DIEL VARIATION OF WATER-EXTRACTABLE CARBOHYDRATE-COMPOSITION OF NATURAL PHYTOPLANKTON POPULATIONS IN KINU-URA BAY

Natural phytoplankton populations from Kinu-ura Bay were incubated adding (NaHCO3)-C-13 under natural light and temperature conditions for 24 h in July 1986, and diel changes in the production of water-extractable carbohydrates were investigated. The water-extractable carbohydrates were separated into fraction A (mol. wt. >6000), B (mol. wt. 1000-6000) and C (mol. wt. <1000) with Bio-Gel P6, and C-13 incorporation into monosaccharide components in each fraction was analyzed by GC/MS. Fraction A consisted of glucose, xylose, galactose, fucose, mannose, rhamnose and arabinose. Only glucose was found in fraction B, which increased rapidly in the afternoon. Fraction C comprised mainly ribose, galactose and glucose. The monosaccharide composition and the molecular weight of the 3 fractions suggested that fractions A and B were cell wall heteropolysaccharide and storage glucan of algae, respectively, while fraction C consisted of low-molecular-weight cellular components, such as free glucose, oligosaccharides, simple glycosides, and coenzyme and/or nucleotides. These 3 fractions all increased during daytime, and decreased during nighttime. Fraction B increased quickly about 8-fold in the afternoon, but underwent an 80% decrease during nighttime. Diel specific variation of fractions A and C was less than that of fraction B. Monosaccharide components of fractions B and C were highly labelled with C-13 in the daytime as compared with those of fraction A. A marked increase in C-13 atom% of most monosaccharide components of fraction A was observed during nighttime, while C-13 atom% of most monosaccharide components of fractions B and C tended to be constant or decrease. This result indicated that most carbohydrates in fraction A were synthesized not only during daytime, but also during nighttime by low-molecular-weight compounds which were highly labelled with C-13.