Production and neutral aldose composition of dissolved carbohydrates excreted by natural marine phytoplankton populations

Natural populations of diatoms were incubated for 4-12 h with (HCO3-1)-C-13 . The production of particulate and dissolved fractions of organic carbon a nd neutral aldoses (NAld) was followed by gas chromatography/mass spectrome try. The extracellular production rate of dissolved organic carbon (DOC) ra nged 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 composit ion with cellular products suggests that glucose in the excreted DNAld is c omposed mainly of storage glucan. On the other hand, the high ratios of dis solved production to total (dissolved and particulate) production of galact ose, rhamnose, fucose, xylose, and mannose probably reflect the active excr etion of heteropolysaccharides by diatoms. By assuming steady-state concent rations, turnover rates of DOC and DNAld can be estimated from the phytopla nkton production of C-13-labeled material. The estimated turnover rates of DNAld through phytoplankton photosynthesis is 4.2-5.1 times higher than tha t of total DOC, which indicates the bioreactive nature of DNAld. This high turnover rate of DNAld mainly resulted from the high turnover rate of gluco se, and it is likely that dissolved glucan is important as a carbon and ene rgy carrier in the marine food web. The fact that the turnover rates of DNA ld, which is considered to constitute heteropolysaccharides, are lower than that of glucose, would suggest that heteropolysaccharides are more resista nt to biological degradation than glucan.