Differentiation Between Auto- and Heterotrophic Activity: Problems in the Use of Size Fractionation and Antibiotics

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Abstract
Size fractionation of samples indicated that 14HCO3- is assimilated by organisms retained on 3 and 1 .mu.m pore size Nuclepore filters. Samples prefractionated before incubation showed that material smaller than 3 and 1 .mu.m represents, respectively, only 12.9 and 1.5% of the amount retained by a 3 .mu.m filter. The glucose-14C-uptake showed that almost 55.6% of the heterotrophic activity was retained by the 3 .mu.m, 8.3% by the 1 .mu.m, 14.4% by the 0.4 .mu.m and 21.3% by the 0.2 .mu.m filter, indicating that the heterotrophic activity was mainly associated with material larger than 3 .mu.m. The photosynthetic rate was not affected by concentrations of gentamycin between 0-100 .mu.g ml-1 but was affected by greater concentrations. Concentrations of 40 .mu.g ml-1 gentamycin suppressed the heterotrophic activity only between 11-24% while concentrations of 250 .mu.g ml-1 gentamycin result in a 50% suppression. These studies indicate that neither the size fractionation by filtration nor the use of antibiotics are generally suitable for the differentiation of auto- and heterotrophic activity.