Microbiome dynamics and phaC synthase genes selected in a pilot plant producing polyhydroxyalkanoate from the organic fraction of urban waste

Effect of composition of volatile fatty acids on yield of polyhydroxyalkanoates and mechanisms of bioconversion from activated sludge

Polyhydroxyalkanoates (PHA) is green biodegradable natural polymer. Here PHA production from volatile fatty acids (VFAs) was investigated in sequential batch reactors inoculated with activated sludge. Single or mixed VFAs ranging from acetate to valerate were evaluated, and the dominant VFA concentration was 2 times of that of the others in the tests. Results showed that mixed substrates achieved about 1.6 times higher yield of PHA production than single substrate. The butyrate-dominated substrates maximized PHA content at 72.08% of VSS, and the valerate-dominated substrates were followed with PHA content at 61.57%. Metabolic flux analysis showed the presence of valerate in the substrates caused a more robust PHA production. There was at least 20% of 3-hydroxyvalerate in the polymer. Hydrogenophaga and Comamonas were the main PHA producers. As VFAs could be produced in anaerobic digestion of organic wastes, the methods and data here could be referred for efficient green bioconversion of PHA.

Calcium enhances polyhydroxyalkanoate production and promotes selective growth of the polyhydroxyalkanoate-storing biomass in municipal activated sludge

Activated sludge from municipal wastewater treatment processes can be used directly for the production of biodegradable polyesters from the family of polyhydroxyalkanoates (PHAs). However, municipal activated sludge typically cannot accumulate PHAs to very high levels and often low yields of polymer produced on substrate are observed. In the present work, it was found that the presence of calcium promotes selective growth and enrichment of the PHA-storing biomass fraction and significantly improved both PHA contents and yields. Calcium addition resulted in PHA contents of 0.60 ± 0.03 gPHA/gVSS and average PHA yields on substrate of 0.49 ± 0.03 gCOD_PHA/gCOD_HAc compared to 0.35 ± 0.01 gPHA/gVSS and 0.19 ± 0.01 gCOD_PHA/gCOD_HAc without calcium addition. After 48 h, three times more PHA was produced compared to control experiments without calcium addition. Higher PHA content and selective biomass production is proposed to be a consequence of calcium dependent increased levels of passive acetate uptake. Such more efficient substrate uptake could be related to a formation of calcium acetate complexes. Findings lead to bioprocess methods to stimulate a short-term selective growth of PHA-storing microorganisms and this enables improvements to the techno-economic feasibility for municipal waste activated sludge to become a generic resource for industrial scale PHA production.

Simplified engineering design towards a competitive lipid-rich effluents valorization

Medium- and long-chain fatty acids and glycerol contained in the oily fraction of many food-industry effluents are excellent candidates to produce biobased high-value triacylglycerides (TAGs) and polyhydroxyalkanoates (PHAs). The typical process configuration for TAGs recovery from lipid-rich streams always includes two steps (culture enrichment plus storage compounds accumulation) whereas, for PHAs production, an additional pretreatment of the substrate for the obtainment of soluble volatile fatty acids (VFAs) is required. To simplify the process, substrate hydrolysis, culture enrichment, and accumulation (TAG and PHA storage) were coupled here in a single sequencing batch reactor (SBR) operated under the double growth limitation strategy (DGL) and fed in pulses with industrial waste fish oil during the whole feast phase. When the SBR was operated in 12 h cycles, it was reached up to 51 wt % biopolymers after only 6 h of feast (TAG:PHA ratio of 50:51; 0.423 Cmmol_BIOP/Cmmol_S). Daily storage compound production was observed to be over 25% higher than the reached when enrichment and accumulation stages were carried in separate operational units. Increasing the feast phase length from 6 to 12 h (18 h cycle) negatively affected the DGL strategy performance and hence system storage capacity, which was recovered after also extending the famine phase in the same proportion (24 h cycle). Besides, the carbon influx during the feast phase was identified as a key operational parameter controlling storage compounds production and, together with the C/N ratio, culture selection. The different cycle configurations tested clearly modulated the total fungal abundances without no significant differences in the size of the bacterial populations. Several PHA and TAG producers were found in the mixed culture although the PHA and TAG productions were poorly associated with the increased relative abundances (RAs) of specific operational taxonomic units (OTUs).

