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Experiments were conducted in batch culture in the same conditions as
those described for Arabian Light Crude. The mixed culture of bacteria
used was the mixture of the strains Pseudomonas and Moraxella isolated
and purified, described in Section 1.3. By sucessive cultures in flasks
with the pure hydrocarbon mixture as the only carbon substrate, the
mixed culture was progressively adapted to grow on these ten hydrocarbon
compounds.
The culture was carried out in batch for 61 V z hours, and we observed
the changes in the biomass, total hydrocarbons, and each compound, and
also the organic substances that passed into the aqueous phase.
Following a lag phase of about 10 hours, a growth acceleration phase
was observed up to the 25th hour, then a linear phase from the 25th to
the 35th hour, and finally the slowdown of bacterial growth. At the end
of the batch, the dry cell weight was 0.5 g.l -1 . The biodegradation
process of total hydrocarbons perfectly matched the microorganism growth
pattern. After 61 Y 2 hours, 82.8 % of the hydrocarbons were degraded.
It appears that the three most volatile compounds, paracymene , decaline
and benzothiophene, could not be found after extraction, from the very
outset of the experiment. These three products must therefore disappear
chiefly during extract evaporation operations. However, a small proportion
passes very rapidly into the aqueous phase in the marine environment,
because the latter contained oxidation products of p-cymene among
others, as well as benzothiophene.
The two n-alkanes, n-hexadecane and octacosane, and the dodecylbenzene
were consumed first. For these three products, which practically
disappeared by the end of the batch, their respective biodegradation
rates after 37 Y 2 hours only of culture were 93 %, 87.5 % and 80 %.
Pristane only started being attacked after 24 V z hours, and was 69.2 %
consumed at the end of the culture. During the last 20 hours, while
practically no alkanes or alkylbenzene remained in the reactor, dimethylnaphtalene
was biodegraded (disappearance rate 67 %) . Phenanthrene and
dibenzothiophene were consumed very little if at all.
These results perfectly confirm those found with Arabian Light Crude,
which showed that alkanes and isoprenoids were attacked first, followed
by mono- and di-aromatics. Similarly, it was observed that triaromatics
and sulfur-containing aromatics were only slightly sensitive
or insensitive to the action of microorganisms. The fact that dodecylbenzene
disappeared fairly rapidly is explained by the presence of the
linear chain which, like the n-alkanes, is readily accessible to
bacteria.
The total organic carbon concentration (TOC) measured in the medium was
385 mg.l -1 . After esterif ication and evaporation of the organic extract,
esters and some other polar compounds were found in a concentration of
221.5 mg.l -1 .
We carried out analyses by gas phase chromatography and GC/MS coupling
in an attempt to identify these products.
Since many products were present in trace amounts, and several of them
were eluted simultaneously and combined in a single peak, we encountered
considerable difficulty in identifying them on the mass spectrometer.
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