EFFECTS OF COLLOIDAL ORGANIC-MATTER ON THE GROWTH OF BACTERIA AND PROTISTS IN LAKE WATER

The impact of colloids of subbacterial size on the growth of bacteria and heterotrophic flagellates in lake water was studied. Colloidal organic matter <0.2 mum but > 100 kD was concentrated from the water of four lakes, and growth of microorganisms was compared in water with and without added colloid concentrate. A tenfold increase in the concentration of colloids had either no effect on bacteria or supported an increase in bacterial yield of less-than-or-equal-to 80% in predator-free samples. Growth of flagellates was also stimulated by the enhanced colloid concentration. In two of the lakes, increased colloid concentration resulted in more flagellates produced per unit of bacteria consumed, indicating that colloids served as a supplementary source of nutrition. However, enhancement of the colloid concentration did not influence flagellate-bacterial predator-prey patterns, suggesting that bacteria were still the major food source of flagellates despite increased abundance of colloids of subbacterial size. In addition, in experiments with water from a eutrophic lake, abundance of viruslike particles (VLP) was monitored. At natural colloid concentration as well as at tenfold enhanced colloid concentration, abundance of VLP decreased in samples containing heterotrophic flagellates, but did not decrease in samples without flagellates. Assuming the decrease in VLP was due to ingestion by flagellates, the calculated contribution of VLP carbon to the total carbon demand of flagellates was only marginal (0.09-0.2%).