EFFECTS OF DIFFERENT MOLECULAR-WEIGHT FRACTIONS OF DISSOLVED ORGANIC-MATTER ON THE GROWTH OF BACTERIA, ALGAE AND PROTOZOA FROM A HIGHLY HUMIC LAKE

Effects of different molecular size fractions (< 1000 MW, < 10 000 MW, < 100 000 MW and < 0.1-mu-m) of dissolved organic matter (DOM) on the growth of bacteria, algae and protozoa from a highly humic lake were investigated. DOM from catchment drainage water as well as from the la ke consisted mostly (59-63%) of high molecular weight (HMW) compounds (> 10 000 MW). With excess inorganic nutrients, the growth rate and yi eld of bacteria were almost identical in all size fractions. However, in < 1000 MW fractions and with glucose added, a longer lag phase occu rred. Without added nutrients both the growth rates and biomasses of b acteria decreased towards the smaller size fractions and the percentag e of dissolved organic carbon (DOC) used during the experiment and the growth efficiency of bacteria were lower than with excess nutrients. The growth efficiency of bacteria was estimated to vary between 3-66% in different MW fractions, largely depending on the nutrient concentra tions, but the highest growth efficiencies were observed in HMW fracti ons and with glucose. The growth of algae was clearly lowest in the < 1000 MW fraction. In dim light no net growth of algae could be found. In contrast, added nutrients substantially enhanced algal growth and i n deionized water with glucose, algae achieved almost the same growth rate and biomass as in higher MW fractions of DOM. The results suggest ed that bacteria and some algae were favoured by DOM, but protozoans s eemed to benefit only indirectly, through bacterial grazing. The utili zation of DOM by bacteria and algae was strongly affected by the avail ability of phosphorus and nitrogen.