Challenges and Perspectives of Polyhydroxyalkanoate Production From Microalgae/Cyanobacteria and Bacteria as Microbial Factories: An Assessment of Hybrid Biological System

Abstract

Polyhydroxyalkanoates (PHAs) are the biopolymer of choice if we look for a substitute of petroleum-based non-biodegradable plastics. Microbial production of PHAs as carbon reserves has been studied for decades and PHAs are gaining attention for a wide range of applications in various fields. Still, their uneconomical production is the major concern largely attributed to high cost of organic substrates for PHA producing heterotrophic bacteria. Therefore, microalgae/cyanobacteria, being photoautotrophic, prove to have an edge over heterotrophic bacteria. They have minimal metabolic requirements, such as inorganic nutrients (CO2, N, P, etc.) and light, and they can survive under adverse environmental conditions. PHA production under photoautotrophic conditions has been reported from cyanobacteria, the only candidate among prokaryotes, and few of the eukaryotic microalgae. However, an efficient cultivation system is still required for photoautotrophic PHA production to overcome the limitations associated with (1) stringent management of closed photobioreactors and (2) optimization of monoculture in open pond culture. Thus, a hybrid system is a necessity, involving the participation of microalgae/cyanobacteria and bacteria, i.e., both photoautotrophic and heterotrophic components having mutual interactive benefits for each other under different cultivation regime, e.g., mixotrophic, successive two modules, consortium based, etc. Along with this, further strategies like optimization of culture conditions (N, P, light exposure, CO2 dynamics, etc.), bioengineering, efficient downstream processes, and the application of mathematical/network modeling of metabolic pathways to improve PHA production are the key areas discussed here. Conclusively, this review aims to critically analyze cyanobacteria as PHA producers and proposes economically sustainable production of PHA from microbial autotrophs and heterotrophs in “hybrid biological system.”

Green Chemistry, Biocatalysis, and the Chemical Industry of the Future

“…Interestingly, the bio-based plastics, polyhydroxyalkanoates (PHAs) can be produced directly from CO 2 using cyanobacteria, but better productivities were observed using a hybrid mixotrophic system comprising an autotrophic cyanobacterium and a heterotrophic bacterium working synergistically. [81] Cyanobacteria can also be used for the production of higher added-value chemicals, such as pharmaceutical intermediates. For example, the light-driven enantioselective reduction of aryl methyl ketones catalyzed by whole cells of a wild-type cyanobacterium, Synechococcus elongatus PCC 7942, was already described by Nakamura et al [82,83] in 2000.…”

Microalgae Xanthophylls: From Biosynthesis Pathway and Production Techniques to Encapsulation Development

“…Microalgae and cyanobacteria are a highly diverse group of photoautotrophs. These microorganisms are present across all aquatic environments [1]. Common microalgae ancestors lived in an aquatic environment approximately 3 billion years ago and in this period, microalgae evolved and diversified [2].…”

Photoautotrophic and Mixotrophic Cultivation of Polyhydroxyalkanoate-Accumulating Microalgae Consortia Selected under Nitrogen and Phosphate Limitation

“…Photoautotrophic PHA production has been reported in diverse strains of cyanobacteria and a few strains of green microalgae, but the PHA biomass content is relatively low compared to that of heterotrophic PHA-producing bacteria [13]. However, PHA accumulation by microalgae can be promoted to satisfactory levels under optimized conditions, such as sufficient carbon and light sources, optimal temperature and pH, and adequate concentrations of nutrients and metal ions in the culture medium.…”

“…Furthermore, supplementation of organic carbon sources, such as simple sugars and volatile fatty acids, allows the microalgae to grow mixotrophically. Nutrient starvation and/or salt stress conditions were also reported to enhance PHA accumulation by the microalgae [7,13,14]. Successive mixotrophic PHA production has been mainly reported with monoseptic culture of microalgae.…”