Suet Ying Wong1, Put Ang, Jr.2
1Marine Science Institute, School of Life Sciences, The Chinese University of Hong Kong, Hong Kong SAR, China
2Institute of Space and Earth Information Science, The Chinese University of Hong Kong, Hong Kong SAR, China
The seasonal and individual variations (between holdfast, basal, middle, top blade regions and receptacles) of mannitol in Sargassum siliquastrum collected monthly over a 30 month period were investigated using HPLC. Seasonal variation of its mannitol content ranged from about 1 to 17% of its dry weight. Mannitol content increased with growth and peaked during the active growing stage in September to October (10.81 ± 4.87 to 13.17 ± 5.60 g mannitol/ 100g dry seaweed). A drop in mannitol content was recorded only at the start of the reproductive period in December and remained stable thereafter. Dramatic decrease in mannitol concentration occurred after the reproductive period and a small increase in mannitol content was identified in the middle of the slow growth stage in April to (5.69 ± 2.11 to 17.87 ± 7.46 g mannitol/ 100g dry seaweed), suggesting that the surplus produced during the slow growth period may be used to maintain the perennial holdfast or to develop new shoots for the next season. On an individual level, more mature basal part of the plant (i.e. holdfast and basal blade region) contained higher levels of mannitol when compared with the younger middle and top blade regions. Receptacles of S. siliquastrum displayed the lowest mannitol content, which appeared to match its low photosynthetic activity. The disproportionally high level of mannitol in the photosynthetically less active holdfast suggested that mannitol may be diffused from the basal blades down to the holdfast to be utilized for the development and elongation of new shoots.