Valorization of Brewery Waste through Polyhydroxyalkanoates Production Supported by a Metabolic Specialized Microbiome

Simple Summary

Raw brewers’ spent grain, a by-product of beer production, is produced at a large scale and is usually used as animal feed or is landfilled. However, its composition shows that this feedstock has the potential for other applications, such as bioplastics production (e.g., polyhydroxyalkanoates). In this way, the aim of this work was to assess the use of raw brewers’ spent grain for polyhydroxyalkanoates production, adding new value to this feedstock. The results confirm the potential of raw brewers’ spent grain to produce polyhydroxyalkanoates, as the population was enriched in the microorganisms able to accumulate these biopolymers. These results will contribute to society’s knowledge and competence via the development of a treatment process for brewery waste of both environmental (productive waste treatment) and economic interest (production of biopolymers), which will certainly attract its application to the brewery industry worldwide.

Abstract

Raw brewers’ spent grain (BSG), a by-product of beer production and produced at a large scale, presents a composition that has been shown to have potential as feedstock for several biological processes, such as polyhydroxyalkanoates (PHAs) production. Although the high interest in the PHA production from waste, the bioconversion of BSG into PHA using microbial mixed cultures (MMC) has not yet been explored. This study explored the feasibility to produce PHA from BSG through the enrichment of a mixed microbial culture in PHA-storing organisms. The increase in organic loading rate (OLR) was shown to have only a slight influence on the process performance, although a high selectivity in PHA-storing microorganisms accumulation was reached. The culture was enriched on various PHA-storing microorganisms, such as bacteria belonging to the Meganema, Carnobacterium, Leucobacter, and Paracocccus genera. The enrichment process led to specialization of the microbiome, but the high diversity in PHA-storing microorganisms could have contributed to the process stability and efficiency, allowing for achieving a maximum PHA content of 35.2 ± 5.5 wt.% (VSS basis) and a yield of 0.61 ± 0.09 Cmmol_PHA/Cmmol_VFA in the accumulation assays. Overall, the production of PHA from fermented BSG is a feasible process confirming the valorization potential of the feedstock through the production of added-value products.

Exploring the Limits of Polyhydroxyalkanoate Production by Municipal Activated Sludge

Municipal activated sludge can be used for polyhydroxyalkanoate (PHA) production, when supplied with volatile fatty acids. In this work, standardized PHA accumulation assays were performed with different activated sludge to determine (1) the maximum biomass PHA content, (2) the degree of enrichment (or volume-to-volume ratio of PHA-accumulating bacteria with respect to the total biomass), and (3) the average PHA content in the PHA-storing biomass fraction. The maximum attained biomass PHA content with different activated sludge ranged from 0.18 to 0.42 gPHA/gVSS, and the degree of enrichment ranged from 0.16 to 0.51 volume/volume. The average PHA content within the PHA-accumulating biomass fraction was relatively constant and independent of activated sludge source, with an average value of 0.58 ± 0.07 gPHA/gVSS. The degree of enrichment for PHA-accumulating bacteria was identified as the key factor to maximize PHA content when municipal activated sludge is directly used for PHA accumulation. Future optimization should focus on obtaining a higher degree of enrichment of PHA-accumulating biomass, either through selection during wastewater treatment or by selective growth during PHA accumulation. A PHA content in the order of 0.6 g PHA/g VSS is a realistic target to be achieved when using municipal activated sludge for PHA production.

Quantification of polyhydroxyalkanoate accumulated in waste activated sludge

Polyhydroxyalkanoate accumulation experiments at pilot scale were performed with fullscale municipal waste activated sludge. Development of biomass PHA content was quantified by thermogravimetric analysis. Over 48 h the biomass reached up to 0.49 ± 0.03 gPHA/gVSS (n=4). Samples were processed in parallel to characterise the distribution of PHA in the biomass. Selective staining methods and image analysis were performed by Confocal Laser Scanning Microscopy. The image analysis indicated that nominally 55% of this waste activated sludge was engaged in PHA storage activity. Thus even if the biomass PHA content reached 0.49gPHA/gVSS, the accumulating fraction of the biomass was estimated to have attained about 0.64gPHA/gVSS. The combination of quantitative microscopy and polymer mass assessment enabled to distinguish the effect of level of enrichment in PHA storing bacteria and the average PHA storage capacity of the accumulating bacteria. The distribution of microbial 16S rRNA levels did not follow a measurable trend during PHA accumulation.

