International Journal of Marine Science 2015, Vol.5, No.27: 1-11
6
the oil depletion percentage (r = 0.63).
In our previous
study, the density of oil-degrading bacteria growth over
2 months on an oil-polluted sandy beach amended
with slow release fertilizer (Osmocote) had increased
to 34.8 % of the total bacterial population (Darmayati,
2010). This showed that, in oil-polluted sediment, the
higher the total number of bacterial cells, the higher
the availability of oil-degrading bacteria.
The amendment of fertilizer 15 g/kg sediment might
be too much for the level of oil pollution 100 g oil /kg
sediment. The depletion rate and bacterial numbers of
biostimulation only treatment was lower than the
combination treatment which applied as low as 7.5 kg
N/ kg (Table 1; Figure 2), although, the initial number
of
bacterial cells in biostimulation only and the
combination treatments was similar (Figure 2). This
may have been caused by the Carbon/Nitrogen (C/N)
ratio of 1000 : 75 which provided better environment
for oil degrading bacteria than the ratio of 1000 : 150.
Xu et al.(2003) found out that an addition 0.8% and
1.5% of slow-release fertilizer, Osmocote consisting
of 18, 4.8, and 8.3% NPK (w/w) to oil polluted
sediment was sufficient to maximize the metabolic
activity of biomass and biodegradation of straight
chain and branch chain n-alkane, respectively. This
equates to a C/N ratio of 1000 : 33 and 1000 : 61
which they say is sufficient to maximize the metabolic
activity of biomass and biodegradation of straight
chain and branch chain n-alkane, respectively. An
excessive nutrient concentration may supress the
growth of bacteria, whereas in combination treatments,
nutrient concentration was lower but still enough for
bacterial growth. Sufficient loading rates of nitrogen
will be necessary if biostimulation is to occur.
According to Gibbs et al., (1975) approximately 4mM
of nitrogen was required to breakdown 1 mg of crude
oil and phosphorus was not become limiting down to a
minimum P/N ratio of 0.02. Loading rates below the
critical concentration will be a waste of resources as
would excessive use which could also promote
secondary impacts such as harmful algal bloom and
oxygen depletion (Bragg et al., 1994; Jackson and
Pardue, 1999).
Slow release fertilizer use (Gramafix) provided not
only NPK in the ratio of 22:7:12, but also Mg, Ca, S
and micro nutrients in the ratio of 2:4:3:1. These
minerals seemed to play an important role in boosting
the catalytic activity of enzymes produced by oil
degrading bacteria. Cookson and John (1995)
mentioned that Mg
2+
and Ca
2+
are metallic ions that
can function as a cofactor in the catalytic enzyme
activity of microbes. Salinity was increased
significantly up to 46 ppt when this SRF was applied
to oil-contaminated sediment (Figure 3D). This may
also have been caused by the addition of numerous
minerals from Gramafix. This high level of salinity
may still be under the tolerable level for marine
oil-degrading bacteria. It can be shown by the growth
observed until 23 days after treatment (
Figure
2 B,C, and
D)
. Mille et al., (1998) investigated the biodegradation
of crude oil by a mixed bacterial community isolated
from marine sediment with varying concentrations of
sodium chloride. Initially the amount of oil degraded
increased with increasing salt concentration, to a
maximum level of 0.4 mol/l NaCl. Thereafter the
amount of oil degraded decreased with increasing salt
concentration, probably as the salt-tolerance limit of
the bacteria was reached.
In the present study, the combination of biostimulation -
bioaugmentation performed better than biostimulation
only. Result of this study indicated that oil degradation
rate was faster and the number of bacterial was higher
during 90 days experiment in both combination
treatments. It may caused by synergistic activity of
exogenous and indigenous oil degrading bacteria and
stimulation by sufficient concentration of nutrient
provided from application of slow release fertilizer
(Gramafix) at 7.5 g N/kg. Availability of carbon and
energy source and favorable environmental conditions,
namely temperature, salinity, oxygen and nutrients,
may also support the growth. In bioaugmentation, the
addition of oil-degrading bacteria boosts biodegradation
rate, whereas in biostimulation, the growth of
indigenous hydrocarbon degraders is stimulated by the
addition of nutrients (mainly N and P) or other growth
limiting nutrients (Nikolopoulou and Kalogerakis,
2010). Nikolopoulou et al., (2013) also mentioned that
the success of oil spill bioremediation depends on the
establishment and maintenance of physical, chemical
and biological conditions that favor enhanced oil
biodegradation rates in the marine environment. This
result was incoherent with previous work (Nikolopolou
et al., 2013; Ueno et al., 2007;
Stallwood , 2005).
