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Microbial polyhydroxyalkanoates (PHA) are biodegradable and biocompatible bio-based polyesters, which are used in various applications including packaging, medical and coating materials. In this study, an extremophilic hydrocarbonoclastic bacterium, previously isolated from saline sediment in the Tunisian desert, has been investigated for PHA production. The accumulation of intracellular PHA granules in Halomonas desertis G11 was detected by Nile blue A staining of the colonies. To achieve maximum PHA yield by the strain G11, the culture conditions were optimized through response surface methodology (RSM) employing a Box-Behnken Design (BBD) with three independent variables, namely, substrate concentration (1–5%), inoculum size (1–5%) and incubation time (5–15 days). Under optimized conditions, G11 strain produced 1.5 g/L (68% of DCW) of PHA using glycerol as a substrate. Application of NMR (1H and 13C) and FTIR spectroscopies showed that H. desertis accumulated PHA is a poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV). The genome analysis revealed the presence of typical structural genes involved in PHBV metabolism including phaA, phaB, phaC, phaP, phaZ, and phaR, coding for acetyl-CoA acetyltransferase, acetoacetyl-CoA reductase, class I polyhydroxyalkanoates synthases, phasin, polyhydroxyalkanoates depolymerase and polyhydroxyalkanoates synthesis repressor, respectively. Glycerol can be metabolized to 1) acetyl-CoA through the glycolysis pathway and subsequently converted to the 3HB monomer, and 2) to propionyl-CoA via the threonine biosynthetic pathway and subsequently converted to the 3HV monomer. In silico analysis of PhaC1 from H. desertis G11 indicated that this enzyme belongs to Class I PHA synthase family with a “lipase box”-like sequence (SYCVG). All these characteristics make the extremophilic bacterium H. desertis G11 a promising cell factory for the conversion of bio-renewable glycerol to high-value PHBV.

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Bio-based ethanol production from enzymatic lignocellulosic biomass degradation serves an efficient technology to combat the problem of depleting fossil fuels. High cost of the enzyme is the major obstacle in preventing the commercialization of this process. Thus, main objective of the present study was to optimize composition of culture parameters for enhancing cellulase production by two halophilic strains Oceanobacillus picture E50 and Bacillus vallismortis J77 using Posidonia oceanica waste (POW) as a substrate. A Plackett-Burman design was conducted to study and evaluate the weight of eight cultural variables on bacterial cellulase production. The optimal culture conditions for maximum enzyme yield were developed by maintaining the variables of POW concentration, NaCl concentration, POW liquor, bacterial strain and inoculum size at their higher levels, while keeping substrate particle size, glucose concentration and incubation time at their lower levels. The highest cellulase production in terms of CMCase activity was 950 U/L following this optimization process. Thus, the results obtained in this study demonstrated the potential of utilizing POW as a substrate for enhanced cellulase production by halophilic bacteria. B. vallismortis J77 produced the highest cellulase activity and therefore could be of great interest for saccharification of Posidonia oceanica biomass for biofuel and biorefinery applications.