DETERMINATION AND PROPERTIES OF ACTIVELY METABOLIZING HETEROTROPHIC BACTERIA IN SEA, INVESTIGATED BY MEANS OF MICRO-AUTORADIOGRAPHY

Substrate transformation and microbial biomass production in aquatic ecosystems depend mainly on the total number of actively metabolizing heterotrophic bacteria. The most common methods used concern the determination of either the colony-forming bacteria or the total number of bacteria including autotrophs and inactive organisms. A micro-autoradiographic method is presented which enables the substrate uptake of single bacteria by means of 3H-amino-acid mixture and Nuclepore filters to be determined. The standardization procedure revealed the greatest success after 3 h incubation with 10 .mu.Ci/ml tritiated amino-acid mixture and an exposure of 14 days to the X-ray film. Preliminary experiments showed inactivation of an active fresh-water population form 100% to 0.6% within 3 h at 28.permill. S. With increasing distance from the shore, the number of colony-forming units decreases from 6 to 0.6% of the total number of active heterotrophic bacteria. The fraction of very small heterotrophic bacteria which cannot be cultured on nutrient media may be responsible for the continuous breakdown of organic matter in offshore regions of the sea.