In order to study the establishment of a spermosphere, the C2H2 reduction activity of N2-fixing bacteria isolated from river sand was examined in a simulated spermosphere in the river sand which contained sucrose, an amino acid mixture, and CN- released from plant seeds. The sand incubated with 10-10 to 10-9 mol CN- 30 g sand-1 exhibited higher C2H2 reduction activity than that without CN-. The change in the most probable number of N2 fixers with increasing quantities of CN- roughly corresponded to that in C2H2 reduction activity. However, the most probable number of non-N2-fixing bacteria decreased except for CN-- tolerant ones. Both C2H2 reduction activity and proliferation of the N2 fixers isolated on a modified Burk's medium were almost similar to those in the bacteria in the sand. In contrast, the proliferation of some nonfixers decreased with an increasing CN- concentration. C2H2 reduction activity of N2 fixers cultured in combination with non-fixers exhibited a clear peak at 10-7 M CN- as for C2H2 reduction activity in the sand. We therefore speculate that cyanide evolved from seeds during a pregermination period may suppress the growth of general bacteria, but may promote the proliferation of N2 fixers, thus contributing to the establishment of a spermosphere.
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