Polyhydroxyalkanoates (PHAs) are polyesters synthesized as a carbon and energy reserve material by a wide number of bacteria. These polymers are characterized by their thermoplastic properties similar to those of plastics derived from the petrochemical industry, such as polyethylene and polypropylene. PHAs are widely used in the medical field and have the potential to be used in other applications due to their biocompatibility and biodegradability. Among PHAs, P(3HB-co-3HV) copolymers are thermo-elastomeric polyesters that are typically soft and flexible with low to no crystallinity, which can expand the range of applications of these bioplastics. Several bacterial species, such as Cupriavidus necator, Azotobacter vinelandii, Halomonas sp. and Bacillus megaterium, have been successfully used for P(3HB-co-3HV) production, both in batch and fed-batch cultures using different low-cost substrates, such as vegetable and fruit waste. Nevertheless, in recent years, several fermentation strategies using other microbial models, such as methanotrophic bacterial strains as well as halophilic bacteria, have been developed in order to improve PHA production in cultivation conditions that are easily implemented on a large scale. This review aims to summarize the recent trends in the production and recovery of PHA copolymers by fermentation, including different cultivation modalities, low-cost raw materials, as well as downstream strategies that have recently been developed with the purpose of producing copolymers, such as P(3HB-co-3HV), with suitable mechanical properties for applications in the biomedical field.

Alginate is a biomaterial produced by Azotobacter vinelandii, a diazotroph that, under nitrogen-fixing conditions, can fix nitrogen under high oxygen levels. In A. vinelandii, alginate is synthesized from fructose-6P via synthesis of precursor, polymerization, and modification/exportation. Due to its viscosifying, gelling, and thickening characteristics, alginate is widely used in food, pharmaceutical, and cosmetical industries. This study aimed to develop a continuous bioprocess and a comparative analysis of alginate production under diazotrophic and nondiazotrophic conditions. Continuous cultures were developed at three dilution rates (0.06, 0.08 and 0.10 h⁻¹). In steady state, the respiratory activity, alginate production, alginate molecular weight and the genes encoding alginate polymerase were determined. Under the conditions studied, the specific oxygen uptake rate and respiratory quotient were similar. The diazotrophic conditions improved the conversion of sucrose to alginate and the specific productivity rate, which was 0.24 ± 0.03 g g⁻¹ h⁻¹. A higher alginate molecular weight (725 ± 20 kDa) was also achieved under diazotrophic conditions, which can be explained by an increase in the gene expression of genes alg8 and alg44 (encoding polymerase). The results of this work show the feasibility of enhancing alginate production (yields and specific productivity rates) and quality (molecular weight) under nitrogen-fixing conditions, opening the possibility of developing a continuous bioprocess to produce alginate with specific characteristics under conditions of diazotrophy.

Azotobacter vinelandii OP is a bacterium that can produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P3HBV), a biodegradable and biocompatible polymer with applications in the biomedical field. This study aimed to evaluate P3HBV production and its 3-hydroxyvalerate (3HV) fraction under different agitation rates and oxygen uptake rates (q_O2) in chemostat cultures of A. vinelandii OP. Steady-state conditions with either oxygen or carbon limitation were established by modulating the agitation rates. Under oxygen-limited conditions (low q_O2 values) biomass and P3HBV concentrations increased to 3.3 g L⁻¹ and 2.1 g L⁻¹, respectively. At higher q_O2 values, the chemostat cultures were limited by carbon, and P3HBV content decreased from 62% to 33% (w w⁻¹). The highest 3HV molar fractions, 33.7 and 36.4 mol %, were observed at both the lowest and highest q_O2 levels, possibly linked to comparable valeric acid consumption rates. An elevated NAD(P)H/NAD(P)⁺ ratio was also observed under oxygen limitation, favoring polymer accumulation by indicating a more favorable intracellular redox state. These findings highlight the impact of nutrient limitation and respiratory activity on the biosynthesis of P3HBV and the 3HV composition by Azotobacter vinelandii OP. Such insights can support the development of tailored bioprocesses to modulate polymer characteristics, enabling a broader range of potential biomedical applications for P3HBV.