An investigation into microbial biotransformations of antimony
Interactions of microorganisms, both prokaryotic and eukaryotic, with the metal
antimony were studied. Of particular interest was the process of biomethylation.
Volatilisation of trimethylantimony from inorganic antimony substrate by mixed
inoculum (of environmental source) enrichment cultures was demonstrated to occur.
Trimethylantimony was the sole volatile antimony species detected in incubations
designed to promote the growth of clostridia, no stibine or other volatile methylated
species were detected. Two Clostridium sp. were isolated from environmental
enrichment incubations and three characterised Clostridium sp. were demonstrated to
possess a biomethylating capability. Up to 21 μg. 1-1 involatile methylantimony species
were detected in the culture medium of monoseptic incubations of the characterised
Clostridium sp. The relative quantities of involatile mono-, di- and trimethylantimony
species produced during the course of the cultivation period is consistent with
trimethylantimony oxide being a final product of antimony biomethylation, with monoand
dimethylantimony species appearing transiently in the cultures as intermediates of
an antimony biomethylation pathway.
The fungi Cryptococcus humicolus, Candida boidinii, Candida tropicalis, Geotrichum
candidum and Saccharomyces cerevisiae were all demonstrated to possess a similar
antimony biomethylating capability. Volatile and involatile methylantimony species
were detected, with involatile species being the predominant form. Both stibine and
trimethylantimony were detected in culture headspace gases of fungal incubations.
Levels of trimethylantimony were higher in incubations supplied with antimony III
substrate, whilst stibine was the predominant volatile antimony species in incubations
supplied with V valency substrate. S. cerevisiae demonstrated the highest stibine
generating capability with up to 0.3% substrate being transformed. Regardless of
substrate, overall antimony biomethylation efficiency (to both volatile and involatile
species) was low, indicating that this biotransformation does not form the primary mode
of resistance to the metal. Less than 0.1% of antimony III substrate was biomethylated
by C. humicolus, the most productive species in terms of formation of methylantimony
compounds. The intracellular accumulation of methylated antimony species further
belies the theory that antimony biomethylation constitutes a resistance mechanism.
Study of C. humicolus revealed the biomethylation process to be enzymatic and
inducible by arsenic but not by antimony. This may indicate that the enzymes of the
arsenic biomethylation pathway are the likely biocatalysts for the biomethylation of
antimony. The low efficiency of antimony biomethylation indicates that this is most
likely a fortuitous process.
A number of Gram-positive cocci isolated from soil and sediment were demonstrated to
bioreduce antimonate to an unknown inorganic antimony III compound concurrently
with lactate oxidation and biomass formation (as measured by protein). Up to 48% of
the supplied antimonate was bioreduced. The demonstration of dissimilatory antimonate
respiration adds this metal to the increasing list of known "unusual" electron acceptors
such as uranium, arsenic, selenium, iron and manganese.
These studies reveal some of the microbial interactions of microorganisms with the
metal antimony, demonstrating the potential that microorganisms have to contribute to
the biogeochemical cycling of antimony through biotransformation processes