Single cell protein production by photosynthetic bacteria grown on the clarified effluents of biogas plant

A comprehensive report on valorization of waste to single cell protein: strategies, challenges, and future prospects

The food insecurity due to a vertical increase in the global population urgently demands substantial advancements in the agricultural sector and to identify sustainable affordable sources of nutrition, particularly proteins. Single-cell protein (SCP) has been revealed as the dried biomass of microorganisms such as algae, yeast, and bacteria cultivated in a controlled environment. Production of SCP is a promising alternative to conventional protein sources like soy and meat, due to quicker production, minimal land requirement, and flexibility to various climatic conditions. In addition to protein production, it also contributes to waste management by converting it into food and feed for both human and animal consumption. This article provides an overview of SCP production, including its benefits, safety, acceptability, and cost, as well as limitations that constrains its maximum use. Furthermore, this review criticizes the downstream processing of SCP, encompassing cell wall disruption, removal of nucleic acid, harvesting of biomass, drying, packaging, storage, and transportation. The potential applications of SCP, such as in food and feed as well as in the production of bioplastics, emulsifiers, and as flavoring agents for baked food, soup, and salad, are also discussed.

Single Cell Protein for Foods and Feeds: A Review of Trends

Introduction:

Predictions on the world’s population in the next few decades suggest that the global demand for animal-derived proteins may not be met if current conventional agriculture approaches are used. One promising solution to this complex crisis lies in the use of single-cell proteins (SCP). SCP refers to the edible biomass of unicellular microorganisms and can be developed as animal feeds or human foods. This paper provides a detailed overview on research towards the production and utilisation of SCPs and trends within the field.

Study Design:

A bibliometric based study was conducted on 425 SCP research articles collected from the Web of Science database, analysing the most cited papers using VOSviewer software, and contributing authors, affiliations and country of origin. Research publications on SCP started in 1961 and has grown steadily over the years.

Discussion:

Emerging research topics within SCP production focused on the use of improved fungal strains, the composition and characteristics of SCPs based on the type of substrates used, industrial production processes and the use of waste for SCP production, which serves the dual purpose of mitigating the cost associated with waste disposal and production of a valuable product.

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

The urgency for a protein transition towards more sustainable solutions is one of the major societal challenges. Microbial protein is one of the alternative routes, in which land- and fossil-free production should be targeted. The photohydrogenotrophic growth of purple bacteria, which builds on the H₂- and CO₂-economy, is unexplored for its microbial protein potential. The three tested species (Rhodobacter capsulatus, Rhodobacter sphaeroides and Rhodopseudomonas palustris) obtained promising growth rates (2.3-2.7 d^-1 at 28°C) and protein productivities (0.09-0.12 g protein L^-1 d^-1), rendering them likely faster and more productive than microalgae. The achieved protein yields (2.6-2.9 g protein g^-1 H₂) transcended the ones of aerobic hydrogen oxidizing bacteria. Furthermore, all species provided full dietary protein matches for humans and their fatty acid content was dominated by vaccenic acid (82-86%). Given its kinetic and nutritional performance we recommend to consider Rhodobacter capsulatus as a high-potential sustainable source of microbial food.

An overview of anoxygenic phototrophic bacteria and their applications in environmental biotechnology for sustainable Resource recovery

Anoxygenic phototrophic bacteria (APB) are a phylogenetically diverse group of organisms that can harness solar energy for their growth and metabolism. These bacteria vary broadly in terms of their metabolism as well as the composition of their photosynthetic apparatus. Unlike oxygenic phototrophic bacteria such as algae and cyanobacteria, APB can use both organic and inorganic electron donors for light-dependent fixation of carbon dioxide without generating oxygen. Their versatile metabolism, ability to adapt in extreme conditions, low maintenance cost and high biomass yield make APB ideal for wastewater treatment, resource recovery and in the production of high value substances. This review highlights the advantages of APB over algae and cyanobacteria, and their applications in photo-bioelectrochemical systems, production of poly-β-hydroxyalkanoates, single-cell protein, biofertilizers and pigments. The ecology of ABP, their distinguishing factors, various physiochemical parameters governing the production of high-value substances and future directions of APB utilization are also discussed.

