Abstract
Natural populations of diatoms were incubated for 4–12 h with H 13 CO 3 −1 . The production of particulate and dissolved fractions of organic carbon and neutral aldoses (NAld) was followed by gas chromatography/mass spectrometry. The extracellular production rate of dissolved organic carbon (DOC) ranged from 4.1% to 6.4% of total (particulate and dissolved) production rate. Glucose was a major component of the excreted dissolved neutral aldoses (DNAld), and galactose, rhamnose, fucose, xylose, and mannose were found as secondary components of the excreted DNAld. The comparison of NAld composition with cellular products suggests that glucose in the excreted DNAld is composed mainly of storage glucan. On the other hand, the high ratios of dissolved production to total (dissolved and particulate) production of galactose, rhamnose, fucose, xylose, and mannose probably reflect the active excretion of heteropolysaccharides by diatoms. By assuming steady‐state concentrations, turnover rates of DOC and DNAld can be estimated from the phytoplankton production of 13 C‐labeled material. The estimated turnover rates of DNAld through phytoplankton photosynthesis is 4.2–5.1 times higher than that of total DOC, which indicates the bioreactive nature of DNAld. This high turnover rate of DNAld mainly resulted from the high turnover rate of glucose, and it is likely that dissolved glucan is important as a carbon and energy carrier in the marine food web. The fact that the turnover rates of DNAld, which is considered to constitute heteropolysaccharides, are lower than that of glucose, would suggest that heteropolysaccharides are more resistant to biological degradation than glucan.