The success of bioprocess implementation relies on the ability to achieve high volumetric productivities and requires working with high-cell-density cultivations. Elevated atmospheric pressure might constitute a promising tool for enhancing the oxygen transfer rate (OTR), the major growth-limiting factor for such cultivations. However, elevated pressure and its effects on the cellular environment also represent a potential source of stress for bacteria and may have negative effects on product formation. In order to determine whether elevated pressure can be applied for enhancing productivity in the case of medium-chain-length polyhydroxyalkanoate (mcl-PHA) production by Pseudomonas putida KT2440, the impact of a pressure of 7 bar on the cell physiology was assessed. It was established that cell growth was not inhibited by this pressure if dissolved oxygen tension (DOT) and dissolved carbon dioxide tension (DCT) were kept below ∼30 and ∼90 mg L(-1), respectively. Remarkably, a little increase of mcl-PHA volumetric productivity was observed under elevated pressure. Furthermore, the effect of DCT, which can reach substantial levels during high-cell-density processes run under elevated pressure, was investigated on cell physiology. A negative effect on product formation could be dismissed since no significant reduction of mcl-PHA content occurred up to a DCT of ∼540 mg L(-1). However, specific growth rate exhibited a significant decrease, indicating that successful high-cell-density processes under elevated pressure would be restricted to chemostats with low dilution rates and fed-batches with a small growth rate imposed during the final part. This study revealed that elevated pressure is an adequate and efficient way to enhance OTR and mcl-PHA productivity. We estimate that the oxygen provided to the culture broth under elevated pressure would be sufficient to triple mcl-PHA productivity in our chemostat system from 3.4 (at 1 bar) to 11 g L(-1)h(-1) (at 3.2 bar).
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