeon Keys. Concentrations of chlorophyll a (0.09 to 0.41 mg g d wt-1) and phycoerythrin (0.06 to 0.36 mg g d wt- 1) were low and reflect the low nutrient regime of the habitats, especially when compared to laboratory cultured plants. Compensation and saturation irradiances were also low (11–38 and 90–127 μmol photon m-2 s-1), indicating acclimation to lower irradiances in their shallow (1–2 m depth) habitats where turbidity can be high. In comparison with other subtropical and warm temperate species of Gracilaria, G. cornea had lower levels of pigment, but similarly high photosynthetic efficiency, demonstrating shade adaptation; it had only limited tolerance to salinities below 20‰ and temperatures below 15 °C. Thus, G. cornea from the Florida Keys in mariculture would require subtropical to tropical temperatures and stable oceanic salinities.