Environmental Engineering Reference
In-Depth Information
waters enriched with naphthalene,
Cycloclasticus
and
Pseudomonas
were found to be abundant,
but
Pseudomonas
appeared in the latter stages
of the enrichment (Niepceron et al.
2010
). It is
envisaged that though there is single carbon and
energy source, both species are able to coexist
presumably, because not all PAHs are oxidized to
CO
2
and H
2
O by a single organism; intermediate
oxidation products are formed, which are utilized
by other microbes as carbon and energy source
(McGenity et al.
2012
). One of the lesser appre-
ciated microbes in the context of oil spill biore-
mediation are fungi found in marine mats (Allen
et al.
2009
) and many of them are also reported
to be salt adapted (Valentin et al.
2006
), which
may play a major role in degradation of coastal
PAH (Frey-Klett et al.
2011
). Filamentous fungal
networks provide the so-called “fungal highway”
of continuous liquid films in which chemoattrac-
tants provide a gradient for directional transport
of hydrocarbon-degrading bacteria to the pollut-
ant (Furuno et al.
2010
). High molecular weight
PAHs strongly adsorb minerals and associated
organic matter and, thus their bioavailability de-
creases. Microbes often circumvent this problem
by either colonizing on the surface of minerals
or producing biosurfactants (biological surface
active agents with dual hydrophobic and hydro-
philic moieties), which minimize the diffusion
time and enhance bioavailability and desorption
of PAHs (Guerin and Boyd
1992
; Perfumo et al.
2010
).
A. borkumensis
produces surfactants that
increase the bioavailability of PAHs for other mi-
crobes. Though it does not use the PAHs as car-
bon or energy source itself, the biosurfactants so
produced may be helpful in reducing the stress
due to accumulation of toxic PAHs (McGenity
et al.
2012
). Knowledge about cooperative be-
havior of microbes in establishing self-sufficient
community, which is pivotal in biodegradation of
petroleum fractions, will help choosing the mi-
crobe or consortium of microbes for bioaugmen-
tation.
In a real-life incident of oil spill, it is essential
to know which type of microbe is best suited for
bioaugmentation. The above discussion throws
some light on potential candidates for bioaug-
mentation, however, to unequivocally decide on
the right candidate, it is essential to know more
about microbial interactions and isolate more of
the still uncultivable marine bacterial species for
oil spill bioremediation. Conventional methods
to cultivate marine bacteria have been unsuccess-
ful, since many of them live as oligotrophs and
do not adapt to high carbon-containing media.
“Extinction culturing” has been used to isolate
hydrocarbon-degrading marine bacteria. In ex-
tinction culturing, microorganisms are grown in
natural sea water as medium at a density rang-
ing from 1 to 10 cells per tube. The recovery of
bacteria by this method is 2-60 % in comparison
to 0.1 % obtained from conventional culturing
methods (Button et al.
1993
). Several culture-
independent rRNA based approaches such as 16S
rRNA gene (rDNA) clone libraries, fluorescence
in situ
hybridization (FISH) with rRNA-targeted
oligonucleotide probes (Pernthaler et al.
2002
),
and denaturing gradient gel electrophoresis
(DGGE) of PCR-amplified rDNA (Baker et al.
2003
) have revealed a surprising diversity in ma-
rine bacteria (Harayama et al.
2004
). It has been
suggested that uncultivability of such bacteria in
axenic culture is due to the unavailability of sec-
ondary factors (metabolites and/or signaling mol-
ecules), which are produced by other microbes.
Metabolites such as biosurfactants,
N
-acyl ho-
moserine lactones, and cyclic AMP (cAMP) have
been documented to increase the cultivability
of bacteria. Extracellular polysaccharides from
Rhodococcus rhodochrous
function as biosur-
factant and encourage the growth of
Cycloclas-
ticus sp.
(Iwabuchi et al.
2002
). The addition of
N
-acyl homoserine lactones helps in cell-to-cell
communication in Gram-negative bacteria and
brings enhanced cultivability of marine bacteria
(Bruns et al.
2002
). Presence of cAMP (cyclic
adenosine monophosphate) also increases the re-
suscitation (recovery of cultivability) of starved
marine bacteria (Bruns et al.
2002
). Anaerobic
degradation of hydrocarbons can take place in
environments where oxygen concentration is
often limiting such as mangroves, aquifers, and
sludge digesters. Despite the absence of oxygen
for the activation of hydrocarbons in anaerobic
species, diverse metabolic pathways exist which
help petroleum hydrocarbon degradation. These
Search WWH ::
Custom Search