RESPONSE OF AQUATIC BACTERIAL-POPULATIONS TO SUBSTRATE ENRICHMENT

The responses of natural bacterial populations in the waters of the Kiel Fjord, Germany, and in Lake Kinneret, Israel, to additions of organic substrates were followed by determining changes over 24 h either in direct cell counts or in H-3-thymidine incorporation, and in the Kiel Fjord additionally in H-3-leucine incorporation. In parallel, 1 mu m filtered water samples were stored for 3 or 4 d in order to starve the indigenous bacterial populations prior to repeating the substrate addition experiments. Generally, upon substrate addition, relatively higher incorporation of radiotracers was noted in the preincubated samples. Growth response to substrate addition even in starved populations was only significant after 24 h. Incorporation rates of H-3-thymidine and H-3-leucine were more sensitive indicators of bacterial response to substrate additions than cell counts. Continued cell replication in unsupplemented controls, and insignificant increase over time of radiotracer incorporation in most of the fresh samples with added supplements, indicated that the indigenous bacterial populations in Kiel Fjord and Kinneret were apparently not substrate limited. Comparison of actual bacterial production after 24 h (direct counts) to that predicted by H-3-thymidine incorporation after 1 h showed that although reasonably good predictions of daily production were obtained in the unsupplemented samples, this was usually not the case when substrates were added.