Established and advanced approaches for recovery of microbial polyhydroxyalkanoate (PHA) biopolyesters from surrounding microbial biomass

Abstract

Downstream processing for recovery of microbial polyhydroxyalkanoate (PHA) biopolyesters from biomass constitutes an integral part of the entire PHA production chain; beside the feedstocks used for cultivation of PHA-production strains, this process is currently considered the major cost factor for PHA production.Besides economic aspects, PHA recovery techniques need to be sustainable by avoiding excessive use of (often precarious!) solvents, other hazardous chemicals, non-recyclable compounds, and energy. Moreover, the applied PHA recovery method is decisive for the molecular mass and purity of the obtained product, and the achievable recovery yield. In addition to the applied method, also the PHA content in biomass is decisive for the feasibility of a selected technique. Further, not all investigated recovery techniques are applicable for all types of PHA (crystalline versus amorphous PHA) and all PHA-producing microorganisms (robust versus fragile cell structures).The present review shines a light on benefits and shortcomings of established solvent-based, chemical, enzymatic, and mechanical methods for PHA recovery. Focus is dedicated on innovative, novel recovery strategies, encompassing the use of “green” solvents, application of classical “PHA anti-solvents” under pressurized conditions, ionic liquids, supercritical solvents, hypotonic cell disintegration for release of PHA granules, switchable anionic surfactants, and even digestion of non-PHA biomass by animals.The different established and novel techniques are compared in terms of PHA recovery yield, product purity, impact on PHA molar mass, scalability to industrial plants, and demand for chemicals, energy, and time.

Recovery of Polyhydroxyalkanoates From Single and Mixed Microbial Cultures: A Review

“…A similar concept must be applied if non-volatile solvents like ionic liquids (IL) are used: these solvents (e.g., 1-ethyl-3-methylimidazolium diethyl- or dimethylphosphate) are capable of interacting with bacterial cell wall components like peptidoglycan and phospholipids via intermolecular H-bonds or electrostatic interactions, dissolving the NPCM and suspending PHA stored inside the cells. But since they are not removable under vacuum due to their negligible vapor pressure, alcohols are required as anti-solvents to precipitate PHA or purifying PHA after filtration (even if filtrating IL’s solution needs large porosity filters because of the intrinsic high viscosity of the solution); contrarily to what happens with high-boiling point solvents like anisole, the purity of the extracted PHA is low (from 30 to 86% if filtration or precipitation are applied, respectively) ( Kobayashi et al, 2015 ; Dubey et al, 2018 ; Koller, 2020 ). For these reasons (low purity and “PHA-suspension” effect) the use of ILs seems more a pre-treatment than a real extraction, oriented to disaggregate the cellular components and then facilitate the action of an organic solvent capable of solubilizing PHA.…”

“…An alternative cell lysis approach recently described to recover PHA from single strains exploits whole microorganisms as cell-lytic agents (namely predatory bacteria of other Gram-negative bacteria), instead of isolated enzymes ( Koller, 2020 ); these predatory bacteria (e.g., the species Bdellovibrio bacteriovorus ) have a hydrolytic arsenal (e.g., extracellular-like PHA depolymerase) capable of breaking the cell walls of the prey (the culture of PHA-producing bacteria) in a relatively short time frame (24 h of “predation”), causing the release of the intracellular PHA in the culture medium ( Martínez et al, 2016 ). To avoid the further hydrolysis of the released PHA granules by extracellular-like PHA depolymerase (that can hydrolyze up to 80% of the PHA produced by the prey), engineering of the predator seems to be the method of choice ( Martínez et al, 2016 ).…”

Ethylic Esters as Green Solvents for the Extraction of Intracellular Polyhydroxyalkanoates Produced by Mixed Microbial Culture

“…The latter is an attractive procedure widely investigated, as evidenced by the great number of scientific papers, reviews or patents on this topic, mainly dealing with PHAs from pure culture [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. In alternative to chlorinated hydrocarbons, which are the best solvents for PHAs, many research studies address the use of green solvents, with low or no toxicity and possibly derived from biochemical conversion, to overcome ecological issues, limitations involving worker safety or stringent regulations on solvent traces in goods for particular applications [ 25 ].…”

The Synthesis, Characterization and Applications of Polyhydroxyalkanoates (PHAs) and PHA-Based Nanoparticles

“…A greater concentration of SDS cause the PHA to solubilize, lowering the yield [102]. However, because this method has a significant risk of generating hazardous halogenated chemicals, it is not widely used for extracting PHA from cells [100].…”

“…The digestion method is an alternative method for extracting PHA from the cell biomass as the solvent method involves the usage of various solvents which can be harmful and toxic to the environment when used on a large scale. When a cell is lysed, the membrane-bound PHA granules are loosened, and the hydrophilic non-PHA cell mass is altered to form a water-soluble material with the help of chemical or enzymatic activity [ 100 ]. It can be of two types: Chemical Digestion or Enzymatic Digestion.…”