Introduction

In quantitative terms, crude oil is one of the most important organic pollutants in marine environment and it has been estimated that worldwide somewhere between 1.7- 8.8×10⁶ tons of petroleum hydrocarbons impact marine waters and estuaries annually (1). Reports have been appearing since last three decades on the biodegradability of crude oil by bacteria which can use hydrocarbons as source of carbon and energy (2). When micro organisms grow in environment rich in hydrocarbon, they undergo many adaptations. One such adaptation is biosurfactant production which is a frequently encountered feature in hydrocarbon degrading bacteria or sometimes even a prerequisite for growth on hydrocarbons (3). Biosurfactant production helps the hydrocarbon degrading bacterium to gain better access to their hydrophobic substrates as it brings about changes like reduction of surface tension of the environment around the bacterium, reduction of interfacial tension between bacterial cell wall and hydrocarbon molecules, membrane modifications like increasing the hydrophobicity of cell wall by reducing the lipopolysaccharide content of cell wall, enhancing the dispersion of hydrocarbon by encapsulation of the hydrocarbon into micelles etc (4,5,6,7). Amongst the above changes reduction in surface and interfacial tension is a universal phenomenon displayed by almost all types of biosurfactant whereas changes like membrane modifications and emulsion formation strictly depends upon type of biosurfactant for e.g. glycolipids, lipopepetides, polymeric or particulate etc. Enhancement in hydrocarbon degradation may occur by cumulative effect of above changes.

Here we present a report on isolation of crude oil degrading marine bacteria from oil contaminated sea water, their screening for crude oil degradation and biosurfactant production.

Methods

Collection of sample

Isolation and screening of crude oil degrading bacteria

Crude oil biodegradation

Biosurfactant production

Statistical analysis

Results And Observation

Isolation and screening of crude oil degrading bacteria

Estimation of crude oil biodegradation

The results from control flasks indicated 15% abiotic loss of crude oil from the medium. Highest crude oil biodegradation was observed with isolate M2 (70%) followed by M1 and M8, both degrading 55% of the added 1% crude oil in the medium. A comparatively lower biodegradation of 40-50% was

found in isolates M3, M4, M5, M6 and M7. Least biodegradation of 30% was recorded with M9 and M10.

The effect of 1% Tween 80 on biodegradation of crude oil (1%) by the bacterial isolates varied drastically. On addition of 1% Tween 80 highest increase in biodegradation was shown by M9 which was otherwise the least efficient degrader. Biodegradation among other isolates in presence of Tween 80 ranged from 25-55%. Enhancement in crude oil biodegradation was observed only with two other isolates namely, M3 and M10. In these isolates the increase in biodegradation was 37 and 50% respectively. However, most of the isolates showed decrease in biodegradation in the range of 10 to 44% after addition of Tween 80. Isolate M8 was indifferent to presence of Tween 80 in this regard (Table 1).

Figure 1

Table 1: Crude oil degradation by bacterial isolates

* Values represent mean of triplicates

Biosurfactant production

Seven out of ten isolates scored positive for biosurfactant production in drop collapsing assay. On addition of the culture supernatant of isolate M2, M3 and M8 on mineral oil, beaded drops were observed even after 24 hrs indicating lack of biosurfactant production. In all other isolates addition of culture supernatant on the mineral oil lead to formation of flat drop after 60 sec and/or 24 hrs varying according to the isolate and therefore they were considered biosurfactant producers (Table 2).

Figure 2

Table 2: Biosurfactant studies on bacterial isolates

* Mean values indicated with standard error

NA: Not Applicable

Each of the seven isolates which scored positive for biosurfactant production also emulsified toluene but to a varying extent. Not all the emulsions formed by these isolates were stable after 24 hrs. Highest emulsifying index of 68% was observed with M6 after 10 min which remained stable after 24 hours. Similarly isolates M4 and M7 registered stable emulsification activity of 68.23% and 63.85% respectively. Emulsification activity of isolates M1, M5 and M10 was 56.75%, 47.75% and 62.44% respectively after 10 min which drastically declined after 24 hrs. M9 showed emulsification activity of 49.35% only after 10 minutes while M2 did not show any such activity.

The seven biosurfactant producers tested for oil displacement activity did not yield appreciable activity. No displacement activity was observed in M9, despite scoring positive for production of biosurfactant and emulsification of toluene (Table 2). Biosurfactant produced by none of the isolates had hemolytic properties. This was evident as no zone of clearance was observed in the blood agar plates inoculated with the isolates

Characterization of the bacterial isolates

The ten isolates recovered from oil contaminated sea water varied in their characteristics. There were similarities observed in colony morphology and pigmentation of few isolates but they differed in biochemical properties (Table 3). Isolates like M3, M5 and M6 were found to have similar colony morphology and pigmentation. Likewise, M7 and M10 were also similar in appearance, shape and color. Cocci and coccobacilli were the dominant cell morphology and most of the isolates were Gram positive. Only two isolates namely M1 and M8 were gram negative. Pigmentation of colonies varied from yellow, orange, and pink to beige.

