situations, as opposed to chemical dispersants, are currently the limiting factors that
prevent their application in the field (Patel et al. 2019).
A recently isolated B. Subtilis BL27 (Wang et al. 2019b) was found to be
enhanced by SDS and Tween 80 while being indifferent to the addition of
biosurfactants rhamnolipid and surfactin. Further, addition of CTAB and TTAB
were found to be highly toxic.
Biosurfactants produced by one species of microorganisms may damage the cell
membranes of other microbial species or strains. Therefore, when using microbial
consortia, this aspect needs to be considered. The use of naturally occurring communities in bioremediation efforts (Perera et al. 2019) may help to overcome this
hurdle. In their studies, a natural biofilm-producing bacterial–fungal consortium
(comprising Aspergillus sp. MM1 and Bacillus sp. MM1) demonstrated synergistic
degradation of hexadecane (Perera et al. 2019) and crude oil (Perera et al. pers.
comm). While both were biosurfactant producers, Bacillus sp. MM1 produced
comparatively higher biosurfactant than the consortium, indicating it reduces its
production in the presence of the fungus. This is in conformity with a previous report
(Benoit et al. 2015) which indicated that surfactin production by B. subtilis was
reduced when co-cultivated with Aspergillus. This could be due to the active
adaptation of the Bacillus to the fungus. Surfactin is not only a powerful
biosurfactant, but an antifungal agent (Sarwar et al. 2018).
These reports highlight the need for good understanding of the requirements of
the organism or community, prior to their use in a bioremediation system.
11.8 External Factors Affecting Biodegradation
In addition to the presence of suitable microbes, several other factors also affect the
biodegradation of petroleum hydrocarbons. The concentration of crude oil in the
polluted site greatly affects the biodegradation capacity of the microbes, with
increasing concentrations above 2% reported to decrease biodegradation efficiency
(Chen et al. 2017). Similarly, environmental factors such as pH and temperature
directly affect bacterial survival and growth, consequently affecting biodegradation.
The availability of oxygen is also vital in aerobic biodegradation. A knowledge of
the factors that influence bioremediation is thus valuable in developing cost-effective
bioremediation strategies (Varjani 2017).
11.8.1 Temperature
Cui et al. (2019) in their study observed that while biodegradation rates for PHs
increased with increasing temperatures up to ~30
C, there was no significant
difference between 30 and 40
C in their studies using activated sludge in an airlift
loop bioreactor. These results are similar to our own findings, using an Aspergillus–
11 Microbial Bioremediation of Petroleum Hydrocarbons
283
prevent their application in the field (Patel et al. 2019).
A recently isolated B. Subtilis BL27 (Wang et al. 2019b) was found to be
enhanced by SDS and Tween 80 while being indifferent to the addition of
biosurfactants rhamnolipid and surfactin. Further, addition of CTAB and TTAB
were found to be highly toxic.
Biosurfactants produced by one species of microorganisms may damage the cell
membranes of other microbial species or strains. Therefore, when using microbial
consortia, this aspect needs to be considered. The use of naturally occurring communities in bioremediation efforts (Perera et al. 2019) may help to overcome this
hurdle. In their studies, a natural biofilm-producing bacterial–fungal consortium
(comprising Aspergillus sp. MM1 and Bacillus sp. MM1) demonstrated synergistic
degradation of hexadecane (Perera et al. 2019) and crude oil (Perera et al. pers.
comm). While both were biosurfactant producers, Bacillus sp. MM1 produced
comparatively higher biosurfactant than the consortium, indicating it reduces its
production in the presence of the fungus. This is in conformity with a previous report
(Benoit et al. 2015) which indicated that surfactin production by B. subtilis was
reduced when co-cultivated with Aspergillus. This could be due to the active
adaptation of the Bacillus to the fungus. Surfactin is not only a powerful
biosurfactant, but an antifungal agent (Sarwar et al. 2018).
These reports highlight the need for good understanding of the requirements of
the organism or community, prior to their use in a bioremediation system.
11.8 External Factors Affecting Biodegradation
In addition to the presence of suitable microbes, several other factors also affect the
biodegradation of petroleum hydrocarbons. The concentration of crude oil in the
polluted site greatly affects the biodegradation capacity of the microbes, with
increasing concentrations above 2% reported to decrease biodegradation efficiency
(Chen et al. 2017). Similarly, environmental factors such as pH and temperature
directly affect bacterial survival and growth, consequently affecting biodegradation.
The availability of oxygen is also vital in aerobic biodegradation. A knowledge of
the factors that influence bioremediation is thus valuable in developing cost-effective
bioremediation strategies (Varjani 2017).
11.8.1 Temperature
Cui et al. (2019) in their study observed that while biodegradation rates for PHs
increased with increasing temperatures up to ~30
C, there was no significant
difference between 30 and 40
C in their studies using activated sludge in an airlift
loop bioreactor. These results are similar to our own findings, using an Aspergillus–
11 Microbial Bioremediation of Petroleum Hydrocarbons
283