Microalgal biodegradation of pentachlorophenol.
Pentachlorophenol (PCP) is a chlorophenol with a pronounced biocidal activity that
has led to its use in a number of applications. It was introduced in the 1930s as a
preservative for timber and lumber and since then has found wide use as a biocide in
agricultural and industrial applications. Many different physical, chemical and
biological methods have been tried for the removal of PCP from wastewater.
However, using microalgae for the removal of PCP and other organochlorine
compounds from water may prove to be a cheaper alternative and give complete
degradation of the compounds. The aim of this project was to study the efficiency
of microalgae to degrade PCP.
An algal strain named VT -1 and a bacterial strain named AT -14 were isolated from
PCP containing conditions in the laboratory. The growth of VT -1 in the presence of
PCP was compared with Chlorella emersonii and Chlorella vulgaris in two different
autotrophic media. It was observed that VT-1 had the highest IC50 value of 25-26mg
}-l PCP and EC50 value of 11.3mg }-1 PCP in S&K medium. With glucose as an
additional carbon source the IC50 value for VT-1 in S&K medium was 29-30mg t 1
PCP. Bacterium AT-14 could grow in the presence of PCP, only with glucose as a
carbon source. Mineralization of PCP by VT -1 and the two Chlorella strains was
compared by using 14C_PCp. With all the three algae exposed to 14C_PCp, only VT-1
showed release of 14C02, which was evidence of mineralization of PCP by VT-1
which occurred only in the presence of light. Bacterium AT-14 did not produce
14C02. However, the consortium of VT-1 and AT-14 showed enhanced 14C02
evolution in the presence of glucose.
The release of chloride ions from PCP can also indicate PCP dehalogenation and
degradation. The evolution of 14C02 lagged behind chloride release (90 %) indicating
that dechlorination of PCP could be the first step in its biodegradation. Breakdown
of PCP was also followed by its extraction from the cells and medium. Normally
dichloromethane (DCM) was used to extract PCP. The changes in the label extracted
in DCM and iso-butanol were studied under different light condjtions, which showed
that the 14C counts in DCM reduced and those in iso-butanol extract increased with
time. The 14C counts in the iso-butanol extract could be a metabolite of PCP which
is more hydrophilic. VT-1 appeared not to degrade PCP completely, since only 15%
of 14C was recovered as 14C02. It appears that intermediates are formed which are
distributed in the growth medium and in the biomass.
It can thus be concluded that VT -1 is tolerant of PCP, appears to dechlorinate PCP
and then releases some part of it as CO2.