Figure 3

Table 3: Biochemical characteristics of the bacterial isolates

Discussion

The appearance of colonies on the MM2 agar plate sprayed with ethereal solution of crude oil showed that contaminated sea water at ship yard in Alang coast harboured crude oil degrading bacteria. The bacterial isolates were designated as crude oil utilizers (16).

The overlay technique also confirmed that hydrocarbon degrading bacteria were ubiquitously present; their population size might be small in non-polluted area but in the hydrocarbon polluted area like the ship yards where crude oil pollution is common the population of crude oil degraders was dominating (17). Ten out of eighteen isolates showed profuse growth on screening through overlay technique.

Colonies of most of the isolates were mucoid and fused together in dense growth areas. This might be because of the exopolysaccharide production which leads to mucoid colony morphology. It has been reported that there is a close relationship between mucoid colony morphology and the ability to grow on crude oil. (18). The biodegradation of crude oil by bacterial isolates was on a very wide scale. Where on one hand 70% of crude oil was degraded by isolate M2, the isolates M9 and M10 degraded only 30% of the added crude oil. This might account for the varying ability of the isolates to survive in a single concentration of crude oil (17). The 1% crude oil added to the medium might be higher then the tolerance limit of M9, M10, M3 and M5 thus slowing down their growth and hence biodegradation, whereas the same concentration might not be high enough to affect the growth of the other isolates negatively and hence they could degrade it efficiently in the range of 50-70% (Table 1).

On addition of 1% Tween 80 the results obtained in case of bacterial isolates namely M10, M3 and M9 were in agreement with the published reports as degradation of crude oil was enhanced in the media of these isolates (19). The enhancement could be owed to the reduction of surface tension of the media and interfacial tension between hydrocarbon and cell surface by Tween 80 being a chemical surfactant (20). But there was decrease as well in degradation of crude oil on addition of 1% Tween 80 in most of the bacterial isolates namely, M2, M5, M4, M7, M1 and M6 which may be due to the toxic effects of Tween 80 on bacterial cells. Tween 80 beyond a certain concentration is poisnous to bacteria (21). So probably concentration of Tween 80 added in media i.e. 1% was inhibitory for these isolates retarding their growth and hence the degradation of crude oil.

70% of the bacterial isolates showed biosurfactant producing ability through drop collapsing assay, emulsification of toluene and oil displacement assay (Table 2). This accounts for the natural adaptation of biosurfactant production in many hydrocarbon degrading bacteria for better bioavailability of their substrates as hydrocarbons are not easily soluble being hydrophobic (20).

The supernatant obtained on centrifugation of the media contained biosurfactant and supernatant of few biosurfactant producing isolates formed good emulsion with toluene. In almost all isolates the emulsification activity was greater then 50-55% which accounts for good emulsification. However most of the emulsions with toluene were not stable as emulsification activity reduced in most of the isolates in 24 hrs. Differences in emulsification indices reflected different interactions among biosurfactant and hydrocarbon which explained why emulsions of some isolates with toluene were more stable than others. This observation emphasizes upon selection of specific biosurfactant for particular hydrocarbon pollution (22). The results of oil displacement assay indicated a very feeble activity from most of the isolates (Table 2). This could be attributed to the very low concentration of biosurfactant in the supernatant, as this assay is sensitive to as low as 10 nmol of biosurfactant concentration (14).

In our study the results of biosurfactant production could not be correlated to crude oil degradation in all the isolates as the observations made were contradictory. The highest crude oil degrader, M2 did not produce detectable levels of biosurfactant. This result is not in agreement with the published reports which say that biosurfactant producing bacteria are efficient crude oil degraders as well (23). The mode of hydrocarbon uptake is different for different bacteria and biosurfactants mostly enhance the attachment of the hydrocarbon to the substrate .Thus if biosurfactant production does not augment the mode of hydrocarbon uptake by the cell it may not always ensure enhanced biodegradation. Another reason might be that biosurfactants have been mostly reported to desorb the hydrocarbons from soil in mesocosm studies making them more and more bioavailable to the microorganisms in the soil but in experimental liquid media where hydrocarbons are easily available to the bacteria biosurfactants may not have any significant role to play (24).

In the hemolysis assay, none of the isolate gave clear zone on the blood agar. This may be because the biosurfactants falling under the category of lipopeptides and lipoproteins mainly show the property of hemolysis of mammalian blood so none of the bacterial isolates might be producing biosurfactant belonging to this category (6).

On the basis of colony morphology, staining and biochemical characteristics M1 was a member of genus Acinetobacter. Similarly M2 and M6 belonged to genera Marinococcus but they differed at species level due to differences in biochemical properties and pigmentation. Isolate M5 belonged to Micrococcus genera, while M3 was identified as Planococcus. Both M4 and M9 belonged to genus Rhodococcus. M8 was Methylobacterium, M7 and M10 due to their distinct colony morphology and biochemical features were similar with members of Noccardia genus (25, 26 and 27).