Review of literature
2.6. Recent expansions in the bioremediation of crude oil
2.6.3. Advantage of the microbial consortium over pure isolates
Like pure bacterial culture, a consortium's biodegradability and secondary metabolite productivity depend on physiological conditions. Hence, a designed bacterial consortium needs to be optimized to obtain the appropriate culture growth condition to achieve optimal results. Suganthi et al. isolated 3 hydrocarbon degraders (Shewanalla chilikensis MG452729, Halomonas hamiltonii MG452731, and Bacillus firmus MG452730) from oily sludge and used these isolates as a consortium for further bioremediation studies. Culture conditions such as pH, incubation time, temperature, biomass concentration and oily sludge concentration were optimized using the OFAT technique. The authors explored the bacterial growth, selective enzyme activity and biosurfactant yield as response factors for the analyses. The optimized culture condition obtained were: pH 7, 35 °C, 1 % (w/v) oily sludge concentration, 15 % (v/v) biomass concentration and 7 days of incubation time. Under the optimized condition, authors reported 96 % biodegradation of total petroleum hydrocarbon concentration and remarkably high hydrocarbon degradative enzyme activity, i.e., 68 U/mL oxidoreductase activity, 80 U/mL lipase activity 46 U/mL catalase activity (Suganthi et al., 2018). Summarily, the use of syntrophic microbes that share either mutualistic or commensalism-based interaction is the preliminary screening criteria of microbes before considering consortia members. Later, the growth conditions of microbes are optimized to achieve cumulative maximum growth and metabolite production.
strain IITR48, which raised to 96.5 % when performed by microconsortium. Other PAH such as benzo(b)fluoranthene and fluorene also showed 1.4 folds and 1.6 folds higher biodegradability in the consortium than axenic culture. The authors highlighted the role of coordinated metabolic activity in the consortium that led to better results (Kumari et al., 2018). Table 2.8. summarizes the comparative change in biodegradation ability of compounds by a few bacterial species when grown in microconsortium over axenic culture.
Table 2.8. Comparison of biodegradation ability of various toxic compounds by pure (axenic) culture and their consortium
S.no. Compounds Microbes used Reaction conditions Efficiency Ref
1. Natural rubber
Rhodococcus pyridinivorans
Consortium (indigenous soil- inhabiting microbes including Rhodococcus pyridinivorans)
Incubated in MSM supplemented with dried latex glove pieces (0.6 %, w/v) as a sole carbon source at 30
°C, 150 rpm for 4 weeks.
9.36 %
18.38 %
(Nawong et al., 2018)
2.
Saturated fractions of oily sludge
S. acidaminiphila B. megaterium B. cibi
P. aeruginosa B. cereus
Consortium (all the strains as
mentioned earlier) Incubated with 1 % oily sludge as the sole carbon source, kept at 100 rpm, and 30 °C for 40 days.
91.7 % 89.0 % 89.7 % 86.7 % 88.4 % 90.7 %
(Cerqueira et al., 2011)
Aromatic fractions of oily sludge
S. acidaminiphila B. megaterium B. cibi
P. aeruginosa B. cereus
Consortium (all the above- mentioned strains)
33.2 % 39.6 % 64.3 % 39.5 % 40.3 % 51.8 %
3. Phenanthrene
Bacillus sp. ASP1 Pseudomonas sp. ASP2
Stenotrophomonas fsmaltophilia ASP3
Staphylococcus sp. ASP4 Geobacillus sp. ASP5 Alcaligenes sp. ASP6
Consortium (all the strains mentioned above)
Incubated with Phenanthrene (300 ppm) using 4 % (v/v) inoculum at 37°C, and 150 rpm for 120 h
29 % 38.66 % 52 %
38 % 43 % 43 % 76 %
(Patel et al., 2013)
4. Crude oil
Raoultella ornithinolytica PS Bacillus subtilis BJ11 Acinetobacter lwoffii BJ10 Acinetobacter pittii BJ6 Serratia marcescens PL
Consortium (all the strains as mentioned above)
Incubated in 10 % (v/v) inoculum, 0.4% crude oil (w/v), 30 °C,180 rpm for 10 days
83.5 % 81.1 % 75.80 % 74.90 % 70.00 % 94.00 %
(Bidja Abena et al., 2019)
5. Bisphenol A
Pseudomonas knackmussii Consortium
(Inherent microbes from contaminated river sediment, predominantly Pseudomonas knackmussii)
Basal salt medium (BSM) supplemented with 10 ppm of BPA,150 rpm at 30 °C
100 % in 7 days
100 % in 28 h (Peng et al., 2015)
6. Octachlorodibenzo -p-dioxin (OCDD)
P. mendocina NSYSU
Consortium (Inherent microbes from contaminated soil with bioaugmentation of
P. mendocina NSYSU)
NB broth, room temperature (20 °C) for the 65-day incubation
68 %
62 % (reduced due to competition with indigenous microbes)
(Tu et al., 2014)
Along with metabolic activity, increased biosurfactant production was also reported by Alves et al.
during co-culturing of biofilm-forming bacterial strains Pseudomonas aeruginosa ATCC 27853 with model biosurfactant producing Pseudomonas sp. While during the axenic study, the overall rhamnolipid production was reported to be 53.5 mg/L, a 2.4 times increase in the biosurfactant yield (i.e., 129 mg/L) was stated in the presence of co-culturing with biofilm-forming Pseudomonas aeruginosa, expressing its role as inducer and stimulator in the consortium (Alves et al., 2019). Hence, microbes complement one another in a consortium by acting as an inducer or stimulator of essential metabolic pathways.
