Crude oil is a complex mixture of hydrocarbons, including paraffins, aromatics, resins, and asphaltenes in which paraffins typically form the major part. most of the reservoirs in the world are matured due to continuous production of crude oil over the past several decades. this results in the production of heavier hydrocarbons primarily containing long chain paraffins (wax) in the reservoir. these waxes are present in the dissolved state at in situ reservoir conditions. the separation and deposition of these waxy components in the production and surface facilities are predominant when the system temperature reduces below the wax appearance temperature during the flow of crude oil from the reservoir to the surface. the precipitation of waxy crystals may pose a serious threat for flow assurance and for enhanced oil recovery (eor). it is, therefore, necessary to address various challenges posed by long chain paraffins using an economical, versatile, and eco-friendly technique. in the current scenario, microbial degradation of hydrocarbons has gained considerable attention because of its environmentally friendly and operationally safer process than the other conventional methods for the sustainable development. the microorganism used for the degradation and the nutrients required for the growth of microorganism are inexpensive, and easy to obtain and handle in the field. microbial technique reduces the secondary treatment required in other conventional wax removal techniques due to several metabolites produced during the processes. the objective of this work is to study the degradation of paraffin waxes (c16h34 to c36h74), model crude oil and waxy crude oil using biosurfactant producing mesophilic strains, such as pseudomonas aeruginosa (p. aeruginosa) and pseudomonas fluorescens (p. fluorescens), and a thermophilic strain bacillus subtilis (b. subtilis) at various environmental conditions. initially, the study has been carried out to determine the effect of the biosurfactant production and degradation of the long chain paraffins using two different mesophilic strains (p. aeruginosa and p. fluorescens). subsequently, the study has been extended for waxy crude oil, model crude oil, and long chain paraffins using a thermophilic strain (b. subtilis). finally, the performance of the thermophilic strain for the viscosity reduction of hydrocarbon was examined at high pressure and high temperature condition. the first part of the study involves the performance of the biosurfactant producing mesophilic strains, such as p. aeruginosa and p. fluorescens in the presence of long chain paraffins (c16h34 and c20h42). the fourier transform infrared spectroscopy (ftir) has been used to determine the functional group of the biosurfactants produced by the two mesophilc strains with c16h34 (c16) and c20h42 (c20). in all the cases, spectra showed the presence of alkyl group (2700 - 3000 cm−1), alcoholic group (oh) (3400 cm−1), carboxylic acid (c=o) (1700 cm-1) and glycosidic bond (co-c) (1100 cm-1). the characteristics of the biosurfactant produced by the two organisms with different substrates has been determined using gas chromatography- mass spectrometry (gc-ms). it is inferred that the biosurfactant produced by p. aeruginosa contains predominantly c10 and c12 fatty acids, whereas that produced by p. fluorescens contains c8 and c10 fatty acids, which are the homologues of rhamnolipid. the latter organism produced 9.8 and 8.2 g l-1 of rhamnolipid with c20 and c16 as substrates, respectively, and the former produced 5.7 and 11.5 g l-1, respectively. the performance and potential of the biosurfactant for crude oil applications involves determining the physico-chemical properties, such as critical micelle concentration (cmc), cell surface hydrophobicity (csh), surface tension (st), and emulsification activity. p. aeruginosa reduced the surface tension against c16 and c20 to 15 and 29 mn/m (from initial 71 and 70 mn/m), respectively, whereas p. fluorescens reduced it to 27 and 18 mn/m (from initial 69 and 70 mn/m), respectively. the biosurfactant produced from both the microorganisms are stable upto 100ºc temperature, 8 mpa pressure, 20% (w/v) salinity and alkaline ph. the next part of the study discusses about the degradation and viscosity reduction of long chain paraffins (c16 and c20) using p. aeruginosa and p. fluorescens. the degradation of paraffins is studied using carbon profiling and reduction in viscosity pre- and post-degradation of paraffins. the carbon profiling has been obtained using gc-ms, and ftir results. p. aeruginosa has degraded about 93% of c16 and 77% of c20 in 1 day, whereas p. fluorescens degraded 78 and 85%, respectively. it is inferred that p. aeruginosa is effective in degrading c16, and p. fluorescens is effective in degrading c20. the viscosity of the culture supernatant with c16 is reduced by 53 and 47% with p. aeruginosa and p. fluorescens, respectively, whereas with c20 it is reduced by 53 and 65% in 1 day, respectively. these results show the potential of these microorganisms to degrade waxes. this study also presents the information on the activity of enzymes responsible for the hydrocarbon degradation. the use of pseudomonas species have shown their use in potential application for bioremediation, oil-spill treatment, and flow assurance. next part involves the performance of kinetic studies by monitoring the production of biosurfactant, degradation of hydrocarbons and the growth of microorganisms with both the