Microbial food from light, carbon dioxide and hydrogen gas: Kinetic, stoichiometric and nutritional potential of three purple bacteria

Harnessing nature's palette: Exploring photosynthetic pigments for sustainable biotechnology

Photosynthetic microorganisms such as cyanobacteria, microalgae, and anoxygenic phototrophic bacteria (APB) have emerged as sustainable and economic biotechnology platforms due to their ability to transform energy from light into chemicals through photosynthesis. The light is absorbed by photosynthetic pigment-protein antenna complexes which are composed of pigments such as bacteriochlorophylls (BChl) and carotenoids in APB, and chlorophylls (Chl), phycobiliproteins (PBP), and carotenoids in cyanobacteria and microalgae. These photosynthetic pigments are essential in the physiology of photosynthetic microorganisms and offer significant health benefits due to their potent antioxidant activity, with properties that include anticancer, antiaging, antiproliferative, anti-inflammatory, and neuroprotective effects. This review first provides an overview of current advances in photosynthetic pigment synthesis and the latest strategies to increase pigment content in cyanobacteria, microalgae, and APB. It then delves into the pigment production process, covering biosynthetic pathways, critical environmental parameters, and extraction methods. Finally, the potential marketability of photosynthetic pigments together with current limitations are discussed.

Current trends and possibilities of typical microbial protein production approaches: a review

Global population growth and demographic restructuring are driving the food and agriculture sectors to provide greater quantities and varieties of food, of which protein resources are particularly important. Traditional animal-source proteins are becoming increasingly difficult to meet the demand of the current consumer market, and the search for alternative protein sources is urgent. Microbial proteins are biomass obtained from nonpathogenic single-celled organisms, such as bacteria, fungi, and microalgae. They contain large amounts of proteins and essential amino acids as well as a variety of other nutritive substances, which are considered to be promising sustainable alternatives to traditional proteins. In this review, typical approaches to microbial protein synthesis processes were highlighted and the characteristics and applications of different types of microbial proteins were described. Bacteria, fungi, and microalgae can be individually or co-cultured to obtain protein-rich biomass using starch-based raw materials, organic wastes, and one-carbon compounds as fermentation substrates. Microbial proteins have been gradually used in practical applications as foods, nutritional supplements, flavor modifiers, and animal feeds. However, further development and application of microbial proteins require more advanced biotechnological support, screening of good strains, and safety considerations. This review contributes to accelerating the practical application of microbial proteins as a promising alternative protein resource and provides a sustainable solution to the food crisis facing the world.

Recovery of purple non-sulfur bacteria-mediated single-cell protein from domestic wastewater in two-stage treatment using high rate digester and raceway pond

Wastewater resources can be used to produce microbial protein for animal feed or organic fertiliser, conserving food chain resources. This investigation hasemployed thefermented sewage to photoheterotrophically grown purple non-sulfur bacteria (PNSB) in a 2.5 m3 pilot-scaleraceway-pond with infrared light to produce proteinaceous biomass. Fermented sewage with synthetic media consisting of sodium acetate and propionic acids at a surface-to-volume (S/V) ratio of 10 m2/m3 removed 89%, 93%, and 81% of chemical oxygen demand, ammonium nitrogen, and orthophosphate, respectively; whereas respective removal in fermented sewage alone without synthetic media was 73%, 73%, and 72% during batch operation of 120 h. The biomass yield of 0.88-0.95 g CODbiomass /g CODremoved with protein content of 40.3 ± 0.3%-43.9 ± 0.2% w/w was obtained for fermented sewage with synthetic media. The results revealed enhanced possibility of scaling-up the raceway reactor to recover resources from municipal wastewater and enable simultaneous high-rate PNSB single-cell protein production.

