Abstract
Dissolved humic substances (HS) are refractory organic compounds in water systems. HS make up ~50% or more of dissolved organic carbon (DOC) in freshwater ecosystems, yet their origins remain unresolved. Recent analytical techniques have revealed that aquatic HS are mainly composed of fulvic acids having molecular weights less than 10,000.
High-performance size exclusion chromatography (HPSEC) with fluorescence detection can separate HS in Lake Biwa basin system into three molecular size classes, i.e., high(MW:5400)-, middle(4000)- and low(2000-1000)-molecular-weight classes.HS in Lake Biwa and outflow river waters contains much amounts of the low-molecular-weight class as compared to those in inflow river waters. This indicates a possibility of in situ generation of low-molecular-weight HS in Lake Biwa. To certify this possibility, we made biodegradation experiments of phytoplanktons (Cryptomonas tetrapyrenoidosa and Chlamydomonas reinhardi) at 20C in the dark. Degradation rapidly proceeded, producing a new organic fraction. This fraction coincided in HPSEC retention time with the low-molecular-weight class of HS in Lake Biwa and remained stably during the long experimental period (100days). It is thus certain that biodegradation of phytoplanktons makes an important contribution to the generation of organic compounds having low-molecular-weights in Lake Biwa.