pseudomonas species as a function of time. two types of models were used for kinetic studies, namely the monod and the logistic models. for all the cases, the logistic model has matched with the experimental values satisfactorily (regression coefficient > 0.98) while the monod model was found to be in agreement with the data only at the initial time periods. the next part encompasses the performance of the biosurfactant producing b. subtilis in the presence of the long chain paraffins, namely c16h34 (c16), c20h42 (c20), c24h50 (c24), c28h58 (c28) c32h66 (c32) and c36h74 (c36), model crude oil and waxy crude oil at various temperatures, such as 35, 50 and 75˚c. the ftir spectra of the produced biosurfactant confirms the presence of cyclic lipopeptide. mass spectra of the biosurfactant indicates that the chemical nature of the produced lipopeptide is c14 fatty acid, which is a homologue of surfactin. the biosurfactant produced at 50˚c has found to be higher than that produced at 35 and 75˚c. b. subtilis produces 4.3 and 3.4 g l-1 of surfactin in the presence of waxy crude oil and model crude oil as substrates, respectively at 50˚c. the maximum surfactin production in the presence of paraffins, viz., c16, c20, c24, c28, c32 and c36 as substrates at 50˚c are 5.3, 5.2, 4.5, 4.6, 3.8 and 3.8 g l-1, respectively. the surfactin production has decreased with increase in carbon number of the substrate. the produced biosurfactant is able to withstand pressures upto 10 mpa, temperature upto 120ºc, salinity upto 10%, w/v, and ph above 8. b. subtilis in the presence of waxy crude oil has been observed to show high microbial adherence, improved surface tension reduction and emulsification activity, production of higher amount of biosurfactant ‘surfactin’, and improved biosurfactant stability. this indicates the potential of microorganism in tackling oil-spill, wax degradation, flow assurance and eor. next part involves the utilization of b. subtilis for the degradation of various long chain paraffins at 35, 50 and 75ºc. the degradation of c16, c20, c24, c28, c32 and c36, at 50ºc and in 10 days were 98, 94, 91, 85, 80 and 78%, respectively, whereas the respective values in 1 day were 77, 71, 68, 64, 63 and 60%. increase in carbon number increases the time required for degradation process, and decreases the percentage degradation of paraffins. though, the extent of degradation reduces with the increase in carbon number, about 60 to 70% of c32 and c36 has degraded in 1 day. this shows that the b. subtilis has good degradation capabilities at 50˚c on pure long chain paraffins. the viscosity of pure paraffins has reduced with decrease in carbon number and increase in degradation time. the relationship between the activation energy required to initiate the degradation and the activity of enzymes responsible for the degradation has also been analyzed. activation energy has increased with decrease in enzymatic activity, and with increase in carbon number. the further part presents the utilization of b. subtilis for the degradation of the waxy crude oil and model oil. the effectiveness of the b. subtilis in degrading the model oil is compared with the waxy crude oil. the extent of degradation was determined by gc-ms analysis. it is observed that 70% of the waxy crude oil and 49% of the model crude oil has degraded at 50˚c in 1 day. the extent of crude oil degradation was observed to be 85 and 80% after 10 days of incubation at 35 and 75˚c, respectively. the corresponding values with model oil was observed to be 82 and 76%. as the molecular weight of the crude oil increases, the percentage of recalcitrant waxy molecules increases, and makes the substrate more hydrophobic. presence of high amount of saturates may be one of the reasons for the less degradation observed with model oil when compared to waxy crude oil. the degradation of crude oil and model oil at 50˚c is observed to be more than the degradation occurred at 35 or 75˚c, and it is in-line with the bacterial growth rate with respect to the temperature. finally, this work targets to examine the ability of b. subtilis, in reducing the viscosity of waxy crude oil at different pressure and temperature in order to mimic the hydrocarbon degradation at reservoir conditions. this study give insights into the flow properties of hydrocarbon post-degradation, which would help in the development of better reservoir models for microbial eor. experiments have been carried out at 0.1, 1, 5 and 10 mpa at 25, 50, 75 and 100oc using a high pressure rheometer for 12 hours. the maximum bacterial growth was observed at 50°c, and 1 mpa, which corresponds to the higher reduction in the viscosity and maximum increase in the api gravity of the crude oil. generally, the viscosity of crude oil will increase with the increase in pressure. however, with the help of b. subtilis, the significant reduction in the viscosity of crude oil has been observed at 0.1, 1 and 5 mpa as compared to higher pressure of 10 mpa. studies indicated that elevated pressure can have an adverse effect on the metabolism of bacteria and make the organism inactive, which may be the reason for lower reduction in the viscosity at higher pressure of 10 mpa. the present study will add value in understanding the microbial degradation of waxy crude oil and for possible robust model development of microbial degradation of complex mixtures of hydrocarbon systems suitable for upstream oil and gas applications, such as enhanced oil recovery and flow assurance.