Interestingly, apart from improved biosurfactant production, different isoforms of biosurfactant have also been reported during consortia study over axenic growth. Ibrar et al. constructed a microbial consortium to enhance the overall biodegradation activity and biosurfactant yield in this approach. More than 60 % biodegradation of glyceryl tributyrate (GT) was obtained by using a microconsortium comprising 4 strains of Lysinibacillus spp. (HC_B, HC_C, HC_4, and HC_4L), Paenibacillus sp.
(HC_A), Gordonia spp. (HC_8A) and Cupriavidus sp. (HC_D). Whereas the axenic growth of Cupriavidus sp. exhibited poor biodegradative activity of 30-45 %, the other bacterial species (Lysinibacillus, Paenibacillus, and Gordonia) exhibited 45-60 % biodegradation. Thus, heptapeptide isoforms during consortium growth led to increased emulsification and biodegradation activity (Ibrar and Zhang, 2020). Kanaly et al. also reported new metabolic activity in Rhodanobacter sp., which otherwise could not grow on benzo[a]pyrene as the sole substrate. On the other hand, as a part of the consortium, by the action of mineralization and solubilization of benzo[a]pyrene by other members of the consortium, it could utilize intermediates of catabolism and hence was reported to be actively participating in the overall degradation, exhibiting a two-fold higher biodegradative activity (Kanaly et al., 2002). Such induction of new metabolite production elucidates the assertive effect of microconsortium over pure cultures.
Consortium also reveals commensalism within members, where one species initiates the degradation of contaminant present, so that rest co-surviving species can thrive on catabolic metabolites released in the environment and degrade the contaminant more effectively. Wanapaisan and the group also suggested a synergistic effect of the microbial consortium over pure culture. In this study, pyrene
hydrocarbon was used as a model high molecular weight contaminant to analyze the bioremediation activity of consortium procured from mangrove sediments. The study stated that the inherent sediment microconsortium was primarily enriched with Mycobacterium spp. strains (PO1 and PO2), capable of utilizing pyrene as a C source. However, the other components of the consortium, i.e., Novosphingobium pentaromativorans PY1, Ochrobactrum sp. PW1 and Bacillus sp. FW1 strains were not able to grow in pyrene enriched agar. Such different growth patterns of various consortium components revealed the co-existence of non-pyrene degraders. Further bioinformatics study explained the occurrence of genes in strains PY1 and PW1 responsible for the catabolism of intermediates of the pyrene degradation pathway. However, strain FW1 lacked genes involved in the biodegradation of pyrene or its intermediates. Interestingly, FW was responsible for the assimilation of pyrene, improving its bioavailability for other bacteria due to its ability to produce biosurfactants. Hence, such diverse bacteria in the consortium (PO1, PO2, PY1, PW1 and FW1) led to > 80 % pyrene degradation within 72 h of incubation, which was more than 2 folds higher than the biodegradation activity of axenic Mycobacterium spp. (PO1, PO2) (Wanapaisan et al., 2018).
Though indigenous microbes are major bioremediating agents; however, the low bioavailability and biotoxicity of hazardous contaminants at high concentrations, the biodegradation is severely compromised. Exogenously augmented microbes add the genetic and metabolic diversity to the contamination site, and hence significant tolerance is attained by the consortium, and their degradation capability is also broadened. Hence, in a few cases, augmenting the contaminated site with new microbes is vital to improving overall survivability and thus bioremediation activity (Yuan et al., 2018).
Ebadi et al. explored the bioremediation activity in a harsh salt-rich contaminated site. Consortia enriched with oil-degraders and biosurfactant producers, Pseudomonas aeruginosa strains (T4, T27, T30, and E1) were prepared. The obtained results stated that in the presence of 30 g/kg of initial crude oil concentration, with an increase in salinity from 0 to 300 mM, the biodegradation rate constant, k (day-1) of inherent consortium varied from 0.002 to 0.0009, which got modified to 0.0049 to 0.0035, expressing the augmenting role of Pseudomonas strains. In concordance, the authors also investigated the effect of catabolic enzyme dehydrogenase in the given experimental conditions. The results reported an approximate 2 fold boost in the enzyme activity in the case of bio augmented (6.35 ± 0.62 µg g-1 h-
1) than inherent consortium (3.41± 0.59 µg g-1 h-1) (Ebadi et al., 2017). Various researchers have also reported the enhancement in the catabolic and degradative enzyme activity in the presence of metabolically enriched consortia (Loureiro et al., 2020; Suganthi et al., 2018).
Thus, the microbial consortium is highly significant in the overall biodegradation of contaminants over pure cultures due to the cooperative and mutualistic effect among members that leads to an increase in metabolic activity and sometimes induces the production of novel pathways for the contaminant degradation and metabolite production. The development of synthetic microbial consortium has been successfully exploited in various environmental remediation and industrial metabolite production applications. Nevertheless, potential insight in this domain is challenging due to the unavailability of smart system biology, bioinformatics and metagenomics tools.