Decomposition process of organic matter derived from freshwater phytoplankton

We investigated the biodegradation process of freshwater phytoplanktonic organic matter using incubation experiments, with special reference to changes in three major biomolecules: neutral aldoses, amino acids and fatty acids. The concentration of neutral aldoses decreased drastically relative to amino acids and fatty acids during the early decomposition phase (days 0–7), owing largely to the rapid decomposition of storage carbohydrate. This resulted in a temporary decrease in the C/N ratio of organic matter. During the late phase (days 7–60), however, the rate of decrease in neutral aldoses slowed, while the considerable decrease in amino acids and fatty acids continued. The inconspicuous change in amino acid composition was probably due to the fact that no protein and/or peptide is composed of a limited species of amino acid. Although the compositional variety of organic matter among the phytoplankton was clearly observed at the start of decomposition, it became obscure in the course of 60 days. This indicates that while the organic composition of the labile fraction of phytoplanktonic organic matter varies depending on the phytoplankton groups, the refractory fraction has similar composition. The addition of bacterial organic matter is likely another reason for the similar composition of the remaining organic matter at day 60.