Abstract:
Frankia and many of the nitrogen fixers found in the nodules of the Casuarinacaea lineage,
belong to actinobacteria, which are largely a group of aerobic, filamentous, gram-positive
bacteria which harbor high G+C content in their respective genomes. Actinobacteria have
been earmarked for their bioremediation potential and have been identified as strong
candidates superior to fungi and gram-negative bacteria in their capacity to remediate
polluted environments. It is important to ascertain the contribution of actinobacteria in sites
of environmental pollution, namely areas of heavy metal contamination such as agricultural
land with strong fertilizer usage, industrial effluents and waste disposal sites. The potential
role of Frankia and auxiliary actinobacteria colonizing actinorhizal plants, in bioremediation,
is largely unknown, with the exception of a few of studies.
A study was carried out, to assess the cadmium tolerance of Frankia and co-colonizing
actinobacteria inhabiting root nodules of the actinorhizal tree Casuarina equisetifolia. The
actinobacteria, Frankia, Micromonospora and Streptomyces, were isolated from root
nodules of Casuarina plants and identified preliminary by colony and mycelial morphology.
It is of significance that this is the first reported isolation of Streptomyces from Casuarina
equisetifolia. Furthermore, the actinobacteria were able to grow on N-free media, signifying
that all three actinobacteria were likely nitrogen fixers. The isolated actinobacteria were
demonstrated to possess cadmium lowering capacities of up to 10 mg/l. It appears that
actinobacteria from root nodules of Casuarina equisetifolia are diazotrophs with capabilities
to lower cadmium from an external medium. A comprehensive molecular characterization
of the three diazotrophs using nif, 16s rDNA and glutamine synthetase gene sequences is
being carried out at present.
