Physiological, kinetic, and process engineering aspects of polyhydroxyalkanoate biosynthesis by extremophiles

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Various extremophilic microbial species originating from diverse extreme environments such as hot springs, biotrickling filters, salterns, glaciers, or heavily polluted habitats, are reported in the current literature to display more or less pronounced potential for polyhydroxyalkanoate (PHA) biopolyester biosynthesis. In context with the current quest for new strategies towards enhanced PHA production processes, the chapter summarizes the present state of research on PHA production under such extreme conditions of temperature, salinity, pH-value, or levels of precarious compounds, which, for most mesophilic PHA-accumulating species, are strongly growth inhibiting or even life threatening. The chapter exposes that employing extremophiles as PHA producers displays a double-edged sword: On the one hand, extremophiles can easily be subjected towards long-term continuous cultivation processes, which considerably enhances overall productivity of the process and also reduces the energy demand in biopolymer production regarding sterility precautions, cooling, or heating; this advantage paves the way towards more cost-efficient PHA production. As a further benefit, many extremophilic PHA producers turned out to utilize industrial waste streams as 2nd-generation feedstocks for PHA production, which not only safes feedstock costs, but, moreover, contributes to value-added, safe treatment of waste materials. Furthermore, in some cases, applying extremophiles even facilitates downstream processing for PHA recovery by exposing PHA-rich cells to conditions favoring their disintegration. On the other hand, one has to get in grips with a range of remaining challenges, such as improving the metabolic capability of extremophiles, recycling of precarious spent fermentation broth, various process engineering aspects, and adaptation of bioreactor materials and process controlling devices to conditions shortening their life span. A considerable number of promising reports on novel PHA production processes based on phylogenetically highly versatile extremophilic organisms is summarized and comparatively assessed in the chapter at hand. It is shown that some processes were already established under controlled bioreactor conditions, using both continuous and discontinuous cultivation regimes. When analyzing the products obtained by these processes, it becomes clear that PHA from extremophiles can easily compete with PHA from usually applied mesophilic biopolymer production strains in terms of material properties. In some cases, “extremophile PHA” even outperforms the processibility of PHA from well-established production processes, e.g., by displaying extraordinarily high molecular masses. Based on the available data, one can currently conclude that especially those extremophile PHA production processes using robust, extremophile organisms, copiously accessible raw materials, and continuous cultivation mode hold realistic promise for future industrial-scale realization. Nevertheless, the route towards routinely implementing extremophile PHA producers is still cumbersome. Further assessment of novel and underexplored extremophile production strains with superior kinetics for growth and PHA accumulation on a range of inexpensive carbonaceous substrates will be needed in order to fully profit from the natural wealth of extremophile biopolyester producers. As suggested by individual studies, metabolic bottlenecks can be coped with genetic engineering in order to boost the kinetic performance of extremophiles. Considering the fact that the scientific community already has a rather clear picture about feasible inexpensive feedstocks to be used for cost-efficient PHA production, further process optimization has to emphasis on enhanced productivity and energy efficiency. Here, the application of robust extremophiles provides the chance to minimize energy for sterilizing, cooling, or heating.

Alkaliphiles; Archaea; Bacteria; Biopolymers; Extremophiles; Haloarchaea; Halophiles; Metalophiles; Polyhydroxyalkanoates (pha); Psychrophiles; Stress factors; Thermophiles

KOLLER, M.; OBRUČA, S.; PERNICOVÁ, I.; BRAUNEGG, G.

1. 9. 2021

978-1-53613-439-1

Polyhydroxyalkanoates: Biosynthesis, Chemical Structures and Applications

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