Growth of Marine Bacteria at Limiting Concentrations of Organic Carbon in Seawater1

Growth responses of several species of heterotrophic marine bacteria to limiting concentrations of lactate, glycerol, and glucose in seawater were determined in a chemostat. In all cases, threshold concentrations of the limiting substrates were found below which the organisms were unable to grow. This phenomenon is explained on the basis of a positive feedback mechanism, which is abolished below a certain minimum population density. The resulting inhibitory effect on growth leaves a corresponding concentration of the limiting substrate unattacked. According to their growth parameters, two types of species could be distinguished, one adapted to the marine environment by its ability to grow at low substrate concentrations and one inactive in natural seawater but surviving. The implications of these results on the rates of microbial transformations and on the occurrence and concentrations of dissolved organic material in the sea are discussed.