Production of single cell protein from manure as animal feed by using photosynthetic bacteria

To reduce the cost of protein feedstock for animal feed, the use of single cell protein (SCP) produced from waste of animal agriculture is an interesting choice. This study reveals that chicken manure was the best substrate for SCP production by submerged fermentation using photosynthetic bacteria compared to swine, cow, and buffalo manure. Regression analysis showed that the productions were found to be significantly influenced by chicken manure content, inoculum size, and cultivation time. Response surface methodology based on central composite design generated the optimal condition (15% chicken manure, 30% inoculum size and cultivation time for 14 days) at which biomass, protein, and carotenoid productions were increased by 92.3%, 21.6%, and 18.2%, respectively. The percentage of error between the predicted and actual values for biomass, protein, and carotenoid productions were 1.56%, 2.64%, and 2.09%, respectively, which indicates the precision of the model. To verify the quality of SCP, the bacterium was cultured in a photobioreactor to investigate amino acid composition, protein, and nucleic acid contents. The SCP yielded 62.7% protein with essential amino acids including lysine, methionine, threonine, phenylalanine, leucine, isoleucine, valine, histidine, and low nucleic acid content of 4.52%. This study suggests an alternative SCP production for animal feed as well as the strategy for animal waste management.

Volatile fatty acids impacting phototrophic growth kinetics of purple bacteria: Paving the way for protein production on fermented wastewater

Nutrient losses in our food chain severely surpass our planetary boundaries. Resource recovery can contribute to mitigation, for instance through converting wastewater resources to microbial protein for animal feed. Wastewater typically holds a complex mixture of organics, posing a challenge to selectively produce heterotrophic biomass. Ensuring the product's quality could be achieved by anaerobic generation of volatile fatty acids (VFAs) followed by photoheterotrophic production of purple non-sulfur bacteria (PNSB) with infrared light. This study aimed to determine the most suitable PNSB culture for VFA conversion and map the effect of acetate, propionate, butyrate and a VFA mixture on growth and biomass yield. Six cultures were screened in batch: (i) Rhodopseudomonas palustris, (ii) Rhodobacter sphaeroides, (iii) Rhodospirillum rubrum, (iv) a 3-species synthetic community (i+ii+iii), (v) a community enriched on VFA holding Rb. capsulatus, and (vi) Rb. capsulatus (isolate 'v'). The VFA mixture elevated growth rates with a factor 1.3-2.5 compared to individual VFA. Rb. capsulatus showed the highest growth rates: 1.8-2.2 d^-1 (enriched) and 2.3-3.8 d^-1 (isolated). In a photobioreactor (PBR) inoculated with the Rb. capsulatus enrichment, decreasing sludge retention time (SRT) yielded lower biomass concentrations, yet increased productivities, reaching 1.7 g dry weight (DW) L^-1 d^-1, the highest phototrophic rate reported thus far, and a growth rate of up to 5 d^-1. PNSB represented 26-57% of the community and the diversity index was low (3-7), with a dominance of Rhodopseudomonas at long SRT and Rhodobacter at short SRT. The biomass yield for all cultures, in batch and reactor cultivation, approached 1 g COD_Biomass g^-1 COD_Removed. An economic estimation for a two-stage approach on brewery wastewater (load 2427 kg COD d^-1) showed that 0.5 d SRT allowed for the lowest production cost (€ 10 kg^-1 DW; equal shares for capex and opex). The findings strengthen the potential for a novel two-stage approach for resource recovery from industrial wastewater, enabling high-rate PNSB production.

Biomass production from fermented starch wastewater in photo-bioreactor with internal overflow recirculation

A photo-bioreactor with internal overflow recirculation was applied to treat real fermented starch wastewater and convert it to photosynthetic biomass for further utilization. The photo-bioreactor was operated at a hydraulic retention time of 10days by circulating mixed liquor through overflow pipes and penetrating light through infrared transmitting filter. During the operation of 154days, the average BOD and COD removals were 95% and 88%, respectively. Majority of photosynthetic bacteria was found attached on pipes as biofilm contributed to 82% of total biomass production. Photosynthetic biomass yield was 0.51g dried solid/g BOD removed and crude protein content of 0.58g/g dried solid. Rhodopseudomonas palustris was found in the photosynthetic system as the predominant bacterial group by denaturing gradient gel electrophoretic analysis (DGGE) and 16S rDNA sequencing method.

Growth characteristics of Rhodopseudomonas palustris cultured outdoors, in an underwater tubular photobioreactor, and investigation on photosynthetic efficiency

The underwater tubular photobioreactor is a fully controlled outdoor system to study photosynthetic bacteria. Before growing bacteria cells outdoors, two modified van Niel medium (vN-A, vN-B) were tested under artificial light. During exponential growth, the specific growth rates were 0.0416 and 0.0434 h(-1), respectively; vN-B was chosen for outdoor experiments. The growth behavior of Rhodopseudomonas palustris was investigated under a natural light-dark cycle (sunrise-sunset, 15L/9D) and a forced light-dark cycle (9:00-19:00, 10L/14D). Relationships between solar radiations, daily growth rates, and biomass output rates were also investigated. After determining the elemental biomass molar composition and its combustion heat, some trends of photosynthetic efficiency (PE) were obtained over daylight. The PE trends were always of the oscillatory type, with the exception of that achieved at low biomass concentration. Under a natural light/dark cycle, the maximum PE (11.2%) was attained at sunset, while under a forced light/dark cycle, the highest PE (8.5%) was achieved in the morning. Three initial biomass concentrations were investigated (0.65, 1.01, and 1.54 g l(-1)). The stoichiometric equation for bacteria cells indicated that 87.7% of the carbon of acetic acid was converted to biomass and only 12.3% was lost as CO(2).

