Oligotrophic bacteria on organic debris and plant roots in a paddy field soil

Abstract

The distribution of heterotrophic bacteria on organic debris and roots of rice plants in a paddy field were studied. The heterotrophic bacteria consisted of two main groups: those which grew on full-strength nutrient broth (NB) and those which did not grow on NB but on a 100-fold dilution of NB (DNB). The latter group was called ‘DNB organisms’ and were considered to be oligotrophic. In both manured and unmanured soils, DNB organisms were predominant in the bacterial communities on organic debris and the rice roots throughout most of the entire period of rice cultivation, although a transient decrease in the proportion of DNB organisms was observed immediately after an application of manure. Morphological and physiological characteristics of DNB isolates from organic debris and rice roots were studied: five types of cell shape were observed, (1) regular rods, (2) filament-forming rods, (3) irregular rods, (4) prosthecate organisms and (5) large oval cells. Regular rods (42% of the total DNB isolates) and irregular rods (46%) were abundant. The ecological roles of DNB organisms in paddy soil are discussed in relation to their physiological characteristics.