Mini review: antimicrobial compounds produced by bacteria associated with marine invertebrates

The marine environment is considered one of the most important ecosystems with high biodiversity. Microorganisms in this environment are variable and coexist with other marine organisms. The microbes associated with other marine organisms produce compounds with biological activity that may help the host's defense against invading organisms. The symbiotic association of bacteria with marine invertebrates is of ecological and biotechnological importance. Biologically active metabolites isolated from bacteria associated with marine invertebrates are considered potential sources of natural antimicrobial molecules for treating infectious diseases. Many studies have been conducted to screen the antimicrobial activity of metabolites produced by bacteria associated with marine invertebrates. This work provides an overview of the advancements in antimicrobial compound research on bacteria associated with marine invertebrates.

Ruminations on sustainable and safe food: Championing for open symbiotic cultures ensuring resource efficiency, eco-sustainability and affordability

Microbes are powerful upgraders, able to convert simple substrates to nutritional metabolites at rates and yields surpassing those of higher organisms by a factor of 2 to 10. A summary table highlights the superior efficiencies of a whole array of microbes compared to conventionally farmed animals and insects, converting nitrogen and organics to food and feed. Aiming at the most resource-efficient class of microbial proteins, deploying the power of open microbial communities, coined here as 'symbiotic microbiomes' is promising. For instance, a production train of interest is to develop rumen-inspired technologies to upgrade fibre-rich substrates, increasingly available as residues from emerging bioeconomy initiatives. Such advancements offer promising perspectives, as currently only 5%-25% of the available cellulose is recovered by ruminant livestock systems. While safely producing food and feed with open cultures has a long-standing tradition, novel symbiotic fermentation routes are currently facing much higher market entrance barriers compared to axenic fermentation. Our global society is at a pivotal juncture, requiring a shift towards food production systems that not only embrace the environmental and economic sustainability but also uphold ethical standards. In this context, we propose to re-examine the place of spontaneous or natural microbial consortia for safe future food and feed biotech developments, and advocate for intelligent regulatory practices. We stress that reconsidering symbiotic microbiomes is key to achieve sustainable development goals and defend the need for microbial biotechnology literacy education.

Potential of enriched phototrophic purple bacteria for H2 bioconversion into single cell protein

Single cell protein (SCP) has emerged as an alternative protein source, potentially based on the recovery of carbon and nutrients from waste-derived resources as part of the circular economy. From those resources, gaseous substrates have the advantage of an easy sterilization, allowing the production of pathogen-free SCP. Sterile gaseous substrates allow producing pathogen-free SCP. This study evaluated the use of an enriched phototrophic purple bacteria (PPB) consortium for SCP production using H2 and CO2 as electron and C sources. The influence of pH (6.0-8.5), temperature (15-50 °C) and light intensity (0-50 W·m-2) on the growth kinetics and biomass yields was investigated using batch tests. Optimal conditions were found at pH 7, 25 °C and light intensities over 30 W·m-2. High biomass and protein yields were achieved (~ 1 g CODbiomass·g CODH2consumed-1 and 3.9-4.4 g protein·g H2-1) regardless of the environmental conditions, being amongst the highest values reported from gaseous streams. These high yields were obtained thanks to the use of light as a sole energy source by the PPB consortium, allowing a total utilization of H2 for growth. Hydrogen uptake rates varied considerably, with values up to 61 ± 5 mg COD·d-1 for the overall H2 consumption rates and 2.00 ± 0.14 g COD·g COD-1·d-1 for the maximum specific uptake rates under optimal growth conditions. The latter value was estimated using a mechanistic model able to represent PPB growth on H2. The biomass exhibited high protein contents (>50 % w/w) and adequate amino acid profiles, showing its suitability as SCP for feed. PPB were the dominant bacteria during the experiments (relative abundance over 80 % in most tests), with a stable population dominated by Rhodobacter sp. and Rhodopseudomonas sp. This study demonstrates the potential of enriched PPB cultures for H2 bioconversion into SCP.