From Organic Wastes and Hydrocarbons Pollutants to Polyhydroxyalkanoates: Bioconversion by Terrestrial and Marine Bacteria

The use of fossil-based plastics has become unsustainable because of the polluting production processes, difficulties for waste management sectors, and high environmental impact. Polyhydroxyalkanoates (PHA) are bio-based biodegradable polymers derived from renewable resources and synthesized by bacteria as intracellular energy and carbon storage materials under nutrients or oxygen limitation and through the optimization of cultivation conditions with both pure and mixed culture systems. The PHA properties are affected by the same principles of oil-derived polyolefins, with a broad range of compositions, due to the incorporation of different monomers into the polymer matrix. As a consequence, the properties of such materials are represented by a broad range depending on tunable PHA composition. Producing waste-derived PHA is technically feasible with mixed microbial cultures (MMC), since no sterilization is required; this technology may represent a solution for waste treatment and valorization, and it has recently been developed at the pilot scale level with different process configurations where aerobic microorganisms are usually subjected to a dynamic feeding regime for their selection and to a high organic load for the intracellular accumulation of PHA. In this review, we report on studies on terrestrial and marine bacteria PHA-producers. The available knowledge on PHA production from the use of different kinds of organic wastes, and otherwise, petroleum-polluted natural matrices coupling bioremediation treatment has been explored. The advancements in these areas have been significant; they generally concern the terrestrial environment, where pilot and industrial processes are already established. Recently, marine bacteria have also offered interesting perspectives due to their advantageous effects on production practices, which they can relieve several constraints. Studies on the use of hydrocarbons as carbon sources offer evidence for the feasibility of the bioconversion of fossil-derived plastics into bioplastics.

Effect of the organic loading rate on the PHA-storing microbiome in sequencing batch reactors operated with uncoupled carbon and nitrogen feeding

Over the last years, in a search for sustainable and biodegradable alternatives to petrol-based plastics, biotechnological applications turned to the potentialities of mixed microbial cultures (MMC) for producing polyhydroxyalkanoates (PHAs). Under a feast and famine regime, an uncoupled carbon (C) and nitrogen (N)-feeding strategy may be adopted by dosing the C-source at the beginning of the feast and the N-source at the beginning of the famine in order to stimulate a PHA storage response and microbial growth. Even though this strategy has been already successfully applied for the PHA production, very few information is to date available regarding the MMC operating in these systems and the influence of Organic Loading Rate (OLR) on their selection and enrichment. To fill the gap, this study investigated the effect of the OLR on the selection of PHA-accumulating microorganisms in a sequencing batch reactor (SBR) operated with an uncoupled C and N feeding strategy. The SBR cycle length was set at 12 h and four OLRs values (4.25, 8.50, 12.75 and 18 gCOD L^-1 d^-1) were tested by changing the concentration of the feeding solution, made of a synthetic mixture of acetic (85% of the overall COD) and propionic (15%) acids. The PHA-storage yield increased by increasing the OLR (up to 0.69 COD/COD at 12.75 gCOD L^-1 d^-1) but significantly decreased (0.27 COD/COD) at 18 gCOD L^-1 d^-1 concomitantly with a longer feast phase and a lower PHA content in the biomass at the end of the feast phase. The selective pressure induced by the applied OLRs strongly influenced the microbiome composition revealing a high content of putative PHA-storing bacteria, such as Rhodobacter, Thauera and Paracoccus, in the SBR operated at OLRs 4.25, 8.50 and 12.75 g COD L^-1 d^-1 (up to 97.4% of total reads) and a low content (5.4%) in the SBR at 18 g COD L^-1 d^-1where the predominance of genus Nitrinicola was instead observed.

Ammonium-assimilating microbiome: A halophilic biosystem rationally optimized by carbon to nitrogen ratios with stable nitrogen conversion and microbial structure

The contradiction between theoretical metabolism of ammonium assimilation and experiential understanding of conventional biosystems makes the rational optimization of the ammonium-assimilating microbiome through carbon to nitrogen (C/N) ratios perplexing. The effect of different C/N ratios on ammonium-assimilating biosystems was investigated in saline wastewater treatment. C/N ratios significantly hindered the nutrient removal efficiency, but ammonium-assimilating biosystems maintained functional stability in nitrogen conversions and microbial communities. With sufficient biomass, higher than 86% ammonium and 73% phosphorus were removed when C/N ratios were higher than 25. Ammonium assimilation dominated the nitrogen metabolism in all biosystems even under relatively low C/N ratios, evidenced by the extremely low abundances of nitrification functional genes. Different C/N ratios did not significantly change the bacterial community structure of ammonium-assimilating biosystems. It is anticipated that the ammonium-assimilating biosystem with advantages of clear metabolic pathway and easy optimization can be applied to nutrient removal and recovery in saline environments.