Microbial ecology of the closed artificial ecosystem MELiSSA (Micro-Ecological Life Support System Alternative): reinventing and compartmentalizing the Earth's food and oxygen regeneration system for long-haul space exploration missions

MELiSSA is a bioregenerative life support system designed by the European Space Agency (ESA) for the complete recycling of gas, liquid and solid wastes during long distance space exploration. The system uses the combined activity of different living organisms: microbial cultures in bioreactors, a plant compartment and a human crew. In this minireview, the development of a short-cut ecological system for the biotransformation of organic waste is discussed from a microorganism's perspective. The artificial ecological model--still in full development--that is inspired by Earth's own geomicrobiological ecosystem serves as an ideal study object on microbial ecology and will become an indispensable travel companion in manned space exploration.

The MELISSA pilot plant facility as as integration test-bed for advanced life support systems

The different advances in the Micro Ecological Life Support System Alternative project (MELISSA), fostered and coordinated by the European Space Agency, as well as in other associated technologies, are integrated and demonstrated in the MELISSA Pilot Plant laboratory. During the first period of operation, the definition of the different compartments at an individual basis has been achieved, and the complete facility is being re-designed to face a new period of integration of all these compartments. The final objective is to demonstrate the potentiality of biological systems such as MELISSA as life support systems. The facility will also serve as a test bed to study the robustness and stability of the continuous operation of a complex biological system. This includes testing of the associated instrumentation and control for a safe operation, characterization of the chemical and microbial safety of the system, as well as tracking the genetic stability of the microbial strains used. The new period is envisaged as a contribution to the further development of more complete biological life support systems for long-term manned missions, that should be better defined from the knowledge to be gained from this integration phase. This contribution summarizes the current status of the Pilot Plant and the planned steps for the new period.

Modeling stability of photoheterotrophic continuous cultures in photobioreactors

Continuous cultures of the purple non-sulfur bacterium Rhodospirillum rubrum were grown in a cylindrical photobioreactor in photoheterotrophic conditions, using acetate as carbon source. A new kinetic and stoichiometric knowledge model was developed, and its ability to simulate experimental results obtained under varying incident light fluxes and residence times is discussed. The model accurately predicts the stable, unstable, or oscillating behavior observed for the reactor productivity. In particular, the values of residence time corresponding to a subcritical bifurcation with a typical hysteresis effect are calculated and analyzed. The robustness of the proposed model allows the engineering operating domain of the photobioreactor function to be set and offers a promising tool for the design and control of such photoheterotrophic processes.

MELISSA: a loop of interconnected bioreactors to develop life support in space

The development of a loop of interconnected continuous bioreactors, aimed to provide life support in space, is reported. The complete loop concept consists of four bioreactors and one higher plant compartment. For its realization the continuous and controlled operation of the bioreactors is characterized, up to the pilot scale level, first for each individual reactor, second for the interconnected reactor operation. The results obtained with the two more advanced bioreactors in the Micro Ecological Life Support System Alternative (MELISSA) loop are described more specifically. These reactors consist of a packed-bed reactor working with an immobilized co-culture of Nitrosomonas and Nitrobacter cells, and an external loop gas-lift photobioreactor for the culture of the cyanobacteria Spirulina platensis. Their individual operation for long duration runs has been achieved and characterized, and their interconnected operation at pilot scale is reported.

Biomass production and studies on Rhodopseudomonas palustris grown in an outdoor, temperature controlled, underwater tubular photobioreactor

A temperature controlled underwater tubular photobioreactor was studied for 6 months in outdoor conditions to determine biomass production of Rhodopseudomonas palustris 42OL. Biomass output rate was very high during the study period. In July, productivity was about 75.0 g reactor(-1) d(-1) for a short period (5 days). The biomass yield averaged 0.7 g biomass dry weight (d.w.)xg acetic acid(-1). We also obtained a mean photosynthetic efficiency of 7.6% in winter and 7.1% in summer. Biomasses rich in pigments and a biodegradable thermoplastic polymer [poly-beta-hydroxybutyrate (PHB)] were obtained. During the summer, PHB was about 4.0% of the biomass d.w. The bacteriochlorophyll/carotenoid ratio was lower at midday than either in the morning or in the evening. The reduction in this ratio may have been a response of the Rhodopseudomonas cells to high irradiance (about 900 W m(-2)). A reduced rate of protein synthesis continued even in the dark. PHB and carbohydrates, synthesized in excess during the day, were used as substrates for night protein synthesis.