Cryo-EM structure of a monomeric RC-LH1-PufX supercomplex with high-carotenoid content from Rhodobacter capsulatus

In purple photosynthetic bacteria, the photochemical reaction center (RC) and light-harvesting complex 1 (LH1) assemble to form monomeric or dimeric RC-LH1 membrane complexes, essential for bacterial photosynthesis. Here, we report a 2.59-Å resolution cryoelectron microscopy (cryo-EM) structure of the RC-LH1 supercomplex from Rhodobacter capsulatus. We show that Rba. capsulatus RC-LH1 complexes are exclusively monomers in which the RC is surrounded by a 15-subunit LH1 ring. Incorporation of a transmembrane polypeptide PufX leads to a large opening within the LH1 ring. Each LH1 subunit associates two carotenoids and two bacteriochlorophylls, which is similar to Rba. sphaeroides RC-LH1 but more than one carotenoid per LH1 in Rba. veldkampii RC-LH1 monomer. Collectively, the unique Rba. capsulatus RC-LH1-PufX represents an intermediate structure between Rba. sphaeroides and Rba. veldkampii RC-LH1-PufX. Comparison of PufX from the three Rhodobacter species indicates the important residues involved in dimerization of RC-LH1.

Dehazing redox homeostasis to foster purple bacteria biotechnology

Purple non-sulfur bacteria (PNSB) show great potential for environmental and industrial biotechnology, producing microbial protein, biohydrogen, polyhydroxyalkanoates (PHAs), pigments, etc. When grown photoheterotrophically, the carbon source is typically more reduced than the PNSB biomass, which leads to a redox imbalance. To mitigate the excess of electrons, PNSB can exhibit several 'electron sinking' strategies, such as CO₂ fixation, N₂ fixation, and H₂ and PHA production. The lack of a comprehensive (over)view of these redox strategies is hindering the implementation of PNSB for biotechnology applications. This review aims to present the state of the art of redox homeostasis in phototrophically grown PNSB, presenting known and theoretically expected strategies, and discussing them from stoichiometric, thermodynamic, metabolic, and economic points of view.

Purple bacteria screening for photoautohydrogenotrophic food production: Are new H2-fed isolates faster and nutritionally better than photoheterotrophically obtained reference species?

Photoautohydrogenotrophic enrichments of wastewater treatment microbiomes were performed to obtain hypothetically high-potential specialist species for biotechnological applications. From these enrichment cultures, ten photoautohydrogenotrophic species were isolated: six Rhodopseudomonas species, three Rubrivivax members and Rhodobacter blasticus. The performance of these isolates was compared to three commonly studied, and originally photoheterotrophically enriched species (Rhodopseudomonas palustris, Rhodobacter capsulatus and Rhodobacter sphaeroides), designated as reference species. Repeated subcultivations were applied to improve the initial poor performance of the isolates (acclimation effect), which resulted in increases in both maximum growth rate and protein productivity. However, the maximum growth rate of the reference species remained 3-7 times higher compared to the isolates (0.42-0.84 d^-1 at 28 °C), while protein productivities remained 1.5-1.7 times higher. This indicated that H₂-based enrichment did not result in photoautohydrogenotrophic specialists, suggesting that the reference species are more suitable for intensified biomass and protein production. On the other hand, the isolates were able to provide equally high protein quality profiles as the references species, providing full dietary essential amino acid matches for human food. Lastly, the effect of metabolic carbon/electron switching (back and forth between auto- to heterotrophic conditions) initially boosted µ_max when returning to photoautohydrogenotrophic conditions. However, the switch negatively impacted lag phase, protein productivities and pigment contents. In the case of protein productivity, the acquired acclimation was partially lost with decreases of up to 44 % and 40 % respectively for isolates and reference species. Finally, the three reference species, and specifically Rh. capsulatus, remained the most suitable candidate(s) for further biotechnological development.

Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes

Biohydrogen (BioH₂) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH₂ from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH₂ synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH₂ production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH₂ productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH₂ systems are finally addressed, allowing the technology to become more innovative and commercial.