Rapid recovery of mixed culture polyhydroxyalkanoate production system from EPS bulking using azithromycin

The long-term stable operation of the mixed culture polyhydroxyalkanoate (PHA) enrichment stage is the guarantee for the continuous synthesis of PHA, however extracellular polymeric substances (EPS) sludge bulking occurred from time to time may cause the operation fail. In order to solve this problem, as a quencher of signal molecules and antibiotic, azithromycin (AZM) was used in the two systems with different modes to recover the sedimentation capacity of the sludge. The results showed that AZM addition resulted in the reduction of polysaccharide /protein (PS/PN) ratio in EPS and significant improvement of the sedimentation capacity of the sludge. Quorum quenching of AZM or aiiA gene maintained the sedimentation ability of the sludge in a relay mode. By adding AZM, the growth of Thauera and Flavobacterium, which caused sludge bulking, was inhibited. Paracoccus, a strong PHA producer, has been enriched to ensure that the maximum PHA synthesis of the system.

Dynamics of PHA-Accumulating Bacterial Communities Fed with Lipid-Rich Liquid Effluents from Fish-Canning Industries

The biosynthesis of polyhydroxyalkanoates (PHAs) from industrial wastes by mixed microbial cultures (MMCs) enriched in PHA-accumulating bacteria is a promising technology to replace petroleum-based plastics. However, the populations’ dynamics in the PHA-accumulating MMCs are not well known. Therefore, the main objective of this study was to address the shifts in the size and structure of the bacterial communities in two lab-scale sequencing batch reactors (SBRs) fed with fish-canning effluents and operated under non-saline (SBR-N, 0.5 g NaCl/L) or saline (SBR-S, 10 g NaCl/L) conditions, by using a combination of quantitative PCR and Illumina sequencing of bacterial 16S rRNA genes. A double growth limitation (DGL) strategy, in which nitrogen availability was limited and uncoupled to carbon addition, strongly modulated the relative abundances of the PHA-accumulating bacteria, leading to an increase in the accumulation of PHAs, independently of the saline conditions (average 9.04 wt% and 11.69 wt%, maximum yields 22.03 wt% and 26.33% SBR-N and SBR-S, respectively). On the other hand, no correlations were found among the PHAs accumulation yields and the absolute abundances of total Bacteria, which decreased through time in the SBR-N and did not present statistical differences in the SBR-S. Acinetobacter, Calothrix, Dyella, Flavobacterium, Novosphingobium, Qipengyuania, and Tsukamurella were key PHA-accumulating genera in both SBRs under the DGL strategy, which was revealed as a successful tool to obtain a PHA-enriched MMC using fish-canning effluents.

Effects of sulfide availability on the metabolic activity and population dynamics of cable bacteria in freshwater sediment

Cable bacteria occur in many natural environments, and their electrogenic sulfide oxidation (e-SO_x) may influence sediment biogeochemistry. The environmental factors determining the growth and diversity of cable bacteria are poorly known, especially in freshwater sediments. We conducted a laboratory incubation experiment, using freshwater sediments with different sulfide supply levels, to study how sulfide availability in sediment affects the metabolic activity and population dynamics of cable bacteria. A moderate increase in the sulfide availability in sediment significantly promoted metabolic activity and the proliferation of the cable bacteria population, as revealed by enhanced e-SO_x intensity and increased bacteria abundance. In high-sulfide treatments there was a more significant increase in the population of cable bacteria in the deeper sediment layers, indicating that increased sulfide availability may expand the vertical scale impact of cable bacteria activities on sediment biogeochemistry. The relative proportions of co-existing species in the cable bacteria population also changed with sulfide supply levels, indicating that sulfide availability can be involved in determining the interspecies relationships of cable bacteria. Our findings provide new insight into the relationship between sediment sulfide availability and the growth, depth distribution, and species composition of cable bacteria, implying the consideration of regulating environmental sulfide availability as a potential management practice for the development of cable bacteria-based environmental biotechnologies.

Optimization of aerobic dynamic discharge process for very rapid enrichment of polyhydroxyalkanoates-accumulating bacteria from activated sludge

The aerobic dynamic discharge (ADD) process has the potential to reduce the enrichment period of polyhydroxyalkanoates (PHA)-accumulating bacteria in PHA production using mixed microbial cultures (MMCs). This study aimed to efficiently enrich PHA-accumulating bacteria from activated sludge within a fixed period of 2 d by optimizing operating conditions of the ADD process. Based on the results, enrichment with separate feeding of carbon and nutrients in the feast and famine phases, respectively, and a settling duration of 10 min after the feast phase in the sequencing batch cycle for 12 h was found to be optimal. The MMC enriched at optimum conditions could store as much as 68.4 wt% of PHA. Dechloromonas and Zoogloea were identified as potential PHA-accumulating bacteria responsible for enhancing PHA accumulation ability in the enriched MMC. The optimized ADD process will facilitate the consecutive use of daily generated waste activated sludge for PHA production.

