Enhancement of nitrogen-fixing activity of Enterobacteriaceae strains isolated from Sago palm (Metroxylon sagu) by microbial interaction with non-nitrogen fixers

This study was done to determine microbial interactions between nitrogen-fixing bacteria (NFB) and different indigenous bacteria, both of which were isolated from the same samples of sago palm. Co-culture of NFB and indigenous bacteria, isolated using nutrient agar medium, into a nitrogen-free Rennie medium showed significantly higher nitrogen-fixing activity (NFA) than single inoculations of NFB did in almost all combinations. A reduced oxygen status also enhanced the NFA of NFB. All NFB preferred simple sugars as their substrates for NFA and showed extremely low levels of NFA in starch, hemicellulose, and pectin-containing media. NFA was markedly stimulated in the consortium of starch-degrading Bacillus sp. strain B1 and NFB, ranging from 0-1.5 to 150-270 nmol C2H4 culture(-1) h(-1). The consortium of hemicellulose-degrading Agrobacterium sp. strain HMC1 or Flexibacter sp. strain HMC2 and NFB also showed enhanced NFA, ranging from 0-0.1 to 16-38 nmol. In contrast, no enhanced NFA was detected in the consortium of pectin-degrading Burkholderia sp. strain BTI or Paenibacillus sp. strain P1 and NFB. These results may indicate that beneficial microbial interactions occur in sago palm to enhance nitrogen-fixing activity through collaborative utilizations of starch, hemicellulose and their degradation products.