Scaling-up microbial community-based polyhydroxyalkanoate production: status and challenges

Conversion of organic waste and wastewater to polyhydroxyalkanoates (PHAs) offers a potential to recover valuable resources from organic waste. Microbial community-based PHA production systems have been successfully applied in the last decade at lab- and pilot-scales, with a total of 19 pilot installations reported in the scientific literature. In this review, research at pilot-scale on microbial community-based PHA production is categorized and subsequently analyzed with focus on feedstocks, enrichment strategies, yields of PHA on substrate, biomass PHA content and polymer characterization. From this assessment, the challenges for further scaling-up of microbial community-based PHA production are identified.

Parallel artificial and biological electric circuits power petroleum decontamination: The case of snorkel and cable bacteria

Degradation of petroleum hydrocarbons (HC) in sediments is often limited by the availability of electron acceptors. By allowing long-distance electron transport (LDET) between anoxic sediments and oxic overlying water, bioelectrochemical snorkels may stimulate the regeneration of sulphate in the anoxic sediment thereby accelerating petroleum HC degradation. Cable bacteria can also mediate LDET between anoxic and oxic sediment layers and thus theoretically stimulate petroleum HC degradation. Here, we quantitatively assessed the impact of cable bacteria and snorkels on the degradation of alkanes in marine sediment from Aarhus Bay (Denmark). After seven weeks, cable bacteria and snorkels accelerated alkanes degradation by +24 and +25%, respectively, compared to control sediment with no cable bacteria nor snorkel. The combination of snorkels and cable bacteria further enhanced alkanes degradation (+46%). Higher degradation rates were sustained by LDET-induced sulphide removal rather than, as initially hypothesized, sulphate regeneration. Cable bacteria are thus overlooked players in the self-healing capacity of crude-oil contaminated sediments, and may inspire novel remediation treatments upon hydrocarbon spillage.

Formation of microbial products by activated sludge in the presence of a metabolic uncoupler o-chlorophenol in long-term operated sequencing batch reactors

Metabolic uncouplers are widely used for reducing excess sludge in biological wastewater treatment systems. However, the formation of microbial products, such as extracellular polymeric substances, polyhydroxyalkanoate and soluble microbial products by activated sludge in the presence of metabolic uncouplers remains unrevealed. In this study, the impacts of a metabolic uncoupler o-chlorophenol (oCP) on the reduction of activated sludge yield and formation of microbial products in laboratory-scale sequencing batch reactors (SBRs) were evaluated for a long-term operation. The results show the average reduction of sludge yield in the four reactors was 17.40%, 25.80%, 33.02% and 39.50%, respectively, when dosing 5, 10, 15, and 20 mg/L oCP. The oCP addition slightly reduced the pollutant removal efficiency and decreased the formation of soluble microbial products in the SBRs, but stimulated the productions of extracellular polymeric substances and polyhydroxyalkanoate in activated sludge. Furthermore, the significant reduction of electronic transport system activity occurred after the oCP addition. Microbial community analysis of the activated sludge indicates dosing oCP resulted in a decrease of sludge richness and diversity in the SBRs. Hopefully, this study would provide useful information for reducing sludge yield in biological wastewater treatment systems and behaviors of activated sludge in the presence of uncouplers.

Agroindustrial waste as a resource for volatile fatty acids production via anaerobic fermentation

This study evaluated the feasibility of the anaerobic digestion as a sustainable valorisation strategy for volatile fatty acids production from agroindustrial waste (cucumber, tomato and lettuce). High bioconversion efficiencies were reached by operating the reactors at 25 °C, 3 g VS·d^-1·L^-1 with pH adjustment. Cucumber fermentation achieved the highest bioconversion (52.6%), whereas tomato degradation was the least efficient bioprocess (40.1%) due to the low pH (5.6) that partially inhibited the hydrolytic and acidogenic activities. In all cases, carboxylic acid profiles were mainly composed of volatile fatty acids with even carbon number. The developed microbial community exhibited high hydrolytic and acidogenic activities associated to carbohydrates degradation. This microbial population was dominated by Firmicutes phylum and showed a lack of acetogenic bacteria related with CH₄ production, resulting in a remarkably high VFAs accumulation.